Systematic Survey of Short X-Ray Transients by MAXI

Home , MAXI (ISS Experiment)

Tokyo Institute of Technology

Doctoral Thesis

Systematic Survey of Short X-ray Transients by MAXI

Author: Supervisor: Ryuichi Usui Prof. Nobuyuki Kawai

A thesis submitted in fulﬁlment of the requirements for the degree of Doctor of Physics

in the

Kawai Grp. Department of Physics

March 2014 Abstract

We present the systematic search of short X-ray transients for the 1327-day data obtained with MAXI (Monitor of All-sky X-ray Image). Short X-ray transients have been detected by past X- ray/γ-ray satellites in large-sky surveys, and their temporal, spectral and spatial characteristics have been studied. They include Gamma-Ray Bursts (GRBs), X-ray bursts from neutron-star binaries, X-ray flares of active stars and X-ray short activities of blazars. In addition shock breakouts of supernovae and tidal disruption events at the center of a nearby galaxy have been also reported recently. Discovery of new class of transients should also benefit astrophysics. To perform the search we used the data obtained by the Gas Slit Camera (GSC) of MAXI, an X-ray detector that monitors almost the whole sky with the arc-like field of view every 92- min orbital period of the International space station. While the search of transients using the GSC data has been already performed in the past, its method required detailed modeling of the instrument and the background, and the propagation of errors and the evaluation of significance was not trivial. In addition, the used data covered only the first 15-month of operation. Thus, we newly develop a simple and versatile analysis program, and conducted the search for short X-ray transients from the MAXI-GSC data obtained since October 2009 until May 2013.

The search consists of two main processes; global search and localization. First we prepare a dataset which excludes data from the galactic plane and the vicinity of 30 bright X-ray sources in order to prevent false detections. Next, as the global search, we extracted candidate transient events which have significant excess X-ray counts over the background assuming the Poisson statistics. Third, the localization process determines the accurate position, X-ray flux and refined detection significance of these candidates. The significance is evaluated by the probability for the observed excess assuming the null hypothesis that it is consistent with the background, which is sampled in the regions surrounding the candidate in the time-vs-detector coordinate plane. Since these excesses include false detections originating from X-rays of solar flares and increasing of background, we carefully removed them and finally obtained 146 short X-ray transient with sufficiently high significance. ii

First we performed cross matching of the detected short X-ray transients with the reported GRBs and some optical/X-ray/γ-ray source catalogs. This study resulted in associations with 11 prompt emissions of GRBs, one GRB’s afterglow, several X-ray bursts from neutron-star X- ray binaries, and flares of active stars. On the other hand, we cannot find significant associations with active galactic nuclei partly because the localization error is not so small considering the source density on the sky.

In addition we studied spectral characteristics and investigate positional and spatial distributions of the detected short X-ray transients. Their hardness ratio of 8-20 keV to 2-8 keV energy bands is larger on average than those of stellar ﬂares, and comparable to those of X-ray bursts and prompt emission of GRBs. The dipole and quadruple moment of the transients imply their positional distribution is isotropic, which is consistent with the expectation of the previous studies. However, V/Vmax test reveals that the spatial distribution is not consistent with a uniform distribution in space. This distribution indicates the detected transients include too many faint events near thresholds. In addition, the log N-log S plot shows excess of the faint transients, and is also inconsistent with the uniform spatial distribution. The excess of short X-ray transients at large distance implies a distribution astrophysically implausible.

To investigate possible origins of such false detections, we examined their correlations with the detected time, detector coordinate and detected position on earth. However, we found no significant dependence on them, meaning that solar flares or variation of charged particles at high latitudes and around the South Atlantic anomaly is not responsible for the false detections. The variations of anti-coincident signal count are confirmed not to significantly affect the false detections except for one case. We found, however, one anomaly. One of the GSC (camera 7) had significantly larger number of transients detections than another (Camera 1) while these two cameras share the same field of view. This indicates that one of the origins of false detections in intrinsic to the camera, probably noise in the detector or readout electronics. Acknowledgements

For the five year of my graduate course, I have been supported and encouraged by many people. I would like to express deep gratitude to my supervisor associate professor N. Kawai who has provided the scientific facts, the way of thinking and the discussion based on his deep knowledges and experiences. I want to thank assistant professor Y. Yatsu, Dr. Y. Saito and Dr. M. Arimoto who have encouraged me and talked about not only scientific discussion but also dairy topics. The members of our laboratory and friends also have uplifted me, so I thank them.

In this thesis, I used the scientific data obtained by MAXI. This mission is operated by the collaboration of the agency, institutes and universities. I am grateful to the all MAXI operation and scientific team who gives me the opportunity for analyzing the valuable and notable data. Especially, I have greatly benefited form the scientific members; Prof. M. Matsuoka, Prof. H. Negro, Dr. T. Mihara, Dr. M. Serino, Dr. M. Sugizaki, Dr. M. Morii and Dr. M. Kimura. This thesis mostly reflects their wide-ranging discussions and constructive suggestions for data analysis and physical interpretations.

Finally, I owe my deepest gratitude to my family. My father, mother and grandmother mentally and ﬁnancially care about me. I hope they will enjoy their life after the childcare about me and brothers. I also thank to my brothers who are my best encouraging sympathizers.

This work has been ﬁnancially supported by Global-COE “Nanoscience and Quantum Physics”. In addition, I thank to developed technologies and cultures which support and encourage me. The template of this thesis is provided by www.latextemplates.com.

iii Contents

Abstract i

Acknowledgements iii

List of Figures vii

List of Tables xii

Abbreviations xiii

1 Introduction 1

2 Background information of short X-ray transient 3 2.1 Observable transient phenomena with MAXI ...... 3 2.1.1 Gamma-ray burst ...... 3 2.1.2 X-ray ﬂash ...... 7 2.1.3 X-ray burst from neutron star binary ...... 8 2.1.4 Active star ﬂare ...... 10 2.1.5 Flare of blazar ...... 11 2.1.6 Tidal disruption event ...... 12 2.1.7 Supernova shock breakout ...... 12 2.2 Information of past transient event surveys ...... 14 2.2.1 Early history of all-sky X-ray surveys ...... 14 2.2.2 All-sky surveys in 1990s ...... 15 2.2.3 Recent surveys ...... 16 2.2.4 Systematic SXT search with MAXI ...... 17 2.3 statistical test of survey study ...... 19 2.3.1 Test of isotropy ...... 19 2.3.2 V/Vmax test ...... 19 2.3.3 LogN-log S plot ...... 20

3 Observatory – MAXI 23 3.1 MAXI ...... 23 3.1.1 Overview of MAXI ...... 23 3.1.2 Data processing ﬂow of MAXI ...... 24 3.2 GSC ...... 25 3.2.1 Overview of GSC ...... 25

iv Contents v

3.2.2 Components of GSC ...... 25 3.2.3 Principle of the position determination ...... 28 3.2.3.1 Detector Coordinate ...... 29 3.2.3.2 Scan Coordinate ...... 30 3.2.4 In orbit performance ...... 31 3.2.4.1 PSF of GSC ...... 31 3.2.4.2 Alignment calibration ...... 32 3.2.4.3 High voltage operation ...... 33 3.2.4.4 Background environment ...... 33 3.3 SSC ...... 38

4 Analysis 41 4.1 Data selection and screening ...... 41 4.1.1 Near the ends of carbon wire ...... 44 4.1.2 Exclusion of region covered by solar paddles ...... 44 4.1.3 Exclusion of the Galactic Plane ...... 45 4.1.4 Exclusion of region around bright sources ...... 45 4.2 Global search ...... 45 4.2.1 Method of global search ...... 45 4.2.2 Determine threshold ...... 48 4.3 Clustering of candidates ...... 49 4.4 Detailed analysis ...... 49 4.4.1 Method of localization ...... 49 4.4.2 Detection threshold ...... 50 4.4.3 Estimation of position error ...... 50 4.5 Calculation of X-ray flux ...... 53 4.6 Check candidates ...... 55 4.6.1 Candidate associated with the solar X-ray leak ...... 55 4.6.2 Exclusion of events in background-enhanced region due to solar X-ray .. 55 4.6.3 Exclusion of events with lack of effective area in the localization process . 56 4.6.4 Exclusion of events in the center and both sides of BEX ...... 57 4.6.5 Exclusion of candidates associated with outbursts of a known X-ray source 58 4.6.6 Manually exclusion of obviously false detection ...... 58 4.6.7 Effect of the exclusions to our analysis ...... 58

5 Discussion 60 5.1 Catalog Matching ...... 60 5.1.1 SXTs associated with GRB sources ...... 61 5.1.1.1 GRB source Catalogs ...... 61 5.1.1.2 Result of cross matching ...... 61 5.1.2 SXTs associated with non-GRB sources ...... 63 5.1.2.1 Non-GRB source Catalogs ...... 63 5.1.2.2 Results of the cross matching ...... 65 5.2 Hardness of the detected SXTs ...... 66 5.3 Spatial distribution ...... 69 5.4 Study of spatial distribution ...... 73 5.4.1 V/Vmax test ...... 73 Contents vi

5.4.2 LogN-log S plot ...... 74 5.5 Origin of the low-ﬂux SXTs ...... 78

6 Conclusion 82

A List of Detected Transients 84

B Event map, light curve and VETO curve 93

C List of Detected Transients 131

Bibliography 138 List of Figures

2.1 Distribution of GRB’s duration time ...... 4 2.2 Spatial distribution of GRB ...... 5 2.3 Afterglow light curves ...... 5 2.4 Distribution of hardness ratio in X-ray and γ-ray ...... 8 2.5 X-ray burst and the progenitor, X-ray binary ...... 9 2.6 Diﬀerence of quasar and blazar ...... 11 2.7 A story and light curve of Swift J1644+57 ...... 13 2.8 Detection method with model ﬁtting ...... 18 2.9 LogN-log S plot of HEAO-1 ...... 21 2.10 LogN-log S plot assembled with various transient surveys ...... 22 2.11 LogN-log S plot obtained in the previous study ...... 22

3.1 JEM, JEM-EF and MAXI ...... 24 3.2 Overview of MAXI ...... 24 3.3 Layout of GSC on MAXI ...... 26 3.4 Picture and top view of a counter ...... 27 3.5 Cross-section views of a counter ...... 27 3.6 Geometry of a counter and collimators ...... 28 3.7 Field of view of MAXI ...... 29 3.8 PSF of GSC along detector direction ...... 32 3.9 PSF of GSC along scan direction ...... 32 3.10 GSC high voltage history ...... 34 3.11 History of background rate of GSC ...... 36 3.12 Schematic view of origin of sun leak event ...... 37 3.13 Angular distribution of sun leak events ...... 37 3.14 An all-sky map of GSC with solar ﬂare ...... 38 3.15 Exploded view of SSC ...... 39 3.16 Schematic view of SSC unit ...... 40

4.1 Result of data screening ...... 43 4.2 Background distribution in the detection coordinate ...... 44 4.3 Example of global search ...... 47 4.4 Example of distribution of null hypothesis probability ...... 48 4.5 Example of position determination ...... 51 4.6 Contour of null hypothesis probability in the source and background ﬂux plane . 52 4.7 Distribution of null hypothesis probability by localization ...... 52 4.8 Estimation of position determination error with Cyg X-2, Perseus Cluster and X Per ...... 53

vii List of Figures viii

4.9 Scatter plot of null hypothesis probability and source ﬂux ...... 54 4.10 Scatter plot of background count rate ...... 56 4.11 Distribution of candidates in the BEX coordinate ...... 57 4.12 Event map of a candidate on February 21 2011 ...... 59 4.13 Distribution of available data ...... 59

5.1 Hardness-ﬂux diagram and distribution of hardness ratio ...... 67 5.2 Hardness distribution of GRBs, XRFs and stellar ﬂares ...... 68 5.3 Correspondence between hardness and spectral parameters ...... 68 5.4 Map of detected transients ...... 70 5.5 Exposure map of GSC ...... 71 5.6 Distribution of cos θ and sin2 b − 1/3 ...... 72 5.7 Histogram of V/Vmax ...... 73 5.8 LogN-log S plot ...... 76 5.9 Corrected log N-log S plot ...... 77 5.10 Histogram of the SXTs to detected time, detector coordinate and GSC camera . 80 5.11 Position distribution of SXTs on earth ...... 81

B.1 Data of SXT091012 ...... 94 B.2 Data of SXT091014 ...... 94 B.3 Data of SXT091105 ...... 94 B.4 Data of SXT091115 ...... 94 B.5 Data of SXT091119 ...... 95 B.6 Data of SXT091202 ...... 95 B.7 Data of SXT091202 ...... 95 B.8 Data of SXT091204 ...... 95 B.9 Data of SXT091205 ...... 96 B.10 Data of SXT091212 ...... 96 B.11 Data of SXT091218 ...... 96 B.12 Data of SXT091227 ...... 96 B.13 Data of SXT091229 ...... 97 B.14 Data of SXT100108 ...... 97 B.15 Data of SXT100120 ...... 97 B.16 Data of SXT100123 ...... 97 B.17 Data of SXT100207 ...... 98 B.18 Data of SXT100217 ...... 98 B.19 Data of SXT100220 ...... 98 B.20 Data of SXT100302 ...... 98 B.21 Data of SXT100315 ...... 99 B.22 Data of SXT100327 ...... 99 B.23 Data of SXT100415 ...... 99 B.24 Data of SXT100418 ...... 99 B.25 Data of SXT100419 ...... 100 B.26 Data of SXT100420 ...... 100 B.27 Data of SXT100425 ...... 100 B.28 Data of SXT100509 ...... 100 B.29 Data of SXT100510 ...... 101 List of Figures ix

B.30 Data of SXT100516 ...... 101 B.31 Data of SXT100524 ...... 101 B.32 Data of SXT100611 ...... 101 B.33 Data of SXT100611 ...... 102 B.34 Data of SXT100708 ...... 102 B.35 Data of SXT100712 ...... 102 B.36 Data of SXT100720 ...... 102 B.37 Data of SXT100808 ...... 103 B.38 Data of SXT100813 ...... 103 B.39 Data of SXT100823 ...... 103 B.40 Data of SXT100829 ...... 103 B.41 Data of SXT100907 ...... 104 B.42 Data of SXT101019 ...... 104 B.43 Data of SXT101020 ...... 104 B.44 Data of SXT101025 ...... 104 B.45 Data of SXT101030 ...... 105 B.46 Data of SXT101102 ...... 105 B.47 Data of SXT101108 ...... 105 B.48 Data of SXT101111 ...... 105 B.49 Data of SXT101208 ...... 106 B.50 Data of SXT101220 ...... 106 B.51 Data of SXT110106 ...... 106 B.52 Data of SXT110110 ...... 106 B.53 Data of SXT110113 ...... 107 B.54 Data of SXT110115 ...... 107 B.55 Data of SXT110116 ...... 107 B.56 Data of SXT110117 ...... 107 B.57 Data of SXT110122 ...... 108 B.58 Data of SXT110123 ...... 108 B.59 Data of SXT110209 ...... 108 B.60 Data of SXT110215 ...... 108 B.61 Data of SXT110216 ...... 109 B.62 Data of SXT110219 ...... 109 B.63 Data of SXT110221 ...... 109 B.64 Data of SXT110225 ...... 109 B.65 Data of SXT110308 ...... 110 B.66 Data of SXT110313 ...... 110 B.67 Data of SXT110406 ...... 110 B.68 Data of SXT110415 ...... 110 B.69 Data of SXT110423 ...... 111 B.70 Data of SXT110426 ...... 111 B.71 Data of SXT110505 ...... 111 B.72 Data of SXT110525 ...... 111 B.73 Data of SXT110525 ...... 112 B.74 Data of SXT110528 ...... 112 B.75 Data of SXT110607 ...... 112 B.76 Data of SXT110618 ...... 112 List of Figures x

B.77 Data of SXT110620 ...... 113 B.78 Data of SXT110624 ...... 113 B.79 Data of SXT110625 ...... 113 B.80 Data of SXT110704 ...... 113 B.81 Data of SXT110726 ...... 114 B.82 Data of SXT110805 ...... 114 B.83 Data of SXT110826 ...... 114 B.84 Data of SXT110826 ...... 114 B.85 Data of SXT110828 ...... 115 B.86 Data of SXT110831 ...... 115 B.87 Data of SXT110902 ...... 115 B.88 Data of SXT110907 ...... 115 B.89 Data of SXT110924 ...... 116 B.90 Data of SXT111005 ...... 116 B.91 Data of SXT111010 ...... 116 B.92 Data of SXT111015 ...... 116 B.93 Data of SXT111021 ...... 117 B.94 Data of SXT111024 ...... 117 B.95 Data of SXT111106 ...... 117 B.96 Data of SXT111113 ...... 117 B.97 Data of SXT111121 ...... 118 B.98 Data of SXT111204 ...... 118 B.99 Data of SXT111212 ...... 118 B.100Data of SXT111212 ...... 118 B.101Data of SXT111229 ...... 119 B.102Data of SXT111229 ...... 119 B.103Data of SXT111230 ...... 119 B.104Data of SXT120122 ...... 119 B.105Data of SXT120131 ...... 120 B.106Data of SXT120202 ...... 120 B.107Data of SXT120207 ...... 120 B.108Data of SXT120208 ...... 120 B.109Data of SXT120213 ...... 121 B.110Data of SXT120307 ...... 121 B.111Data of SXT120310 ...... 121 B.112Data of SXT120310 ...... 121 B.113Data of SXT120310 ...... 122 B.114Data of SXT120325 ...... 122 B.115Data of SXT120416 ...... 122 B.116Data of SXT120419 ...... 122 B.117Data of SXT120424 ...... 123 B.118Data of SXT120517 ...... 123 B.119Data of SXT120529 ...... 123 B.120Data of SXT120604 ...... 123 B.121Data of SXT120608 ...... 124 B.122Data of SXT120619 ...... 124 B.123Data of SXT120622 ...... 124 List of Figures xi

B.124Data of SXT120701 ...... 124 B.125Data of SXT120702 ...... 125 B.126Data of SXT120702 ...... 125 B.127Data of SXT120804 ...... 125 B.128Data of SXT120805 ...... 125 B.129Data of SXT120810 ...... 126 B.130Data of SXT120927 ...... 126 B.131Data of SXT121011 ...... 126 B.132Data of SXT121014 ...... 126 B.133Data of SXT121027 ...... 127 B.134Data of SXT121104 ...... 127 B.135Data of SXT121218 ...... 127 B.136Data of SXT130102 ...... 127 B.137Data of SXT130106 ...... 128 B.138Data of SXT130123 ...... 128 B.139Data of SXT130206 ...... 128 B.140Data of SXT130226 ...... 128 B.141Data of SXT130310 ...... 129 B.142Data of SXT130320 ...... 129 B.143Data of SXT130325 ...... 129 B.144Data of SXT130407 ...... 129 B.145Data of SXT130418 ...... 130 B.146Data of SXT130520 ...... 130 List of Tables

4.1 List of bright sources ...... 46

5.1 GRB catalogs used for source identiﬁcation ...... 63 5.2 Source catalogs used for source identiﬁcation ...... 64 5.3 Statistical test of isotropy ...... 69

A.1 List of detected short X-ray transients ...... 85 A.2 List of GRBs detected by MAXI ...... 91

C.1 List of detected short X-ray transients ...... 132

xii Abbreviations

MAXI Monitor of All-sky X-ray Image ISS International Space Station GSC Gas Slit Camera SSC Solid state Slit Camera SXT Short X-ray Transient GRB Gamma-Ray Burst XRR X-Ray Rich GRB XRF X-Ray Flash LMXB Low Mass X-ray Binary XRB X-Ray Burst YSO Young Stellar Object AGN Active Galactic Nuclei TDE Tidal Disruption Event NXB Non X-ray Background CXB Cosmic X-ray Background SAA South Atlantic Anomaly FoV Field of View FWHM Full Width of Half Maximum

xiii Chapter 1

Introduction

For decades, X-ray/γ-ray observational satellites have been performed all-sky survey missions. One of their main purposes is an investigation of short X-ray transients which have duration time from one-second to one day. Since the early era of the X-ray astronomy, several types of such short transient phenomena have been reported. For example, X-ray bursts which have fast-rise and slow-decay shaped temporal X-ray activities within a few hundred-seconds have been observed from binary systems consisting of neutron stars and ordinary stars. Another short transients are X-ray ﬂares from active stars which have the strong magnetospheric energy release and are emitted from RS CVn type sources (which are binaries of low-mass main sequence stars) and young stellar objects (which are primordially constructed stars).

The most notable category of short X-ray/γ-ray transient is the Gamma-Ray burst (GRB) which was ﬁrst detected by the Vela nuclear experiment satellites in 1960’s (Klebesadel et al. [36]). Following the detection, more than ten missions have reported numerous GRBs. In particular, since the discovery of afterglow phenomena of GRB by the BeppoSAX satellite in 1997, advanced satellites such as HETE-2, Swift and Fermi have performed multi-wavelength observations aimed to reveal the temporal, spectral and cosmological characteristics of GRBs. Furthermore, theoretical investigations with numerical simulations have been performed; nevertheless, the progenitor and mechanism of the extremely bright γ-ray/X-ray emission are still mystery to astronomers.

In 2009, a new X-ray monitoring mission started operation on the International Space Station (ISS), that is Monitor of All-sky X-ray Image (MAXI). The high sensitive gas proportional counters and charge-coupled detectors (CCDs) have already discovered new X-ray sources, such

1 Chapter 1. Introduction 2 as binaries comprising black holes or neutron stars, super soft source associated with a white dwarf binary, classical short X-ray transients and GRBs. Toizumi [75] performed the first short X-ray transient survey to this valuable survey data and reported 29 transients including some known GRBs. In this study they searched the transients with variable detection sensitivity due to large fluctuation of background and positional dependency on the detector and then estimated the significance of detection based on unbinned ligklihood analysis assuming a locally constant background. This method has limitations in two aspects; one is the assumption on the locally flat background, the other is the assumption on the gaussian PSF (point spread function) used in the image fitting. In order to overcome their limitations we use a simple square aperture to evaluate the significance of source detection. Following the criteria, we will systematically perform the second short X-ray transient survey. In addition to the technical developments, our survey will be performed to the data provided by the longer monitoring observation than that of the previous study.

The ﬁrst chapter in this thesis (Chapter 2) will review astrophysical background information of various kinds of short X-ray transients which will be discovered in our analysis. In addition, past surveys to search short transient phenomena with several X-ray/γ-ray astronomical observatories will be introduced to understand the whole context of systematic investigation in the chapter. Next, in Chapter 3, we will show the mission of MAXI and observational characteristic of the X-ray detector, GSC, which provides the data analyzed in our analysis. Thirdly, we will describe method of the transient source search and investigate detection threshold, position determination accuracy and possible fake detections in Chapter 4. Then, results of the analysis and discussions will be reported in Chapter 5. We will discuss cross matching of detected transients with known GRBs, comparison of hardness ratio, statistical test to study positional and spatial distributions, and interpretation to the origin of the detections. Finally, we show the conclusion of our transient search analysis in Chapter 6. Chapter 2

Background information of short X-ray transient

For the last several decades, numerous all-sky surveys for X-ray and gamma-ray transients have been performed and attempted to reveal their astrophysical characteristics. In the ﬁrst part of this chapter, we will introduce physical properties of short X-ray transients (SXTs) which can be detected in our transient search study. Next, we will review the historical surveys especially focusing on the result in the soft (< 50 keV) X-ray energy band. Finally, we will overview statistical study to test isotropy of SXT spatial distribution for the results of the all-sky surveys.

2.1 Observable transient phenomena with MAXI

2.1.1 Gamma-ray burst

Gamma-Ray Bursts (GRBs) are bright γ-ray ﬂashes ﬁrst detected by the Vela satellites in 1960’s (Klebesadel et al. [36]). The main observational features of GRB are isotropic angular distribution in the sky, short duration time in the order of 10−1–102 sec, greatest amount of the emitted energy about 1052 erg and afterglow phenomena subsequently appearing the main prompt emission.

Great contribution was made to the GRB study in the early era by the Burst And Transient Source Experiment (BATSE) instrument mounted on the Compton Gamma Ray Observatory

3 Chapter 2. Background information of short X-ray transient 4

Figure 2.1: Distribution of duration time of GRBs observed by BATSE. Two populations are shown; one peaks at ∼ 0.3 sec and the other at a few ten second.

(CGRO) satellite. The BATSE Large Area Detector is a NaI scintillation counter mounted on each corner of the satellite sensitive to 20 keV – 2 MeV photons. With the & 60% sky coverage, BATSE detected 2704 GRB in the four year observation (Paciesas et al. [55]). These GRBs have isotropic spatial distribution as demonstrated in Figure 2.2 (Paciesas et al. [55]). This feature suggested the GRBs are originated from the cosmological distance rather than our galaxy or nearby galaxies. In addition, Kouveliotou et al. [40] shows the distribution of duration time T90 of the GRBs and indicates two distributions; short GRB with the duration of < 2 sec and long

GRB with > 2 sec. The distribution of T90 which is the time taken to accumulate 90% of a burst ﬂuence starting at the 5% level is shown in Figure 2.11.

Following the launch of the CGRO satellite, BeppoSAX started observation in the X-ray to γ-ray energy bands. The satellite has two wide ﬁeld detectors, Gamma-Ray Burst Monitor (GRBM) and Wide Field Camera (WFC), and the Narrow Field Instruments (NFIs) that observe X- ray and γ-ray sources with the high angular resolution using focusing X-ray telescopes. In 1997, quick follow-up observations with telescopes of the NFIs were performed by a GRB event (GRB970228)2 and revealed decaying X-ray emission in the 2–10 keV band (Costa et al. [11]). This emission was name “afterglow” and provided much more accurate localization than the

1http://www.batse.msfc.nasa.gov/batse/grb/ 2GRB events are named after the date on which they are discovered. Chapter 2. Background information of short X-ray transient 5

2704 BATSE Gamma-Ray Bursts +90

+180 -180

-90

10-7 10-6 10-5 10-4 Fluence, 50-300 keV (ergs cm-2)

Figure 2.2: Spatial distribution of GRBs observed by BATSE in the Galactic coordinate. The colors means ﬂuences of GRBs corresponding to the lower bar.

No. 1, 2001 PANAITESCU & KUMAR L51

Fig. 1.—Radio,Figure optical, 2.3: and X-rayLight emission curves and modelof GRB light curves afterglows for the GRB obtained afterglows 980519,in the 990123, radio, 990510, optical 991208, and 991216, X-ray 000301c, wave- 000926, and 010222 (the definition of the symbols in the lower left-hand corner of the middle graph applies to all panels). The numerical light curves have been obtained by the minimizationlengths. ofx2 between The dashed the model lines emission are and results the radio, ofmillimeter, numerical submillimeter, calculation, near-infrared, whose optical, parametersand X-ray data (only are a listed part of the in used data is shown inPanaitescu this figure). The & parameters Kumar of [ each57]. model are given in Fig. 2. Optical data have been corrected for Galactic dust extinction. The spread around the model curves exhibited by the radio emission of 980519, 991208, 991216, 000301c, and 000926 can be explained by fluctuations due to scatterings by the inhomogeneities in the Galactic interstellar medium (Goodman 1997). Fluxes have been multiplied by the indicated factors, for clarity. circumstellar medium leads to the formation of a quasi- timescale fluctuations in the early X-ray and optical afterglow homogeneous shell extending from 1016 to տ1018 cm. emission, before these fluctuations disperse, and may have al- Regarding the microphysics of shocks,∼ the results presented ready been observed in the optical light curve of 000301c. in Figure 2 indicate that (1) the total electron energy is close to equipartition with protons, (2) the magnetic field strength, 1/2 4. CONCLUSIONS B eB , is within 2 orders of magnitude of the equipartition value,∝ and (3) the power-law distribution of shock-accelerated The findings reported above should shed some light on the electrons does not have a universal index p, with 1.4 ! p ! GRB progenitors. Homogeneous media of low density, below 2.8. Given that the post–jet-break emission falls off astϪp , the 10Ϫ2 cmϪ3, as found for the afterglows 990123 and 980703, shallow decays of the optical emission of 010222 and the radio indicate a galactic halo or a hot component of the interstellar light curves of 991208, 991216, and 000301c require models medium. External particle densities in the 0.1–1 cmϪ3 range, withp 1.5 . Consistency between such hard electron distri- as found for 980519 and 990510, are characteristic of the in- butions∼ and the observed optical spectral slopes implies that, terstellar medium, while larger values of 10 cmϪ3,asdeter- ∼ for these four afterglows, the cooling frequencync was below mined for the afterglows 991208, 000301c, 000926, and the optical domain. 010222, are typical for diffuse hydrogen clouds. Even larger In our modeling, the passage of a high-energy spectral break particle densities, above 100 cmϪ3 ,wouldbeconsistentwith n yields most of the steepening of the optical decay observed the undisturbed, dense molecular clouds expected in the cur- in∗ the afterglows 991208, 991216, and 000301c. This allows us rently popular collapsar model (Woosley 1993; Paczyn´ski to determine the fractional electron energy up tog corresponding 1998; MacFadyen & Woosley 1999) for GRB progenitors. ∗ 12 ton . We find this fraction to be in the range of33– (Fig. 2), Scalo & Wheeler (2001) have emphasized that the winds and close∗ to equipartition, which provides a natural reason for the supernovae occurring in a cluster of massive stars create existence of the break atg . We note that hard electron distri- 10 pc–1 kpc superbubbles whose density can be as low as butions and equipartition electron∗ energies find mutual consis- 10Ϫ3 cm Ϫ3.Furthermore,theyarguedthatvariationsinthe tency in the shock acceleration treatment of Malkov (1999). cluster age and the density of the giant molecular cloud into Assuming a uniform jet, the jet energies and apertures in- which the superbubble expands, as well as the interaction with ferred here and the observed gamma-ray fluences require an the winds from other clusters, may yield circumburst medium efficiency of the gamma-ray mechanism in dissipating the jet densities spanning a few orders of magnitude. kinetic energy and radiating it in the 20 keV–1 MeV range The relativistic kinetic energies of the eight GRBs analyzed that exceeds 50%. However, if the gamma-ray emission arises here show a remarkably narrow distribution, with a mean value from bright patches∼ caused by angular fluctuations (Kumar & of 3 # 10 50 ergs and a dispersion of 1050 ergs. On the other Piran 2000) of the kinetic energy on the jet surface, the actual hand,∼ the distributions of the gamma-ray∼ energy output (as- efficiency can be much smaller, closer to that obtained from suming uniform jets) and of the initial jet aperture are signif- numerical modeling of internal shocks (Spada, Panaitescu, & icantly broader, with a width of 1 order of magnitude. Me´sza´ros 2000). These bright spots could also induce short That half of the afterglows analyzed here are narrower than Chapter 2. Background information of short X-ray transient 6 result using traditional γ-ray detectors. Owing to the accurate localization, van Paradijs et al. [80] discovered the optical counterpart of GRB970228 using a ground-based telescope. After the remarkable discovery, another GRB (GRB970508) accompanied with X-ray and optical afterglow emissions was discovered (Djorgovski et al. [12]). Optical spectroscopic observations revealed that the GRB was associated with a galaxy whose redshift is ∼ 0.835 and had the total emitted γ-ray energy of ∼ 1052 erg assuming the isotropic emission (Metzger et al. [48]). Since the discovery of afterglow, γ-ray emission of GRB has been called as “prompt emission”.

The detections of afterglow phenomena and development of observational technology in the γ- ray and X-ray bands have greatly contributed to the study of GRB. The highlight of the result is suggestion of the GRB’s progenitors as supernova. The supernova-GRB association were suggested from observations of SN1998bw/GRB980425 and conﬁrmed with SN2003dh/SN030329 since the positional coincidence and the supernova-like temporally and spectroscopically behaviors of the optical counterparts of the GRBs (Galama et al. [17], Hjorth et al. [29]). This indicates that GRB phenomena are probably originated from supernova explosions.

Another investigation with developed observatories is determination of the distance of GRBs by observations of the spectra in optical wavelength. As the result, the typical energy released from a GRB is known as 1052–1054 erg assuming the isotropic emissivity. In order to explain the energetics and high γ-ray luminosity within a short time, narrowlly-collimated explosion and mass ejection traveling with the speed exceeding > 99% of the speed of light, “jet”, was introduced. If such jet aligns into our line of sight, brightness of radiation from the jet is enhanced with the relativistic beaming eﬀect, that is derived from the special relativity as time dilation. In addition to the distance estimation, observations of afterglows show temporal evolutions mostly in the X-ray, optical and radio wavelengths which are well represented with power-law function with “break” at the elapsed time of 103–105 sec after the detections of the prompt emission. Figure 2.3 demonstrate the samples of afterglow light curves. This is explained that the deceleration of the relativistic jet due to interaction in circumstellar medium surrounding the progenitor, which is a prediction of the ﬁreball model (Wijers et al. [82]). In conclusion, the typical agreement of GRB emission mechanism is the focused explosion from the relativistically-traveling jet which is driven by a central engine of a supernova.

The GRB prompt emission is one of the SXT candidates for MAXI-GSC. According to the distribution of X-ray ﬂuence3 of prompt emission obtained with the HETE-2 satellite (Sakamoto

3The flux integrated over time. Chapter 2. Background information of short X-ray transient 7 et al. [63]), the typical fluence in the 2–30 keV energy band is 10−7–10−5 erg cm−2. Assuming typical time duration of GRB as 100 sec, the peak X-ray flux of 10−9–10−7 erg cm−2 s−1 is large enough to detect with GSC. In fact, since the first light in August 2009 until May 2013, GSC has discovered 35 probable GRBs (or short X-ray transients), which are summarized in Serino [68]. Almost one third of them (13 events) were confirmed with other observatories such a Swift, Fermi, and Konus-Wind; however 22 events were detected with only MAXI.

X-ray afterglows are also possible SXTs which will be detected by MAXI-GSC. The typical ﬂuxes of afterglow are 10−8 erg cm−2 s−1 at the peak level (the elapsed time ∼ 100 sec after the GRB detection) and 10−12–10−14 erg cm−2 s−1 during the late phase (∼ 105 sec after) in the Swift-XRT 0.3–10 keV energy band. MAXI-GSC can detect such afterglow in the bright time. In addition, late-time (103–105 sec after the prompt emission) X-ray afterglow associated with nearby GRB, e.g., GRB130427A with the redshift of 0.34 (Maselli et al. [46]), may be also detectable with GSC.

2.1.2 X-ray ﬂash

In the traditional study of GRB, some classifications have been introduced; whether the duration time is short or long, type of its host galaxy and whether it accompanies supernova or not. Focusing to the spectral feature, GRBs are identified into classical GRBs, X-ray rich GRBs (XRRs) and X-ray flashes (XRFs). The first suggestion of XRF is Heise et al. [27] which reported peculiar GRBs detected with the BeppoSAX satellite. They selected 17 transients which were detected by Wide Field Camera (covering 2−25 keV) but were not by Gammay-Ray Burst Monitor (40−700 keV) and classified them into XRFs.

The HETE-2 satellite which was launched in 2000 also observed short transients with the X- ray/γ-ray detectors, whose specifications will be described in Section 2.2.3. Using ratios between fluences of the GRBs in X-ray (2–30 keV) and γ-ray (30–400 keV) obtained with HETE-2, Sakamoto et al. [63] classified 45 burst events into 10 GRBs, 19 XRRs and 16 XRFs with the distribution of Figure 2.4. Although various temporal and spectral characteristics were investigated, Sakamoto et al. [63] finally concluded that all three kinds of bursts will arise from the same phenomenon. Sakamoto et al. [65] sophisticated the phenomenalistic study with the bursts obtained with the Swift satellite. Swift has Burst Alert Telescope (BAT) detecting 15– 150 keV photons, which are different from the case of HETE-2. They performed similar analysis to the Swift detected bursts (10 XRFs, 17 XRRs and 14 GRBs) under the re-defined threshold Chapter 2. Background information of short X-ray transient 8 318 SAKAMOTO ET AL.

Fig. 3.—Distribution of the bursts in this study in the [S (2–30 keV ), Figure 2.4: E Fig. 2.—DistributionDistribution of hardness of the fluence ratio of ratio GRBsSE obtained(2 30 keV)/ by theSE HETE-2(30 400 satellite. keV). The SE (30–400 keV )] plane. The dashed line corresponds to the boundary between dashedThe lines dashed divide thelines hard correspond GRBs, XRRs to the and borders XRFs. between hard GRBs and XRRs, XRFs and XRRs. The dot-dashed line corresponds to the boundary between and between XRRs and XRFs. XRRs and GRBs. The solid line is the best linear fit to the burst distribution and 1:282 0:082 of fluence ratio in 25–50 keV and 50–100 keV and also concluded the spectral features of GRB,is given by SE(30 400 keV) (0:722 0:161) ; SE(2 30 keV) Æ . The correlation coefficient of the¼ burst distributionÆ is 0.851. The probability XRR and XRF are similar. However, since Swift provided light curves of X-ray afterglow with S (2–30 keV) and S (30–400 keV): SE (30 400 keV) of such a correlation occurring by chance for the sample size of 45 bursts is E E 8 1:282 0:082 ¼ 1:1 ; 10À . its X-ray Telescope(0:722 (XRT;0:161) 0.3–10; SE keV),(2 30 the keV) difference of temporal. The properties values of were the discovered; Æ Æ the breakslope of the GRB andthe light uncertainty curves appeared in the at much slope earlier that times we find than imply that of a the sig- XRF ones. nificance for the correlation of 15 .Thecorrelationcoeffi- the conclusion reached by Kippen et al. (2003), who found no cient of +0.851 that we find between the two quantities also statistically significant evidence that the T50 and T90 distribu- 2.1.3 X-rayimplies burst a high from significance neutron star for the binary correlation; the probability of tions of BeppoSAX WFC XRFs and long Compton Gamma Ray such a correlation occurring by chance for the sample size of 8 Observatory (CGRO)BATSEburstsdiffer.However,wecau- 45 bursts is 1:1 ; 10À .Thetightnessofthecorrelationimplies A binary system consisting of a neutron star, which is a compact star supported by quantumtion that the size of the current HETE-2 samples of XRFs, XRRs, that there are no bursts in the HETE-2 sample with a high X-ray and GRBs is small, as is the size of the sample of BeppoSAX degeneracy pressure and having the typical radii of 10 km and the mass of 1.4 times solar- fluence and a low -ray fluence, or vice versa. WFC XRFs, and therefore the power of these K-S tests is small. mass (M ), and a star, which is a normal star and not compact, is another SXT candidate. 4. DURATIONS Such system with the low-mass (. 1M ) star is called as low-mass X-ray binary (LMXB) and 5. SKY DISTRIBUTIONS Figure 4 shows the distributions of T (Fig. 4a)andT & 50 90 that with(Fig.4 the massiveb)durationsintheWXMenergyband2–25keVforeach ( 10M ) star is high-mass X-ray binary (HMXB). X-ray bursts are Figure 5 shows the sky distribution in ecliptic coordinates 19 characteristickind phenomena of GRB. of For LMXBs comparison, which have we fast-rise also and show slow-decay the distributions temporal activitiesof of the 44 HETE-2 XRFs, XRRs, and GRBs in this study. The the X-rayof fluxT50 inand a fewT 1–1090 durations sec as shown for inthe Figure BATSE2.5 and bursts spectra (Paciesas which represented et al. withHETE-2 skycoverageisnotuniform,andasaresultitisdifficult 1999). to make a meaningful statement about the sky distributions of blackbody radiationThe durations whose the of temperature the brightest is a few peaks keV. of Observationally, GRBs show such a strong bursts have twothese three kinds of GRBs. Modulo this and the relatively small categoriesdependence labeled Type I on and the Type energy II, however band only in two which sources they have are shown measured the Type II burst.sample size of each of the three kinds of bursts, there is no sta- Hereafter,(Fenimore we means the et Type al. 1995), I bursts as as does X-ray the bursts overall (XRB) duration of LMXB. of the bursts tistically significant evidence that the sky distributions of the (Norris et al. 2000), with both E 0.4.Therefore,onemight three kinds of bursts are different. / À expect the T50 and T90 distributions for XRFs, XRRs, and GRBs to be different, and the T50 and T90 distributions for XRFs and 6. DISTRIBUTION OF SPECTRAL PARAMETERS XRRs to differ from those for long BATSE bursts, particularly WefindthatasimplePLmodelprovidesanadequatefittothe since the durations of the latter were measured in a higher en- spectral data for eight of the 45 bursts in this study (six XRFs ergy band. We have used the Kolmogorov-Smirnov (K-S) test and two XRRs). In the case of the six bursts we classify as XRFs, to investigate this possibility. Applying this test to the T dis- 50 the slope of the power-law index is 2. We interpret this result tributions for XRFs and GRBs, XRRs and GRBs, and XRF and < as follows. The spectral data for theseÀ bursts do not constrain XRRs, we find K-S test probabilities of 0.25, 0.47, and 0.88, E obs ,butthefactthat 2meansthatweareobservingthe respectively. For the T distribution, we find K-S test proba- peak < 90 high-energy power-law portionÀ of their Band-model spectra, and bilities for the same cross-comparisons of 0.97, 0.99, and 0.91, E obs is near or below 2 keV, the lower limit of the WXM energy respectively. Applying the K-S test to the T distributions for peak 50 band. We therefore conclude that these are XRFs. XRFs plus XRRs and long (T > 2s)BATSEbursts,andfor 50 In the case of the four XRRs, the normalization constants, XRFs and long BATSE bursts, we find K-S probabilities of 0.42 K , of the spectra are the lowest among all of the XRRs and and 0.74, respectively. For the T distribution, we find K-S test 15 90 GRBs. We therefore interpret the lack of evidence for E obs in probabilities for the same cross-comparisons of 0.66 and 0.55, peak these bursts as due to the low signal-to-noise ratio of their spec- respectively. tra. In these cases, it is difficult to constrain the break energy, E , These K-S tests reveal no statistically significant evidence 0 and a power-law model gives an adequate fit to the spectral data. that the distributions of T50 and T90 durations for XRFs, XRRs, and GRBs are different, or that the distributions of T50 and T90 durations for XRFs and XRRs plus GRBs are different from 19 Since the attitude control camera was not operational, the celestial co- those for long BATSE bursts. The last result is consistent with ordinates of GRB 010225 are not available. Chapter 2. Background information of short X-ray transient 9

410 A. K. H. Kong et al.

Figure 5. The optical (SAAO) and X-ray (RXTE) burst profiles in various energy bands. The timing resolution is 5 s (optical) and 0.5 s (RXTE/PCA). The decay times depend stronglyFigure on photon energy, 2.5: with decays beingLeft: shorter The at higher energies.light curve of an X-ray burst from GS 1826−24 obtained with the TableRXTE 5. Timing parameters satellite of the simultaneous by Kong X-ray/optical et burstal. by [39 fitting]. linear-rise Right: exponential- Illustration of X-ray binary system. The star at decay model. the left side is ordinary star which provides gas to the compact star located at the right side. Although2±3.5 neutron keV 3.5±6.4 star, keV white 6.4±9.7 keV dwarf 9.7±16 or keV black Optical hole will be located at the center of accretion disk, Peak time (MJD) 50988.82510 50988.82511 50988.82512 50988.82512 50988.82516 Risethe time former (s) 7:2 two^ 0:27 objects:3 ^ 0:27 show:81 ^ 0 X-ray:07 8:52 ^ bursts.0:05 10:7 ^ 0:1 e-folding time (s) 59 ^ 254:3 ^ 0:935:6 ^ 0:820:5 ^ 0:755 ^ 4

The blackbody radius increases to a maximum as the burst rises, cross-correlation analysis, and (ii) modelling of the optical burst by but does not showThe the simultaneous physical drop in mechanismkTbb and increase in ofconvolving X-ray the X-ray burst light curve is with explained a Gaussian transfer with function. mass accretion and nuclear fusion of Rbb that is the overt signature of photospheric radius expansion (see Lewin, van Paradijs & Taam 1995). However, when we plot 3.2.1.1 Cross-correlation. We cross-correlated the optical data the flux F versus F1=4 kT R d 1=2, we do find evidence with X-ray data from different energy bands as well as the total bol thebol accreted= bb = matter as shown in the right panel of Figure 2.5. Gas which mainly consists of for an increase in the X-ray burning/ area on the star (Strohmayer (2±60 keV) X-ray light curve. This allows us to determine the et al. 1997). This is shown in Fig. 7, where the burst begins in the correlation and estimate any time lag between X-ray and optical lower left and evolveshydrogen diagonally to and the upper helium right, and then is transferredvariability. The measurement from the of the star cross-correlation to the function neutron star with forming an accretion across to the left at an essentially constant value until near the end provides a characteristic delay which does not depend on of the burst. This is an indication that indeed the X-ray burning particular model fitting. The results show that the optical burst area is not a constant, but increases with time during the rising lags the X-ray burst by ,4 s, which is marginally larger than 1=4 disk around the neutron star at the Lagrange point in the LMXB system. The gas travels in the phase. Fbol =kTbb eventually reaches a nearly constant value, and expected in this system. The separation of the compact object and the neutron star surface simply cools during the decay phase. companion star is 2±3 light-seconds if we assume an orbital Kilohertz QPOsaccretion between 200 and disk 1200 Hz andwere not cumulates detected period on of ,2 the:1 h (Homer surface, et al. 1998), compressed a neutron star mass of with strong gravitational ﬁeld of the during the burst with an upper limit of 1 per cent (at 99 per cent 1.4 M( and a companion star mass of 0.1±1.1 M(. However, in confidence). We also set upper limits on the presence of any coherent using a cross-correlation method we are essentially limited to the pulsations duringneutron the burst: , 1percentfortheranges100±500Hz star. When the pressure5-s time resolution and of the temperature optical data (the PCA data reached have a much at the limit, nuclear fusion starts. and 600±1200 Hz, and , 3 per cent between 1000 and 4000 Hz higher time resolution). Moreover, the delay appears to vary, from (the Nyquist limit). A detailed timing analysis of the remaining almost nothing at the start of the burst to a few seconds at the simultaneous X-ray/opticalThisreaction data will be presented burns in Paper up II. andpeak. heat This suggests the material that the delay might on be the a function whole of flux. surface with emitting a bright X-ray Therefore a cross-correlation analysis cannot provide a full picture of the delay between the X-ray and optical fluxes, and instead we 3.2.1 X-ray/optical time delay burst. model the optical burst by convolving the X-ray light curve with a Fig. 5 shows the simultaneous optical/X-ray burst in different transfer function. energy bands where there is a few seconds delay at the peak of the burst. In order to quantify this delay, we performed (i) a 3.2.1.2 Transfer function. In order to model the time delay The luminosity of XRBs is limited by the Eddington limit, which is 3.8 × 1038 erg s−1 with q 2000 RAS, MNRAS 311, 405±413 a neutron star whose mass is 1.4 times of the Sun and which has hydrogen-poor atmosphere (Strohmayer & Bildsten [73]). Based on the one-scan detection limit, GSC can detect XRBs which are located at less than ∼ 100 kpc.4 This distance includes the radius of our galaxy, however does not reach other nearby galaxies. Since a neutron star, which is a part of LMXB, is an old star remained after the supernova explosion, the binary often found in globular clusters. A globular cluster is a clump of stars that orbits a galactic core. Globular clusters typically include older stars, white dwarfs and neutron stars. Thus, many LMXBs are discovered in the clusters, such as 4U 0517−40 in NGC 1851 and EXO 1745-248 in Terzan 5.

4One pc (pronounced as “parsec”) is ∼ 3 × 1018 cm. Chapter 2. Background information of short X-ray transient 10

2.1.4 Active star ﬂare

Stars in an early phase of its evolution or a complex environment such as binary systems will show flares which are observed with wide wavelength band. One of such system is RS CVn (Canum Venaticorum) type variable star. It is a class of close binary systems consisting of late- type stars, such as G-M type stars (Hall [23]). These systems show luminous X-ray emissions which will be driven by the chromospherically activity of the stars. The origin of the coronal activity is interpreted as cool large stellar spots (in analogy to sunspots). The flares have fast-rise and slow-decay X-ray light curves and the anomalous luminosity with the orders of magnitude to the solar flares.

An X-ray flare of M dwarf is also driven with the energy release of the magnetic field. The star is a very-low-mass (. 0.1 M ) star with the spectral class of M (the effective temperature of 2000–3000 K) showing the Hα emission line. The surface magnetic field is strong and releases the energy as flares with the large luminosity in the radio-to-X-ray band. The mechanism powering the magnetic field is not fully understood.

The young stellar object (YSO) is a generic name of newly born stars from interstellar molecular clouds. Based on the spectra in infrared and optical wavelengths, the YSOs are classified into sub-classes. The spectral features reflect evolution stage of the YSO; whether the core is surrounded by the clouds, whether the accretion flow and disk is dense or not. The evolved YSOs are named as T Tauri stars after the first identified source. The fast-rise and slow-decay X-ray activities observed from the stars are characterized with the surprisingly luminosity which is brighter than the solar flare by a several order of magnitude. The progenitors of the flares are interpreted as strong magnetic activities on the surface of the star and/or between the star and the accretion disk.

The typical luminosity of these flares is > 1030 erg s−1. Thus, we estimate that flares from nearby stars with 100 pc (∼ 1019 cm) are detectable, assuming the typical detection limit of GSC with one-scan transit (∼ 10−9 erg cm−2 s−1). Since the distance is comparable to the thickness of the galactic plane, GSC will observe X-ray flares from the stars which are isotropically distributed around the solar system. Chapter 2. Background information of short X-ray transient 11

Figure 2.6: Illustration of AGN system. Around the massive black hole located at the center bright region, the accretion disk, truss structure and relativistic jet are generated. The three white allows indicate diﬀerence of our viewing angle. Seeing the AGN with the on-axis directions, we will observe the system as a blazar.

2.1.5 Flare of blazar

Active galactic nucleus (AGN) is a super massive black hole located at the center of galaxy and shows high luminosity in wide waveband from radio to γ-ray. The black hole with the mass 6 10 of 10 –10 M has an accretion disk and often a narrowly-collimated outflow, called as jet. Figure 2.6 shows the schematic picture of the AGN system5. From the aspect of viewing angle, AGNs are classified into sub groups. Blazar is a group of AGNs which have a jet oriented to our line of sight. The apparent luminosity of the blazar’s jet is enhanced with the relativistic beaming since the plasma in the jet is accelerated into sub light-speed ( 99%) and is observed with face-on. A type of blazars with rapid temporal variation of the luminosity is called as BL Lac object after its prototype. In the case of representative BL Lac examples, such as Markarian 421 (Mrk 421), the peak luminosity of the X-ray flare is the order of 10−9 erg cm−2 s−1 (Isobe et al. [31]) that is comparable to or larger than the detection limit of one-scan GSC transit. The time duration of the flares, from sub-day to day (Bartoli et al. [3], Kataoka et al. [33]), is long enough to detect with a scan. The long flares may be continuously detected during several scans of GSC.

5http://www.nasa.gov/centers/goddard/images/content/182566main2 1agn LO.jpg Chapter 2. Background information of short X-ray transient 12

2.1.6 Tidal disruption event

One of the surprising phenomena is anomalous X-ray and γ-ray emission associated with a tidal disruption event. When a star pass through close to a massive object (e.g., black hole), its strong gravitational ﬁeld destroys the star via the tidal force. The debris fall into the massive star making an accretion disk and emitting radiation in multi wavelengths. As the result this phenomena are observed as tidal disruption events (TDEs). The long-term temporal evolution of TDEs was theoretically predicted as a power-law function with the index of −5/3 (Evans & Kochanek [14], Lodato et al. [43]). Classical TDEs have been reported by some observatories that the light curve followed the theory and the spectra were represented as blackbody with the temperature of much less than 1 keV and the peak luminosity of 1042–1044 erg s−1 (Halpern et al. [24], Komossa & Bade [38]).

In 2011 a new type of TDE candidate Swift J164449.3+573451 was discovered by Swift and MAXI with the peak X-ray ﬂux of 7 × 10−9 erg cm−2 s−1. Figure 2.7 shows the long-term light curve obtained by Swift (Burrows et al. [7]). In contrary to the classical TDEs, the source 8 associated with a 10 M supermassive black hole has shown bright X-ray emission with the ﬂux of > 10−10 erg cm−2 s−1 for more than ten days. The observations performed in radio to γ-ray wavelength revealed the relativistic jet which may be driven by the TDE aligned into our line of sight and originated anomalous emission by synchrotron and/or inverse Compton processes (Bloom et al. [5], Burrows et al. [7], Zauderer et al. [85]). The lower panel of Figure 2.7 demonstrates the story of the TDE associated with the jet6. Since the event rate of the TDE was estimated as 0.08–3.9 year−1 (Burrows et al. [7]), additional candidates may be discovered in our analysis.

2.1.7 Supernova shock breakout

Supernovae (SNe) are luminous stellar explosions associated with the death of stars. Based on the spectral features in optical wavelength, SNe are classiﬁed into four main types; Ia, Ib, Ic and II. Here, progenitors of the type Ia SNe are white dwarfs are provided mass from stars in the binaries. When the mass reaches at the Chandrasekhar limit, the dwarf is gravitationally collapsed and drives an explosion following violent nuclear fusion in its core. On the other hand, the other types are triggered in the ending of the evolution by the core collapse of massive star

6http://www.nasa.gov/mission pages/swift/bursts/devoured-star.html doi:Chapter10.1038/nature 2.10374Background information of short X-ray transient RESEARCH SUPPLEMENTARY INFORMATION13

Supplementary Figure 13 | Swift XRT ﬂux light curve of Sw J1644+57, through 17 May 2011. .

× ∼ ∼

Supplementary Table 7 | X-ray Rest Frame Flux and Luminosity of Sw J1644+57.

. × − − − . × − . × − − − . × − . × − − − . × − . × − − − . × −

Figure 2.7: Up: The light curve of Swift J1644+57 obtained by Swift-XRT. Low: The illustration of how to generate the relativistic jet with the tidal disruption event of Swift J1644+57. The black hole at the left side of each panel swallows the distorted star and powers the narrow jet. We observed this phenomenon in face-on. WWW.NATURE.COM/NATURE | 22 Chapter 2. Background information of short X-ray transient 14 whose mass is several times larger than the stelar mass. Observationally, SNe are discovered a few day after its ignition when the accompanied neutrinos will be detected. Colgate [9] and Klein & Chevalier [37] theorized early X-ray and/or ultraviolet emission when shock wave originated from the core breaks out through the stellar surface.

The prompt X-ray emission from a break-out of super nova shock wave was detected in 2008 as the X-ray outburst XRO080109 which associated with the super nova SN2008D (Soderberg et al. [72]). The X-ray light curve was characterized fast-rise with the rise time of 63 sec and exponential decay. The spectral analysis reported the peak ﬂux is 6.9 × 10−10 erg cm−2 s−1 in the 0.3–10 keV band. Since the luminosity is 6.1 × 1043 erg s−1 assuming the host galaxy as NGC2770 (the distance is 27 Mpc), MAXI-GSC will be detectable such shock break outs accompanying SNe in a few Mpc distance.

2.2 Information of past transient event surveys

2.2.1 Early history of all-sky X-ray surveys

In the early era of X-ray astronomy, in 1970s and 1980s, several observatories performed X-ray all-sky surveys. They discovered many X-ray sources that are now well known and short X-ray transients (SXTs) with detecters which were mainly sensitive to soft X-rays (< a few 10 keV). Some of these observatories such as the Uhuru satellite (launched in 1970), the third small astronomy satellite SAS-3 (launched in 1972) have reported discoveries of SXTs (Forman et al. [16], Hoﬀman et al. [30]).

Ariel V, which is an X-ray astronomical observation satellite launched in 1974, performed all- sky survey for 5.5 year and reported 27 SXTs (Pye & McHardy [60]). A detector onboard the satellite is a set of collimated proportional counters with a field of view 0.75◦ × 10.6◦ as called Sky Survey Instrument (SSI). These SXTs were detected with a significance of > 5.5σ, or a threshold flux of > 8 SSI count s−1 = 20 mCrab. Eleven of them were identified as flare stars and known X-ray sources including blazars, X-ray binaries and globular clusters. Pye & McHardy [60] reported that possible counterparts of other unidentified SXTs were gamma-ray bursts and/or yet unknown classes of object.

A systematic all-sky survey were also performed by High Energy Astronomy Observatory 1 (HEAO-1) which was launched in 1977. This observatory had four types of X-ray and soft γ-ray Chapter 2. Background information of short X-ray transient 15 detectors consisting of proportional counters with collimators and scintillator detectors. One of the detectors, Large Area Sky Survey experiment (LASS or called as A-1), had performed observations for six month in the 0.5–20 keV band and discovered ten SXTs (Ambruster & Wood [1]). Although four of them were identified as flare stars, other six transients were unidentified, which may have been gamma-ray bursts. The survey data of another detector, Cosmic X- ray Experiment (CXE or called as A-2), provided five new ones and three sources which were previously found flare stars (Connors et al. [10]).

The Einstein observatory (i.e., HEAO-2 Giacconi et al. [20]) was the ﬁrst imaging X-ray telescope launched in 1978. Detection of SXTs with the Imaging Proportional Counter (IPC) on the observatory was reported in Gotthelf et al. [21]. They discovered 42 SXTs with the limiting sensitivity of 10−11 erg cm−2 s−1 in the 0.2–3.5 keV band. Gotthelf et al. [21] concluded 18 of the transients have harder spectra and slow decaying light curves and were originated from common source like as GRBs.

In addition to the mission by Europe and the United States, Japan and Soviet Union also performed X-ray survey in 1970s and 1980s, From Japan, three mission of X-ray observatories were launched; Hakucho in 1979, Tenma in 1983 and Ginga in 1987. The Gamma-ray Burst Detector (GBD) onboard Ginga have observed 24 SXTs in 1.5–370 keV for about a year (Yoshida et al. [84]). Since GBD had no position sensitivity without any collimators, any counterpart of the SXTs has not been conﬁrmed. In 1989 a Soviet space observatory Granat were launched. The all-sky X-ray instrument WATCH monitored approximately 75% sky in the 8–100 keV and energy band. Castro-Tirado et al. [8] reported six SXT detections, of which three were possible GRBs and two were associated with known sources.

2.2.2 All-sky surveys in 1990s

In 1990s, another developed all-sky survey missions have been launched, such as the CGRO, ROSAT, BeppoSAX and RXTE satellites. The CGRO and BeppoSAX performed surveys in the hard X-ray bands, 20–600 keV of CGRO and 40–700 keV of BeppoSAX, whose results focusing to GRB will be described in Section 2.1.1. One of the remarkable contributors of the soft X-ray survey was ROSAT (the Röntgen Satellite), which was launched in 1990. In the first six months, ROSAT observed the all sky with the Position Sensitive Proportional Counter (PSPC) in the 0.1–2.4 keV. The telescope provided data with ∼ 10−12 erg cm−2 s−1 detective sensitivity and detected 27 X-ray transients (Greiner et al. [22]). Although most of them were identified to flare Chapter 2. Background information of short X-ray transient 16 stars, the small detection rate of ROSAT all-sky survey is consistent with the rate of afterglows expected from the BATSE data.

In 1995, the Rossi X-ray Timing Explorer (RXTE) satellite was launched into low-Earth orbit. The satellite had two instrument which observed soft X-rays, the Proportional Counter Array (PCA) and the All-Sky Monitor (ASM) which detected photons in the 2–20 keV and 2–12 keV energy bands. The PCA is a pointing observable instrument consisting of gas proportional counters and observing in the 2–60 keV. On the other hand, the ASM consists of three set of position sensitive proportional counters and slit masks (Levine et al. [42]). Using the ASM, Smith et al. [71] reported light curves of 15 GRBs. They represented the diﬀerence of temporal variability detected by the ASM and other instrument, such as BATSE, observing in the hard energy bands. One year after the RXTE launch, in 1996, the BeppoSAX satellite started observation with the telescope instruments, the Wide Field Camera (WFC) and Gamma-Ray Burst Monitor (GRBM). The sensitive energy bands of the WFC and GRBM are 2–28 keV and 40–700 keV, respectively. The main achievement of the BeppoSAX focusing to the soft X-ray transient is the discovery of GRB’s afterglow and X-ray Flashes (XRFs).

2.2.3 Recent surveys

The High Energy Transient Explorer (HETE-2) was a satellite aimed to observe GRBs launched in 2000. HETE-2 has FREGATE (NaI scintillation counters), WXM (proportional counters and coded masks) and SXC (X-ray CCDs with corded mask). A burst detected with the FREGATE and/or WXM is localized by the WXM and performed fine localization with SXC. The detection and localization result are immediately reported to the GRB Coordinate Network (GCN)7. Number of the detected GRB with HETE-2 is 250 events in about six year operation (Pélangeon et al. [58]). With the HETE-2 data in the first three years, Sakamoto et al. [63] revealed the temporal and spectral properties of the XRFs and X-ray-rich GRBs. The 45 HETE-2 detected GRBs are characterized by its fluence ratio between the 2–30 keV and 30–400 keV into GRB, X-ray-rich GRB and XRF.

The Swiftsattelite, which was launched in 2004, is the largest contibutor of rapid localization of GRBs. As same as HETE-2, the satellite is specified to observe GRBs and report information (position, flux and significance) of the bursts to the GCN via the Internet. Swift has three observational instruments; Burst Alert Telescope (BAT), X-ray telescope (XRT) and UV/Optical

7http://gcn.gsfc.nasa.gov/ Chapter 2. Background information of short X-ray transient 17

Telescope (UVOT). The BAT detector detects GRBs within the 1.4 steradian FoV and roughly localizes them with their accuracy of sub-degree. When the BAT detects a GRB, the satellite is quickly oriented into the burst and starts observation with the XRT and UVOT. In the five year operation Swift detected 476 bursts by BAT (Sakamoto et al. [66]). Additional XRF samples detected by Swift revealed similarities and differences of the temporal and spectral properties between GRBs and XRFs (see Section 2.1.2 and Sakamoto et al. [65]). Moreover, Swift discovered new type SXTs such as the unexpected X-ray flare associated with tidal disruption event (see Section 2.1.6) and the shock break out of the super nova (see Section 2.1.7).

2.2.4 Systematic SXT search with MAXI

Since 2009, MAXI has been performing all-sky continuous monitoring with ∼ 80% sky coverage in one orbit period of the International Space Station (ISS). With two kinds of X-ray counters, Gas Slit Camera (GSC) and Solid-state Slit Camera (SSC), SXTs including GRBs have been reported by the MAXI observation (The speciﬁcations of MAXI and these detectors will be described in the next chapter).

A systematic search for SXTs has been already performed by Toizumi [75] for the first 15- month observation data of GSC. The method of the search consists of two main processes; rough extraction of SXT candidates and local analysis for the candidates. The former process selects excesses against fluctuation of GSC background which is represented by its simplified model. This selections were sequentially performed to the observation data which were temporally partitioned with every short duration of GSC’s one-scan transit. The partitioned data were represented with the background model which consists of non-X-ray background and cosmic X-ray background as shown in Figure 2.8. On the data, SXTs will appear as excesses with the significant signal to noise ratio against the background supposing that its fluctuation is the Poisson distribution. Next, they performed local analysis to determine accurate positions on the sky and X-ray fluxes of the selected candidates, statistically fitting with point spread function (PSF) of the GSC counter. The PSF of GSC is approximately duplicated by a product of triangular function and Gaussian function, which will be describe in Section 3.2.4.1. The local analysis fitted the X-ray event data of the selected candidates with the function using the un-binned likelihood method instead of the chi-square test so as to estimate the goodness of fit. Finally, the previous systematic search found 29 SXT candidates with the detection threshold Chapter 2. Background information of short X-ray transient 18

Total )

-1 CXB NXB

mm 0.015 -1

0.01

0.005

Count rate (count s (count rate Count -150 -100 -50 0 50 100 150 Detector coordinate (mm)

Figure 2.8: Distribution of X-ray events along the MAXI-GSC detector coordinate and result of model ﬁtting provided by Toizumi [75]. The blue and green lines are model functions of non X-ray background (NXB) and cosmic X-ray background (CXB). The red one is sum of these contributions. probability of 10−8. Four known GRBs and ﬁve possible GRBs were associated with the found SXTs.

Although the previous transient search study seems as success, it possesses complex estimation of propagation of errors. The searching method consists of two processes, selection of excess base on the Poisson-distributed background and local analysis with the PSF-fitting. Here, we note that determination of the detection threshold and estimation of fake detection in the later process depend on whether the former selection process detects these SXT candidates or does not. Furthermore, the analysis has the complicated dependency, nevertheless, the detection significances were estimated independently. Thus, the holistically propagation of errors cannot be simply interpreted. To avoid such complicated situations, we develop new simple and straightforward method to search SXTs from the MAXI-GSC long observational data. The method which will be explained in Chapter 4 supposes the Poisson-fluctuated background of the GSC counter and estimates only significance of excesses originated from SXTs; however it does not apply such fitting analysis with the background model and the PSF of X-ray sources. In Chapter 5 and 6, we will discuss an explicitly determined detection threshold and the estimation of fake detections using the simple method. Chapter 2. Background information of short X-ray transient 19

2.3 statistical test of survey study

2.3.1 Test of isotropy

If celestial objects distribute randomly, the positional distribution on the celestial sphere will be isotropic. To verify the isotropic/anisotropic positional distribution of celestial objects, two statistics hcos θi (Hartmann & Epstein [26]) and hsin2 b − 1/3i (Briggs et al. [6]) have been appreciated. The former is the dipole moment toward the Galactic center is the mean of the cos θi, where θi is an angle between the ith SXT candidate and the Galactic center. Another statistics, hsin2 b − 1/3i , the quadruple moment which tests a concentration in the Galactic plane or at the Galactic poles, where b is galactic latitude. If observation has even exposure and eﬀective area, an isotropic distribution provides both statistics of zero.

Using these statistics, the isotropic distribution of GRBs has been conﬁrmed by several studies. For example, Briggs et al. [6] investigated positional distribution with the 1005 GRBs obtained by the BATSE satellite. The result estimated the diﬀerence of these statistics by < 1σ and concluded the isotropic distribution of GRBs. In the past transient source survey with MAXI also suggested isotropic distribution with the hcos θi = 0.02 ± 0.17 (Toizumi [75]).

2.3.2 V/Vmax test

To investigate uniformity of radial distribution of celestial objects, V/Vmax test was introduced by Schmidt et al. [67]. First we suppose that a source with its luminosity of L and ﬂux F > Flim at a distance r is detectable with an instrument in the Euclidean space. Here, the distance r and the maximum distance at which the same source is detectable rmax is estimated from ( ) L 1/2 r = (2.1) 4πF ( ) L 1/2 rmax = . (2.2) 4πFlim

Next, we deﬁne two spherical volumes;

4πr3 V = (2.3) 3 4πr3 V = max . (2.4) max 3 Chapter 2. Background information of short X-ray transient 20

If we consider the uniform distribution of the sources, half of the source will be detected in the inner half of the volume Vmax and the other half in the outer half. Thus, the average of V/Vmax is expected as V/Vmax = 0.5.

Since in the actual observation we obtained number of X-ray/γ-ray photons using a detector, the V/Vmax value is converted to

( )− V C 3/2 = . (2.5) Vmax Clim

Here, C and Clim are the detected photon count of a source and the detection limit estimated by the signal to noise ratio of the instrument R, respectively. Supposing Poison distributed √ fluctuation of the background count Nb, the Clim is defined as R Nb. Pendleton et al. [59] reported V/Vmax = 0.329 ± 0.011, which is not equal to 0.5, base on the BATSE’s GRB catalog. Following the work, Paciesas et al. [55] also reported the non-half value with the revised 4th BATSE GRB catalog. These results support that GRBs are cosmologically distanced sources and the space is not flat but distorted.

2.3.3 LogN-log S plot

Another leading mark to study the spatial distribution is log N-log S plot. Here, S means a photon or energy fluence observed in any wavelength range and decreases inversely with square of the source distance r, i.e., S ∝ r−2. The N is cumulative number of sources whose fluxes are larger than the corresponding flux S. If objects uniformly distribute in the flat Euclidean space, number of objects within the radius r is proportional to the volume of the space, N ∝ r3. Therefore, the cumulative number of the objects will have relation with the observed flux as N ∝ S−3/2.

The first log N-log S plot of transient X-ray sources is obtained by the HEAO-1 satellite as shown in Figure 2.9 (Ambruster & Wood [1]). It is visually suitable to the S−1 relation rather than S−3/2. The following transient X-ray surveys (Arefiev et al. [2]) provided wide fluence band log N-log S plot which was assembled with archival data of the Ariel V, HEAO-1, WATCH, ROSAT and Einsteinsatellites. The plot Figure 2.10 is globally represented by the single power- law function with the index of −1. The previous study of short transient search with MAXI-GSC provided the similar log N-log S plot as shown in Figure 2.11 (Toizumi [75]). Although the plot 1986ApJ...311..258A

Chapter 2. Background information of short X-ray transient 21

Figure 2.9: LogN-log S plot obtained by HEAO-1. The error was estimated with Poisson distribution. The lines marked S−1 and S−3/2 are best ﬁts to the eye.

obeys the S−1 function in the brighter ﬂuence band (S > 0.8 counts cm−2), another power- law like component with the index of −3/2 appears in the fainter region. They discussed the possible origin of the components as ﬂares of active stars, AGNs and both prompt emissions and afterglows of GRBs. No. 2, 2003 FAST X-RAY TRANSIENTS AND GAMMA-RAY BURSTS 1241

It is known that many flare stars produce short, intense ing the geometric-mean time duration discussed above. X-ray flares (Strassmeier et al. 1993) with thermal spectra. However, because the overall distribution has a slope close Because many FXTs have been identified with flare stars to 1, changes in the assumed duration simply slide the (AW86; C86), we have calculated the FXT distribution that log N log S contribution from the HEAO 1 instruments up results if some FXTs have Crab-like spectra and others have or down along the same curve. The dashed/dotted bounda- thermal spectra. A mixture of 30% thermal-spectrum FXTs, ries show the effect of changing the duration to the extreme with kT 0:7keV,and70%Crab-spectrumFXTsmaxi- possibilities on the HEAO 1 A-2 log N log S distribution. mizes the¼ maximum likelihood function, considering the The ROSAT results from V98 and Einstein results from range kT 0:5 4:5keVand‘‘thermal’’burstfraction GHH96 were not used to determine our net log N log S dis- 0%–100%.¼ tribution. However, they are plotted in Figure 2 for compar- Given this mix of FXT spectra, our best estimate of and ison. We have translated the fluence measured in the 4 A0 yields 0:95 0:1andA0 4 1 10 at 1 crab s. ROSAT energy band to 2–10 keV by assuming a Crab-like Aconservative,3¼ Æ limit bounds¼ð Æ theÞÂ parameters to spectrum with negligible foreground absorption. The events 4 5 0:7 1:2 , A0 10 10 .Inanycase,FXTsarecom- reported by V98 had fluences from 3.5 to 200 mcrab s. We mon,¼½ occurring somewhere¼½ on the sky dozens to hundreds estimated the total number of events per year for ROSAT of times each day. They are 1 or 2 orders of magnitude more events that had no detectable quiescent emission as 6 1 common than classical gamma-ray bursts (Kommers et al. 2 10 yrÀ and for events without optical counterparts Â 5 1 2001). Note that it is impossible to determine whether all the as 4 10 events yrÀ .Wehavetakenthesevaluesas events that make up this distribution fit our strict definition upper Â and lower limits to the FXT rate at this fluence of occurring only once. If some of the FXTs come from (shown as the plus sign in Fig. 2) The Einstein data were also magnetic stars, RS CVn’s, or other nearby objects, they converted to 2–10 keV assuming a Crab-like spectrum may indeed repeat. These repetitions would have been seen (shown as the rhomb in Fig. 2), which is consistent with the by HEAO 1, Ariel-5, etc., if those instruments had better sky spectral shape found by GHH96 for hard events. coverage or longer lifetimes. Our integrated distribution is consistent with a power law 4 0:2 Our distribution ( 0:95 and A0 4 10 ,70%Crab- with a slope 1:0þ0:3.Aspatiallyhomogenousdistribu- like versus 30% thermal¼ spectra) is shown¼ Â in Figure 2. It is tion of identical¼À sourcesÀ (standard candles) would have a not significantly different from the distribution that follows slope 1.5. Our distribution is consistent with the slope of if we assume that all bursts have Crab-like spectra. 0.8 derivedÀ by PM83 from Ariel-5 events. By adding data Figure 2 also shows the segments of the distribution seen from’ several other experiments, we have extended the PM83 by the individual instruments (our analysis approach 2). distribution by several orders of magnitude toward the faint The shaded areas represent 1 errors for the individual end. The ROSAT and Einstein points, derived from V98 experiment log N log S curves, including the normalization and GHH96, are consistent with an extrapolation of our and the statistics of the number of events seen by each log N log S curve. experiment. Figure 3 shows the sky distribution of the events used in Although the duration of the HEAO 1 events is quite Figure 2 (Ariel, HEAO,andWATCHmarkedwithplus uncertain, this uncertainty does not affect the overall distri- signs, and ROSAT and Einstein events marked with trian- Chapter 2.bution.Background We information plot the backward of short X-ray derived transient log N log S assum- 22gles). The sky distribution is consistent with isotropy. The dipole moment of the distribution toward the Galactic center is 0:07 0:08, and the quadrupole moment is 0:31 0:04 (forÆ an isotropic distribution the dipole moment Æ 1 is zero, and the quadrupole moment is 3). We see no significant anisotropy for any subset of these data. However, we need to note that there are no reliable exposure maps for the

Figure 2.10: Fig. 2.Log—ObservedN-log S plot log of shortN log X-rayS distribution transients assembled for FXTs, with the assuming archival dataa of Arielpower-law V, HEAO-1, fluence WATCH, distributionROSAT and [NEinstein>S .A TheS thick ]. The solid thick line solidrepresents line is the the best-fit power law with Ariel V, HEAO-1 and WATCHð data.Þ¼ The0 À index of the function is 0.95. Statistics and systematicbest-fit power errors law of each derived distribution from a are maximum shown as likelihood the shadowed fit to areas.Ariel-5 The, HEAO thick curve is CHAPTE1,andWATCHdata,withR 6. DISCUSSION 0:95 and A 4 104.Theshadowed averaged data of HEAO-1 obtained from¼ the dashed and0 dotted¼ Â curves which mean difference of assumedareas event show duration the measurements to correct HEAO-1 of the data. log N log S distribution from each experiment, including both event statistics and systematic errors. The plus sign denotes ROSAT events (V98), and the diamond represents Einstein FXTs (GHH96).1000 The dashed and dotted curves show the range of distribu- Fig. 3.—Sky distribution of the FXTs included in the log N log S distri- tion that follows from the HEAO 1 A-2 events, given extreme possibilities bution of Fig. 2. The crosses are the Ariel-5, HEAO 1,andWATCHevents; for event duration. MAXI transients triangles are the ROSAT and Einstein events. ] h t 100 n o m 5 1 / 10 s t n e v e [ 1 e t a r

R 0.1 G

0.01 0.1 1 10 100 1000 Fluence [cts cm-2]

FigFigureure 6.1: 2.11:LogNLog-LoNgS-logdisStrplotibut ofion shortof 2 X-ray9 X-ra transientsy transie obtainednts with withthe p MAXIrobab inili thety t previoushreshold of 1study.0! 8. R Thised li plotne s includeshows th 29e b transientsy Swift B whoseAT re detectionsult(15! probability25 keV) us isin moreg the thanßux∼co9nσvlevel.erted to the GSC energy range and normalizedwith the obseving conditions (seetext for details).

Conversion of Swift int o MA X I rat e

Next, we describe the conversion of the GRB rate of the Swift BAT to the MAXI range in the LogN -LogS plot. We obtained the rate conversion factor R for Swift to MAXI as equation 6.1

! T " ! ! " ! F " R = maxi maxi maxi = 0.026 (6.1) Tswift ! swift Fswift where T is the mission life time after launch, ! is the operation e! ciency which is a duty cycle for instruments, and F is the Þeld of view. Table 6.1 shows the value for calculation of conversion factor for Swift BAT to MAXI GSC. The obtained value of rate conversion factor is R = 2.6 " 10! 2.

Conversion of Fl ux int o Fl uence

The ßux measured by Swift BAT must be converted the X-ray ßuence that would have beenmeasured by MAXI GSC.

67 Chapter 3

Observatory – MAXI

3.1 MAXI

3.1.1 Overview of MAXI

Monitor of All-sky X-ray Image (MAXI) is the ﬁrst astronomical mission operated on the International Space Station (ISS). It was delivered to the ISS by Space Shuttle Endeavor on July 16 2009 and installed on the Japanese Experiment Module Kibo Exposed Facility (JEM- EF) on July 24. Since August 15 MAXI has performed nominal operation for more than four years. The mission is reviewed in Matsuoka et al. [47]. Picture of MAXI installed on JEM- EF1 and overview of the MAXI components2 are shown in Figure 3.1 and 3.2 (Matsuoka et al. [47]), respectively. MAXI has two kinds of X-ray cameras, Gas Slit Camera (GSC), and Solid- state Slit Camera (SSC). The information of the GSC instruments is described in Mihara et al. [49], and the in-orbit performance is described in Sugizaki et al. [74]. Tomida et al. [76] and Tsunemi et al. [78] describe the performance of SSC on the ground and in-orbit, respectively. In addition to these observational equipments, MAXI has some controlling and operating systems, e.g., Radiation Belt Monitor, Visual Star Camera, Ring Laser Gyroscope, Loop Heat Pipe and Radiation System, and Mission Data Processor. See publications listed in the web site of MAXI3 to obtain information about these subsystems.

1http://iss.jaxa.jp/kibo/archive/2010/09/good design award 2010.html 2http://www.jaxa.jp/press/2009/08/20090818 maxi j.html 3http://maxi.riken.jp/top/ 23 Chapter 3. Observatory 24

3.1.2 Data processing ﬂow of MAXI

MAXI provides some kind of data; e.g., X-ray observational data and house keeping data. They are processed on data processor of MAXI and transported to ground stations of NASA (National Aeronautics and Space Administration) through data relay satellites. The telemetry data transported to Tsukuba Space Center of JAXA (Japan Aerospace Exploration Agency) is stored on a database, called as MAXI-DB, which is designed to handle MAXI data. From the MAXI-DB, the MAXI science team constructs observational data which include information of

Figure 3.1: Left: Picture of JEM, JEM-EF and MAXI. MAXI mounted in front of the JEM- EF is located at a center lower part in the picture. Right: Overview of MAXI. The left side structure is a docking port of the JEM-EF.

Grapple fixture Loop Heat Pipe & Radiator System for robot arm (LHPRS for SSC cooling)

Payload Interface GPS Unit (PIU) antenna

Visual Star Sensor (VSC) Ring Laser Solid-state Slit Gyro (RLG) Camera (SSC; X-ray CCDs) Gas Slit Camera (GSC; X-ray gas proportional counters)

Figure 3.2: Layout of MAXI instruments. The GSC units are mounted on the front and upper side of MAXI. The cameras on the front corner of MAXI are SSC. Other subsystems are also shown. Chapter 3. Observatory 25 detected X-ray events for each day and each camera of GSC and SSC. Using a function providing real-time data from MAXI-DB, we operate a system to discover and report activities of X-ray sources, which is called as “Nova Search” system.

To be convenient to use the data in MAXI-DB, they are converted to binary ﬁles with the format of FITS (Flexible Image Transport System4), which is a standard data format to analyze and archive the astronomical data of spectra, images and tables contained any information. In addition to our standard scientiﬁc analysis, the FITS formatted data are used to publish daily updated light curves for more than 400 X-ray sources in the MAXI web site. Although the only MAXI science team can access these data, the MAXI on-demand process system5 provides an indirect analysis environment to make images, spectra and light curves in any region and time duration.

3.2 GSC

3.2.1 Overview of GSC

GSC is a set of twelve X-ray sensitive cameras consisting of one-dimensional position sensitive proportional counters and slit-slat collimators. The layout of the counters on MAXI is shown in Figure 3.3 (Mihara et al. [49]). The six GSC cameras (GSC 0, 1, 2, 6, 7 and 8), which point toward tangential direction of the ISS orbit, join a group called as GSC-H (“H” means horizontal). The others (GSC 3, 4, 5, 9, A and B) monitoring the zenith sky are GSC-Z. GSC constantly monitors approximately 85% of the whole sky for every 92-minutes ISS orbit using thin-long ﬁeld of view with the systematic position accuracy of 0.2◦. The capability to obtain spectral information ranges from 2 keV to 30 keV with the energy resolution of 18% at 5.9 keV.

3.2.2 Components of GSC

A camera of GSC consists of three main components; an X-ray detector, front-end electronics and a set of slid-slat collimators. This section explains conﬁguration of these components.

The X-ray photon detector of the GSC camera is a one-dimensional proportional counter ﬁlled with xenon (Xe) gas. Electrons converted from energy of an incident photon (or a charged

4http://fits.gsfc.nasa.gov/ 5http://maxi.riken.jp/mxondem/ Chapter 3. Observatory 26

Figure 3.3: Schematic overview of GSC cameras on MAXI. A set of a triangle and a small cuboid is a GSC camera consisting of collimators and a proportional counter. The ID numbers of the cameras are marked as 0, 1, . . . , 9, A, B. Three pairs are mounted on the side plane of MAXI and the other on the top. An arrow in each counter is a direction of a carbon wire along the detector coordinate. The GSC 0–5 are connected to Mission Data Processor A (MDP- A), and the others to MDP-B. Radiation Belt Monitors (RBMs) are mounted on each center camera. particle such as proton) by photoelectric eﬀect are multiplied and accelerated towards the anode wire with the high voltage in the chamber of the counter. An incident position of a photon or a particle along the wire is determined with a pulse height ratio provided by readouts at both anode ends.

Figure 3.4 shows a picture and a top view of a GSC proportional counter (Mihara et al. [49]). X-ray photons enter the top window of the counter that is sealed with a beryllium foil whose thickness is 100 µm. To support tension of the beryllium foil in vacuum, the window is separated by grid structures which are placed parallel to anode wires and vertically placed at the center of the counter. The body of the counter is made of titanium, since the heat-expansion coeﬃcient is close to that of beryllium. Gas consisting of Xe (99%) and CO2 (1%) with a pressure of 1.4 atm at 0 ◦C is ﬁlled in the sealed chamber. Chapter 3. Observatory 27

Figure 3.4: Picture (left) and top view (right) of a proportional counter unit. The beryllium foil is applied on the inside of the chamber. Anode wires are strained in the longitudinal direction. The thin grid structures parallel to carbon wires and a thick one perpendicular to that are support the foil. The unit of numbers is mm.

Figure 3.5: Cross-section views of a proportional counter unit on the perpendicular (left) and parallel (right) to anode wires. Labels C, B, ST and SB of the wires denote Carbon, Bottom, SideTop and Side Bottom. The wires aligning to separate the gas chamber are ground of the counter. The shaded region is chassis of the counter. The unit of numbers is mm.

Cross-section views of the proportional counter are shown in Figure 3.5 (Mihara et al. [49]). The carbon wires are main sensors and surrounded by the ground wires. The wires labeled with B, ST and SB in Figure 3.5 are readouts of veto signal which rejects events detected with charged particles and γ-rays with anti-coincidence protocol. The geometry of the wires has been determined to obtain adequate eﬃciency for X-rays in the 2–30 keV energy band. In normal operation of GSC, the high voltage of 1650 V is applied to these wires.

Number of readouts of GSC proportional counter is 14; both side of six carbon sensor anodes and two veto anodes.6 Signals from these readouts are processed in font-end electronics boards which include a high-voltage (HV) power supply and House-Keeping (HK) electronics. Each readout is connected to a preampliﬁer through a coupling capacitor. On the other hand, the

6The wires of B0 to B5 in Figure 3.5 are connected in the counter and have one readout as veto signal. ST0, ST1, SB0 and SB1 are also the same. Chapter 3. Observatory 28

Scan angle 3.7 slit 3.1 collimator 64 sheets

118.4 160 22.5 Be window anode direction

Figure 3.6: Geometry of a proportional counter (a box-like structure in the lower side), slat collimators (triangle like sheets in the middle) and a slit (a very narrow rectangle labeled as slit in the upper). The angle φ and “Scan angle” indicate an incident angle in the parallel and perpendicular plane to the anode direction, respectively. The unit of the numbers is mm.

HV supply is connected to all of the anode wires, thus one GSC unit has one HV supply and twelve preampliﬁers. See Mihara et al. [49] to obtain more information of the circuits.

On the proportional counter, sixty-four slat collimators are align in the perpendicular direction to the anodes with the pithes of 3.1 mm. These collimators with the hight of 118.4 mm separate an opening angle of the window of the counter into many ﬁeld of views (FoVs) with the width of 1.5◦ in full width of half maximum (FWHM). A slat is made of phosphor bronze with the thickness of 0.1 mm that stops X-rays up to 30 keV. Above the top of the slats, a slit of two parallel tungsten rods with 3.7-mm separation is located to narrow an opening angle in the plane along the anode direction as 40◦ in FWHM. The integrated geometry of a proportional counter, slat collimators and a slit is shown in Figure 3.6 (Mihara et al. [49]).

3.2.3 Principle of the position determination

GSC determines an X-ray incident direction in two dimensions; a coordinate parallel to the anode wires (called as detecter coordinate) is deﬁned with photon absorbed position, and another coordinate along rotation of the ISS (scan coordinate) is encoded in the transit time within the Chapter 3. Observatory 29

Zenith camera FOV ISS rotation

X-ray sources GSC_3[9] GSC_4[A] * rotation pole o θ -1.5

φ +40o Horizon camera FOV o o +1.5 GSC_5[B] -40 Left camera Slit GSC_0[6] Slat -80o Collimator -40o β MAXI

GSC_1[7] o +40o +80 Acquisition angle:Center camera

GSC_2[8] Xe-gas Right camera Counter ISS orbital motion Anode direction (DETX) 1-D position-sensitivity Earth

Figure 3.7: Left: A ﬁeld of view (FoV) of a GSC unit. A photon from an X-ray source enters to a gap between slat collimators and is absorbed in the proportional counter. The curved arrows mean the width of FoV along the anode direction φ and the scan direction θ. Right: Integrated FoV of GSC is shown as solid arcs. Labels as GSC X[Y] mean the corresponding GSC camera X and the redundancy Y. narrowly collimated FoV. Figure 3.7 (Sugizaki et al. [74]) schematically illustrates the method to determine a photon incident position on the sky.

3.2.3.1 Detector Coordinate

A position of a photon detected on an anode wire is represented as a ratio of signal heights of both readouts. We call such position as Position Measure (PM). A PM parameter has an approximately linear relation with a position where an X-ray absorbed along the anode wire, however not exactly. Thus, ground experiments provided a conversion table of the relation between PM and photon absorbed position. The position converted from the PM value is called as DETX. Although the PM has no unit, the unit of DETX is mm. The DETX coordinate is deﬁned with the origin of the center of the anode wire and the plus direction as the rightward side of the counter (see Figure 3.5).

Note that this DETX position does not correspond to an incident position of the photon on the beryllium window, because the DETX depends on the energy and the incident angle. In the chamber of GSC ﬁlled with Xe gas, an X-ray with higher energy runs on a longer path and ionizes electrons along the path. Photons with sharp incident angles also penetrate into the gas Chapter 3. Observatory 30 with longer paths. Consequently, the wider distribution of absorbed electrons along the anode wire mistakenly shifts the position represented as a DETX value from the incident position on the beryllium window with the length of an approximately mean free path of the X-ray photon. By the correction of the shift with a function of the mean free path, the measured photon energy and the gas pressure we obtain “true” positions of photons called as BEX (X coordinate on the beryllium window) position. The origin and direction of the BEX coordinate is as same as those of the DETX coordinate. The unit of BEX is also mm. We note that since a grid on the upper panel of the proportional counter shadows the center of FoV, a region with |BEX| . 10mm is not sensitive to X-ray photons from sources but background of charged particles and gamma-rays.

The geometry of the GSC camera deﬁnes the FoV along the detector direction seeming as a sector form (see Figure 3.7). Its full width is 80◦ for the camera. As shown in Figure 3.3 the cameras mounted on both sides of MAXI are tilt with 40◦ to the opposite outward direction. Therefore, FoVs of the cameras in each group of GSC-H and GSC-Z make an obtuse (160◦) FoV. The GSC-H and -Z covers the horizontal and zenith sky with the integrated FoV, respectively. Hereafter, an incident angle of X-ray photons along the detector direction is represented with θ for each GSC unit. This angle corresponds to an acquisition angle β for each obtuse FoV.

3.2.3.2 Scan Coordinate

An angle of the incident photon on the plane perpendicular to the anode direction is determined from the attitude of MAXI. A ﬁled of view (FoV) of the GSC proportional counter is sliced lengthwise along the rotation direction of the ISS by the slat collimators. Each narrowed FoV with the width of 3◦ sweeps the sky by the attitude change of MAXI. Viewing from the opposite side, we can convert information of the MAXI attitude to the direction of the FoV covering an arc-like sky at a given time. The incident angle along the scan direction is called as φ.

The attitude information is provided by Attitude Determination System (ADS) consisting of Visual Star Camera (VSC) and Ring Laser Gyroscope (RLG) of MAXI. An accuracy of attitude determination is 0.1 arcmin with VSC when the camera observes more than three stars and 0.1◦ in the worst condition of ADS (Morii et al. [51]). The timing information of MAXI refers the GPS signal with the precision of 0.1 msec. Chapter 3. Observatory 31

3.2.4 In orbit performance

3.2.4.1 PSF of GSC

Point spread function (PSF) describes the response of an image observation system to a point source. Generally, it reﬂects features of the focused optical system and statistical factors of the detector. The PSF of GSC is complex because the shape is deﬁned with the response of the proportional counter and the FoV of the camera on the detector and scan direction, respectively.

The PSF in the detector coordinate represented as the BEX value is a complex function depending on the incident angle φ and the energy of the photon. In principle, the angular response of the slit-and-slat collimator and the position response of the proportional counter determine the PSF along the detector coordinate. As shown in Figure 3.8 (Sugizaki et al. [74]) the PSFs with φ = 5◦ in the soft and hard energy bands are similar to a gaussian function and successfully reconstruct the actual observed proﬁle of Sco X-1. By contrast, one with φ = 33◦ in the 10–30 keV is asymmetric and cannot be approximately represented with a gaussian. Therefore, an accuracy of the position determination of sources in the hard X-ray band (> 30keV) is worser than that in the soft band, which is reasonable for a wide range of the incident angle φ. We note that the width of the PSF, which is approximately represented as a standard deviation σ of a gaussian function, slightly depends on the angle φ. The dependency is almost linear from φ ∼ 0◦ with σ ∼ 1.5 mm to φ ∼ ±40◦ with σ ∼ 2.5 mm.

Meanwhile the PSF in the scan coordinate is defines as a function of effective area of the slit- slat collimator which is determined by only the geometry of the GSC camera. The function of effective area is a triangle as shown in Figure 3.9 (Sugizaki et al. [74]). The area depends on an acquisition angle β toward an X-ray source since the scan-transit time varies with these angles. The transit time of the cameras on the left and right side of MAXI increases from ∼ 40 second with β ∼ 0 (and an incident angle θ ∼ ±40) to ∼ 150 second with β ∼ ±80 (and θ ∓ 40). On the other hand, that of the center camera has small variation as 40–50 second due to the small dependency of the scanned area on the acquisition angle β. According to the actual observation data, the duration time of one-scan transit ranges from 40–200 sec and most frequently takes about 60 sec. Chapter 3. Observatory 32

φ ° φ ° 1 =5 10-30 keV 1 =33 2-10 keV

0.8 0.8

0.6 0.6

0.4 0.4 10-30 keV 2-10 keV Normalized count rate 0.2 Normalized count rate 0.2

0 0-4 -3 -2 -1 0 1 2 3 4 -4 -3 -2 -1 0 1 2 3 4 ∆φ [degree] ∆φ [degree]

Figure 3.8: PSF along the detector direction at each angle β = 5◦ (right) and 33◦ (left) in the 2–10 keV band (solid line) and 10–30 keV (dashed line). To show the PSF a power-law energy spectrum with the photon index of 2.1 is assumed. The observed count rate of Sco X-1 in 2–10 keV (circle) and 10–30 keV (triangle) are plotted with 1-σ error bars.

150 ∆θ [degree] -1.5 -1 -0.5 0 0.5 1 1.5 5 β=5° GSC_1 100 4 GSC_0 GSC_2 ] 2 3 GSC_2 50 GSC_1 Scan−transit time [s]

Area [cm 2

1 0 0 -60 -40 -20 0 20 40 60 −50 0 50 TIME [s] β: Source−acquisition angle [degree]

Figure 3.9: Left: Solid lines represent time variations of eﬀective area during a scan transit for the two cameras at an acquisition angle β = 5◦. The observed count rate of X-ray photon from Sco X-1 is plotted with 1-σ error bars. Right: Dependency of scan-transit time on the angle β for GSC 0, 1 and 2 in the ISS nominal attitude.

3.2.4.2 Alignment calibration

In early operation we found that the position of known X-ray sources are significantly deviated from the true position as large as 0.9◦ in 90% containment radius. Causes of this problem is predicted that the mechanical mis-alignments of the GSC cameras and the chassis of MAXI which reflect an accumulation of tolerances of the MAXI component and a vibration of the launch. Morii et al. [51] performed an alignment calibration of MAXI/GSC with reconstruction of a matrix which converts an attitude in the MAXI coordinate provided from Attitude Determina- tion System to that in the GSC coordinated. The elements of the matrix were determined by localizations of selected bright X-ray sources, e.g., Sco X-1 and Crab nebula, for each GSC camera and each anode wire. As the result of this calibration, the systematic deviation of position accuracy was improved to ∼ 0.12◦ for Sco X-1 and . 0.2◦ for other bright sources. Note that Chapter 3. Observatory 33 since this work was performed with the data in the energy band of 4–10 keV, scientific studies including position determination of X-ray sources is difficult under the present calibration.

3.2.4.3 High voltage operation

In the beginning of the mission, GSC has been operated with the high voltage (HV) of 1650 V in any time except a duration in the SAA and an exposure to the Sun. However, sudden shut downs of GSC units have been reported for the cameras 6, 9 and A (means 10) since 8 September 2009. We ascertain a large amount of discharges in the counters as the cause of the shut down. In order to avoid risk of another accident, the HV of some GSC cameras have been reduced to 1550 V or turned off. In addition, we have ceased the operation of GSC at the high latitude above 40◦ since 23 September 2009. The summary of operation efficiency of GSC is shown in Figure 3.10 (Serino [68]). Now, the HV of the cameras 1, 8 and B (means 11) also have been turned off to preserve redundant counters. In contrast to them, the camera 4 and 5 have been almost perfectly operated with the HV of 1650 V since the first light of GSC.

3.2.4.4 Background environment

One of contributors to background for X-ray detecter is Cosmic X-ray Background (CXB). The CXB is diﬀuse X-ray emission from celestial sources which was discovered by the observation with a rocket (Giacconi et al. [19]). High-resolution observations of Chandra X-ray observatory revealed a lot of extra galactic sources such as active galactic nuclei mostly contribute to the CXB (Setti & Woltjer [70]). Spectral CXB property in Moretti et al. [50], the ﬂux in the 2–10 keV band, which is included in the GSC energy band, is 5 × 10−12 erg s−1 cm−2 deg−2.

Another contributor of background is called as Non X-ray Background (NXB). The NXB is mainly caused by detections of incident charged particles such as protons and electrons. In- coming to the proportional counters of GSC, these particles ionize the Xe gas as same as X-ray photons. Although most of such “fake” events by the high energetic particles is ignored to perform anti-coincidence with veto triggers, reminded events contribute to the background of the GSC camera. Such charged particle mostly distributes in area of the SAA and in radiation belt at high latitudes. Radiation Belt Monitors (RBMs) onboard MAXI measures ﬂux of the charged particles and ceases the operation of GSC to prevent from high count rate due to the particles. Chapter 3. Observatory 34

100 80 camera 0 camera 6 60 40 20 efficiency [%] 100 0 80 camera 1 camera 7 60 40 20 efficiency [%] 100 0 80 camera 2 camera 8 60 40 20 efficiency [%] 100 0 80 camera 3 camera 9 60 40 20 efficiency [%] 100 0 80 camera 4 camera A 60 40 20 efficiency [%] 100 0 80 camera 5 camera B 60 40 20 efficiency [%] 0 2009 2010 2011 2012 2013 2010 2011 2012 2013 year year

Figure 3.10: The operation history of GSC cameras. The 100% eﬃciency means the GSC has been operated for the whole orbit of the ISS. The black and gray lines represent the high voltage conﬁguration of 1650 V and 1550 V, respectively. Chapter 3. Observatory 35

High energetic photons, i.e., γ-ray, also contribute the NXB in the MAXI operation. The possible origins of the photons is astrophysical γ-ray emitters, such as active galactic nuclei and pulsars. As same as the case of charged particles, incoming γ-rays are rejected by the anti-coincidence. However, γ-rays scattered by components of detectors and coming from side or back of the detectors do not trigger the veto system and detected as background. In fact, the origin of these γ-ray is not only celestial sources, but also Cs137 source mounted on the bottom of Soyuz spacecraft. The source on Soyuz is component of a height indicator which is used during Soyuz landing on ground. When Soyuz attached on the nearest port of the ISS from MAXI, background of GSC has signiﬁcantly increased. The history of the background as shown in Figure 3.11 obviously demonstrates the drastic variation of the dairy background rate.

The X-ray emissions in the solar system are also effective to background of MAXI. The Sun is the most powerful X-ray (and charged particle) source in the environment of the ISS. As mentioned in section Section 3.2.4.3, since GSC is not operated when the FoV sees the Sun, the solar X-ray is not dominant background. However, when solar flare emits a great amount of X-rays, background suddenly increases around the Sun region whose area is larger than the HV shutting down duration. In addition to the direct effect of the Sun, a reflection of the solar X-ray with the solar paddle of the ISS is also detected by GSC. The solar paddles almost always cover a part of FoV of some GSC cameras, especially those mounted on the left side of MAXI, and reflect solar X-rays. The effect of the reflection is significant after the solar flare and appears belt-like background increasing in all-sky images.

In the geometry of GSC camera, there is a gap between two units of the GSC camera as shown in Figure 3.12. This brings up an unexpected background increasing during solar ﬂares. An X-ray photon entering to the slit of GSC unit with the particular incident angle along the scan direction travels above the slat collimators and in the gap between the units. Finally, the photon penetrates into the beryllium window of the next proportional counter without any interruption by the grid on the counter. Although this problem is not normally considered, when the ﬂaring Sun is close to the area where photons can penetrate into the next counter, bright source-like excesses come up on the GSC observational data. We call such excesses as “sun leak events”.

The excesses often trouble daily operation, health check of the counters and transient source searching system. Fake detections of the excesses can be ignored with check of the position of the Sun, since this problem only occurs when the ﬂaring Sun aligns with the possible path of the sun leak event. In addition, such excesses are only detected with the anode wire labeled as Chapter 3. Observatory 36 The Astrophysical Journal Supplement Series,207:36(12pp),2013August Hiroi et al.

5 realBGD 6 Soyuz undocked 4 Soyuz docked 3 5 2 4 Rate (cnts/s) 1 5 3 simBGD 4 2

3 BGD rate (cnts/s) 2 1 300 Rate (cnts/s) Rate 1 100 200 400 500 1.1 ratio (= realBGD/simBGD) VC count (cnts/s) 1 Figure 3. Correlation between the 4–10 keV background rate and the VC count Ratio 0.9 rate for GSC_4. The red squares (shown slightly shifted, for visual clarity) and solid curve represent the data points and the best-fit model of a third-order 55000 55200 55400 55600 55800 56000 56200 polynomial, respectively, when Soyuz was undocked, while the blue circles and MJD dotted curve show the data and the model when Soyuz was docked from 2009 December 22 to 2010 May 11 (MJD 55187–55327). Figure 2. Top: background rate in the 4–10 keV band observed with GSC_4 = Figure 3.11: Top:binned History in one-day of increments the actual (real background data). The plotted rate error of bars the indicate GSC 4 the counter 1σ in(A the color 4–10 version of this figure is available in the online journal.) keV. The plotteduncertainties.The data has 1-σ dot-dashederror bar lines in indicatea one-day the boundaries bin size. of The the phases. dot-dashed The lines indicate the dock/undockdocked timings/undocked of the status Soyuz of the spacecrafts.Soyuz spacecraft Middle: is responsible The for same the drastic plot is provided by changes in the daily background rate. Middle: same as the top panel but produced simulations to makefrom simulations. source catalog Bottom: in the Hiroi ratio of et the al. real [28 background]. Bottom: rate toThe the simulatedratio of the actualpart of data the field of view of each camera was occulted by the and simulation. one. The simulated background data are consistent with the real data with an space shuttle and the solar paddles of the ISS. rms scatter of 1%. ∼ We first investigate the long-term variability of the back- (A color version of this figure is available in the online journal.) C5 in Figure 3.5. Thus, we plot positions of the excesses which are detected withground large rate. X-ray The top panel of Figure 2 displays a one-day averaged background rate as a function of time, derived from events in the PSF byof onlythe map the correspond C5 anode to wire the in regions the plane with the of the longest detection exposure coordinatethe dataθ and in the 4–10 keV band taken with the GSC_4 counter. near the poles around the rotation axis of the ISS orbit. One can clearly see that the MAXI/GSC background rate has 1 1 scan coordinated φ. This is approximately represented with a quadratic functiontwo as separate shown in levels of 3–4 counts s− and 2–2.5 counts s− in this camera. We find that this long-term variability of the back- Figure 3.13. 3. ANALYSIS ground is strongly correlated with whether the Soyuz spacecraft 3.1. Background Reproduction was docked or undocked to the ISS.SinceSoyuz has an altime- An example of an all-sky map is shown in Figure 3.14. There are some blank regionster containing in the a radioactive source, the MAXI/GSC background For source detection, we performed an image analysis of rate is significantly increased when Soyuz is attached to the near- map. Circular regionsthe projected at the upper sky image left and from lower all the right whole side MAXI are/GSC an uncovered data, as regionest ISS alongport the to MAXI, and vice versa. Furthermore, it is found detailed in Section 3.2.Basically,wesearchedforexcesssignals ◦ that an increment of the background rate depends on the in- rotation axis of the ISS. Since the FoV along the detector coordinate is not 180 , GSC cannot over the profile of the background, which consists of the non- dividual Soyuz spacecraft docked at that time. The difference monitor regions nearX-ray the background axis. Some (NXB) GSC andcameras the cosmic have not X-ray been background operated, henceof high-voltage the blank level between 1650 V and 1550 V also affects (CXB). Thus, to achieve the best sensitivity, it is critical to the background rate. Thus, we divide the 37 month period into including the galacticmodel center the isbackground not covered image with with any the GSC fewest cameras. systematic To preventerrors thenine proportional different phases according to the presence of Soyuz and the that enable us to detect even the faintest sources as statistically high-voltage level of the GSC, and create the background model counter from unprocessiblesignificant high signals. count The rate NXB the high of the voltage GSC of shows GSC complex is shut downseparately in the curved for each phase. behaviors (Sugizaki et al. 2011). The rate of the NXB is highly belt-like region traversing longitudinally. This map includes effects of solar flares;Next, enhanced we examined the short-term variability of the NXB on variable, depending on the environment such as conditions of timescales less than one day. In the previous study (Hiroi et al. emission of solar X-rays,other spacecraft reflection attached from the to solar the ISS paddleand the and orbital a sun position leak event. and The2011 semicircular), we considered the dependence of the background rate on attitude of the ISS. the cut-off rigidity parameter (COR), which is determined solely orange-colored regionWe at thusthe lower constructed side of a the background figure is model a direct of MAXI effect/ ofGSC the solarby flares. the orbital The position of the ISS.ThecorrelationoftheNXB based on the detailed calibration of the on-board data as with the COR is known to be not perfect, however. Alternatively, source-like excess atdescribed the right below. sideHere, of the we region do not is distinguish a sun leak the event. NXB The from two orangewe have belts found at that the instantaneous background rate is well the CXB. We only treat their sum, since it is not easy to separate the upper side are reflection of solar X-rays from the solar paddles of the ISS. estimated from the so-called VC count, which represents the the two components from the observed data. This is mainly rate of events simultaneously detected by the carbon–anode due to the fact that the MAXI/GSC field of views always look cells and the tungsten–anode cells for particle veto, available at the sky and seldom look at the Earth. For this purpose, as a monitor count in the house keeping data of the GSC we analyze the on-board data that can be regarded as blank (Mihara et al. 2011;Sugizakietal.2011). The reason why sky by excluding regions around the 70 brightest sources at the VC works as a good monitor of the NXB is that high energy high Galactic latitudes listed in the seven month MAXI/GSC particles hitting the GSC counter should be detected by both 19 catalog (Hiroi et al. 2011)andtheGalactic-centerregionwith the inner (carbon) and outer (veto) cells, unlike X-ray events. l < 50◦ and b < 10◦.Thesamescreeningdescribedin Figure 3 plots examples of the correlation between the VC and | | | | Section 2 is adopted for event extraction. To avoid complexity the background rates in the 4–10 keV band for GSC_4 in two in the exposure correction, we discard any data taken when a different phases. The blue circles and red squares correspond to the data obtained when Soyuz was docked to the ISS port and undocked, respectively. The error bars represent the 1σ 19 We confirm that when our new 37 month catalog is used to exclude even fainter sources in this process, the resultant background profile in the detector standard deviation in the background rate including statistical coordinate is little (< 1%) changed. fluctuations. A tight correlation is confirmed in each phase. The

3 Chapter 3. Observatory 37

Solar X-ray

Slit

Slats

Beryllium window

Proportional counter

Figure 3.12: Geometry of a GSC camera pair and trajectory of the sun leak event photon. An incident X-ray to the slit of the right counter with an angle penetrates into the region between the cameras and enters into the beryllium window of the left proportional counter. Finally the photon is detected with the C5 anode wire of the left counter. The box at the center is a supporting structure of the slat collimators which is a triangular frame without any block of the incident photons.

20.5 20 19.5 19 18.5 18 (degree) θ 17.5 17 16.5 16 -40 -30 -20 -10 0 10 20 30 40 φ (degree)

Figure 3.13: Angular distribution of sun leak events in the φ-θ coordinate is plotted as crosses. The dashed line is a ﬁtted quadratic function. Chapter 3. Observatory 38

Figure 3.14: Example of all-sky map obtained with GSC in March 7 2011 17:30 – 23:30 UT, when M-class ﬂares were reported. X-ray events are plotted in the galactic coordinate (the origin corresponds to the galactic center and the equator line is the galactic plane) with pseudo colored; red in 2–4 keV, green in 4–10 keV and blue in 10–20 keV. Red crosses indicate positions of known X-ray sources.

3.3 SSC

Solid-State Camera (SSC) is another X-ray detector of MAXI which is sensitive to photons in the 0.5–12 keV energy band. It has two cameras; one (called as SSC-Z) is placed to monitor the zenith sky, the other (SSC-H) sees +20◦ above the horizontal direction. The layout of the SSC cameras are shown in Figure 3.15 (Tomida et al. [76]). Each camera has CCD units, preampliﬁers, multiplexers, a collimator and slit unit, and a calibration source. Information of systems and in-orbit performance were reported in Tomida et al. [76] and Tsunemi et al. [78]. In addition, recent scientiﬁc result with SSC were also published; e.g., Kimura et al. [35] and Morii et al. [52].

In a SSC camera, 16 CCDs which are front-illuminated p-type are aligned in 2 × 8 array. The design of CCD wafer and electronics of the SSC determine observable energy band as 0.5–12 keV. Although the surface of the CCD is coated with 0.2-µm aluminum, the edge of the CCD wafer is not coated. In orbit, the Sun light illuminates the CCD and imposes careful handling of scientiﬁc analysis to the MAXI science team.

Twenty-four slat collimators and a slit are located above the CCD unit as shown in Figure 3.16 (Tomida et al. [76]). They ﬂatten the ﬁeld of view as 1.5◦ × 90.0◦. Principle to determine positions of X-ray sources with SSC is similar to the case of GSC, alternating the proportional Chapter 3. Observatory 39

Figure 3.15: Exploded view of SSC consisting of two cameras. Each camera mounted on the aluminum stand has a CCD array, preampliﬁers, slats collimators and a slit. counter and the CCD array. With the fan-beam FoV, SSC covers 71% of entire sky in a single orbit of the ISS. Note that, however, the leak of the Sun light during provides scientiﬁcally unavailable data in the day-time. Chapter 3. Observatory 40

θ3

collimaotor sheets 2.7mm

θ1 slit 2.4mm

103mm

θ2 91.7mm

5.0mm CCD CCDs CCDs

208mm

Figure 3.16: Schematic view of a SSC camera consisting of a CCD array, stats collimators and a slit. Left: the geometry perpendicular to the rotation direction of the ISS. The scale ◦ ◦ is modiﬁed easy to see. θ3 is the width of the long FoV with 90 . θ2 = 1.5 is the angular resolution at the acquisition angle of 0.0◦. Right: side view of the camera parallel to the rotation ◦ direction. The width of narrow FoV is θ1 = 3 . Chapter 4

Analysis

We have developed a new procedure to detect short X-ray transients from the archived MAXI- GSC data. In this chapter, we describe the analysis method to search for short transients which are detected in one-orbit transit of the ISS every 92 minute. The method is consisted of three major steps (data selection, global search and localization) and some small procedures (clustering, ﬂux calculation and candidate selection).

4.1 Data selection and screening

We analyzed event data of the GSC which has been obtained in 1327 days from October 1 2009 to May 20 2013. We ignore data in the early operation from August to September 2009 which included difference of operation configuration and some troubles (e.g., high voltage breakdown) as mentioned in Section 3.2.4.3. Since some cameras (GSC 6, 9, A and B) had been damaged and have been turned off, we do not use the data provided by these cameras. Another camera GSC 3, which was damaged on March 26 2010 and has been operated without veto anti coincidence, is omitted from our analysis too. Thus, in our study to search for transients, we use the seven healthy GSCs (Zenith: GSC 4 and 5, Horizontal: GSC 0, 1, 2, 7, and 8).

The event data of GSC is processed with the most up-to-date software (version 1.4) and calibration data base (updated on 25 February 2013) which are provided by the MAXI science team. Each data ﬁle formatted with the FITS contains table of daily X-ray events detected with one of the GSC counters. The table in the ﬁle consists of columns of CAMERAID, MAXITIME, BEX and PI and others which are not used in our analysis. Here, CAMERAID is the ID number

41 Chapter 4. Analysis 42 of each GSC camera such as 0, 1,..., 9, 10 (camera A) and 11 (camera B). MAXITIME is the photon arrival time stamped by the clock of MAXI. BEX is the incident position of X-ray photon projected back on the beryllium window of the proportional counter assuming the energy dependent mean free path in the Xe gas. The energy channel of X-ray events are stored in the PI (Pulse Invariant) column. The value of PI is proportionally converted to the energy of X-ray photon in unit of keV.

First, we make ROOT formatted files from the FITS formatted event files with the conversion program provided by the MAXI science team. ROOT1 is a framework to handle and analyze enormous amount of scientific data collected by detectors associated with particle accelerators of CERN. It provides functions to make histograms and graphs, to fit these data with statistical processing and to store the detected event data structured like trees. Next, we extract X-ray events in the 4–10 keV energy band from the ROOT data. This energy band is chosen because the detection efficiency of the GSC proportional counter is almost 100% in the 4–10 keV band (Matsuoka et al. [47]). Finally, to perform global transient search described in the Section 4.2, we make two-dimensional histograms in the MAXITIME-BEX plane. The histogram is made for each event files with the MAXITIME bin size of 4.0 s and the BEX of 1.0 mm. An angular resolution corresponding to the bin is ∼ 0.3◦ × 0.3◦, which is small enough to determine the position of transient events in the MAXITIME-BEX coordinate with our analysis.

In order to avoid false source detection caused by leaking photons from bright persistent sources or the sun, and ﬂuctuations of the background, we created mask histograms for the following items.

1. Regions near both ends of the carbon anode wires of the counter (|BEX| ≥ 120 mm) 2. Regions covered by the solar paddle of the ISS 3. Galactic plane (galactic latitude |b| < 10.0◦) 4. Neighborhood of bright persistent sources (the radius of 6◦ around Sco X-1, and 4◦ around other bright sources)

Figure 4.1 is a result of the data screening with these criteria. We note some of these items in the following subsections.

1http://root.cern.ch Chapter 4. Analysis 43

5 100 4.5 4 50 BEX (mm) 3.5 0 3 2.5 -50 2 1.5 -100 1 25000 30000 MAXITIME - 324604000.0 (s)

100

50 BEX (mm)

-50

-100 25000 30000 MAXITIME - 324604000.0 (s)

Figure 4.1: An example of result of data screening. The data was provided with the GSC camera 0 on April 15 2010. Up: The two-dimensional histogram shows an X-ray event map in the MAXITIME-BEX coordinate. Events in blank regions are excluded with a mask as shown in the lower panel. Low: X-ray events in the blue colored region are available to our analysis. We do not use events in the yellow and red ﬁlled region which correspond to the galactic plane and persistent sources, and solar paddle of the ISS, respectively. In the white ﬁlled duration, GSC has not been operated. Chapter 4. Analysis 44

0.025 2-10 keV

0.02 -1

mm 0.015 -1

0.01 Counts s

0.005 CXB

0 -100 -50 0 50 100 DETX [mm]

Figure 4.2: The actual background distribution along the anode wire direction in the 2–10 keV energy band. The solid line shows an expected contribution of CXB. The dip at DETX = 0 mm is a shadow of the grid structure on the center of the short side of the proportional counter.

4.1.1 Near the ends of carbon wire

GSC has the anti-coincidence system using the veto anodes placed in parallel to the main anode wires in the sides and bottom to reduce particle events and γ-rays which contribute to non X- ray backgrounds (NXBs). The distribution of X-ray background consisting of NXB and cosmic X-ray background (CXB) along anode wire is shown in Figure 4.2 (Sugizaki et al. [74]). In |BEX| > 120 mm the background clearly increase. Because the both ends of the anodes are not guarded by the veto anodes (see Figure 3.5), and directly detect events due to the NXB. In consequence, events of the NXB may be accidentally detected as a transient candidate. Thus, we ignore events associated with the both ends of the anodes, |BEX| > 120 mm.

4.1.2 Exclusion of region covered by solar paddles

Main energy source of the ISS is the photovoltaic arrays, which referred as the Solar Array Wings. The arrays attached to the truss structures of the ISS rotate around an axis along the paddle and another axis along the truss to keep an appropriate angle toward the Sun. According to the geometry, part of the GSC FoV is covered by the arrays for certain periods in the orbit. Since the covered region is not useful for scientiﬁc analysis, we ignore the regions by masking event maps in the MAXITIME-BEX coordinate. The regions covered by the paddles are calculated from the auxiliary data of the ISS. This data includes rotation angles along the Chapter 4. Analysis 45 paddle and the truss with every second. We assume that the paddle is face-on to a GSC camera to conservatively exclude the covered regions. The covered areas of the GSCs mounted on the left side and the center of MAXI are larger than that for the cameras mounted on the right side.

4.1.3 Exclusion of the Galactic Plane

Since the PSF of GSC is not suﬃciently small to identify X-ray sources around the galactic center and the plane, we ignore these regions in our analysis. We exclude the galactic plane region with the galactic latitude b less than +10◦ and larger than −10◦, which includes almost all of the galactic center region. In the MAXITIME-BEX two-dimensional histogram, sometimes, the part of galactic plane occupies large area as shown in Figure 4.1. The percentage of area excluded with this criterion is about 17% of the whole sky.

4.1.4 Exclusion of region around bright sources

Bright, persistent and known X-ray sources may cause erroneous detection of transients in their neighbourhood with the method used in our analysis. To avoid such false detections, we exclude X-ray events associated with the bright X-ray sources. The list of these sources is shown in Table 4.1. They are chosen from the MAXI/GSC 37-month source catalog (Hiroi et al. [28]) with the ﬂux of > 30 mCrab. The threshold of the ﬂux is less than the detection limit of GSC (& 50 mCrab) during one-orbit scan transit. Considering the PSF of GSC, we chose a radius of excluded region as 4◦ in the sky coordinate for each source. Since photons of Sco X-1, the brightest X-ray source in the MAXI-GSC energy band, leak from 4◦ excluding region, the radius must be larger, 6◦, to reduce false detections around the source.

4.2 Global search

4.2.1 Method of global search

We search candidates of transient sources estimating probability of an assumption that number of X-rays detected in a test region is consistent with the number of the expected background counts. First, as shown in Figure 4.3, a foreground region and two background regions are placed on the MAXITIME-BEX histogram. The widths along the MAXITIEME coordinate Chapter 4. Analysis 46

Table 4.1: List of bright sources Name R.A. Dec. ﬂux∗ SMC X-1 19.27 -73.44 26.3 Perseus Cluster 49.95 +41.51 52.7 X Per 58.85 +31.05 46.9 LMC X-2 80.12 -71.96 23.0 LMC X-3 84.73 -64.08 14.1 LMC X-1 84.91 -69.74 13.7 MAXI J0556-331 89.20 -33.20 11.6 Mrk 421 166.11 +38.21 15.1 NGC 4151 182.64 +39.41 10.1 Virgo Cluster 187.71 +12.39 16.2 Coma Cluster 194.95 +27.98 21.0 Cen A 201.37 -43.02 24.8 Sco X-1 244.98 -15.64 14780.1 4U 1626-67 248.07 -67.46 21.4 Her X-1 254.46 +35.34 26.3 Swift J1753.5-0127 268.37 -1.45 41.0 4U 1822-371 276.44 -37.11 33.0 HETE J1900.1-2455 285.04 -24.92 26.6 4U 2127+119 322.49 +12.17 13.8 Cyg X-2 326.17 +38.32 541.3 ∗ unit of mCrab in 4–10 keV in Hiroi et al. [28] of the foreground and background regions are 0.75 and 1.0 times of the scan-transit time, respectively. The gap between the regions has the width of 0.25 times of the scan-transit time. The width along the BEX for each region is ﬁxed as 6 mm, which is ∼ 80% of the width of the PSF. An example of the global search corresponding to a known GRB (GRB 100510A) is shown in Figure 4.3.

Second, we attempt null hypothesis that number of X-ray events in a test region has a Poisson distribution whose mean is the number of background events. The mean, Nb, is estimated with the following equation;

N1 + N2 Nb = × Af . (4.1) A1 + A2

Here, N1 and N2 are numbers of events in the left-side and right-side background regions, A1 and A2 are areas of the regions, respectively. In addition, we deﬁne Nf as the number of events in the foreground region with the area of Af . The areas of foreground and background regions are calculated from the mask to consider the partial lack due to data screening criteria as mentioned in Section 4.1. Chapter 4. Analysis 47

30 5

BEX (mm) 4 25 3 20 2

15 1 0 69600 69800 70000 MAXITIME - 326765000.0 (s)

Figure 4.3: An example of global search performed data obtained on 10 May 2010. Boxes on the histogram which represent an event map are foreground (solid) and background (dashed) regions. The horizontal axis is the MAXITIME coordinate with the shift to clearly see. There is no X-ray event in the right side blank region.

Third, we calculate a probability to establish the null hypothesis: number of X-ray events has a Poisson distribution with the mean of Nb. The probability Pnull is estimated from the upper tail of the cumulative Poisson-distribution function with the number of foreground events Nf as, ∫ +∞ − N xe Nb P = b dx null x! Nf Γ(N + 1,N ) = 1 − f b . (4.2) Nf !

Here, the value x is a dummy variable representing number of foreground events. In Equation 4.2 which is called as “survival function”, the second term is the cumulative distribution function of the Poisson distribution represented with an incomplete gamma function Γ(x, y). For example, the test case shown in Figure 4.3 has Nb = 7.0 and Nf = 94, thus the null hypothesis probability −87.9 Pnull is very small, 10 . If the probability is small enough to reject the null hypothesis, we allege that the events in the foreground region is signiﬁcantly detected as a transient source with this global search process.

Hereafter, we name a series of the processes as trial. Subsequently, we iterate the next trial with Chapter 4. Analysis 48

104 1000 GSC 4 100 10 1 104 104 1000 GSC 0 1000 GSC 5 100 100 10 10 1 1 104 104 1000 GSC 1 1000 GSC 7 100 100 10 10 1 Number of events 1 104 104 1000 Number of events GSC 2 1000 GSC 8 100 100 10 10 1 1 −20 −15 −10 −5 0 −20 −15 −10 −5 0 Log null hypothesis probability Log null hypothesis probability

Figure 4.4: An example of the distribution of null hypothesis probability. These results are obtained from data on 15 April 2010. A signiﬁcant detection with log Pnull ∼ −17.5 in the “GSC 4” panle is GRB100415A. new foreground and background regions whose positions are moved from that of the previous trial by the half width of the previous foreground region along the MAXITIME and BEX coordinates.

4.2.2 Determine threshold

In the global search we perform about 3.7×108 trials for 1327-days 7-cameras data. Histograms of Pnull obtained from the data on 15 April 2010 are shown in Figure 4.4 as one result of the global search. The histograms have a power-law like distribution in the higher probability region −6 −7 and reach at zero at Pnull ∼ 10 –10 . In data obtained on other days similar distributions are confirmed. A possible origin of the distribution is a fluctuation of Poisson-distributed background. According to the distribution, we decided that the threshold of null hypothesis probability of candidates is 10−5, so as to tolerate trials with less significance. The trials with −5 the probability of Pnull < 10 are applied to the detailed analysis. The global search with this threshold will trigger ∼ 1 false detection for each one-day and one-camera data which includes a several 105 trials. Chapter 4. Analysis 49

4.3 Clustering of candidates

In the global search, bright transients may be redundantly detected with signiﬁcant null hypothesis probabilities since a test region covers other neighboring regions. These redundancies must be ignored as much as possible to save calculation time in the following analysis steps. In the MAXITIME-BEX coordinate, we assume that a group of trials in a rectangular region associates with a possible transient. To prevent from incorrect grouping, we chose the width of the region as 70% of the scan-transit time and the width of the PSF along the MAXITIME and BEX coordinates. Then, we determine a new position associating with the possible candidate as the position of the trial which has minimum null hypothesis probability in the group. After −5 the global search, the trials with Pnull < 10 were applies grouping process and were reduced into 306081 trials.

4.4 Detailed analysis

4.4.1 Method of localization

Since positions of the trials are roughly determined in the global search process, we perform precise position determination with the following steps. We perform similar calculation of null hypothesis probability as the global search with a picture-frame-like background region around the roughly determined position. The shape of the background region and foreground region are shown in Figure 4.5. Here, the background region has the inner and outer width as 1.25 and 3.25 times of the PSF (the scan-transit time along the MAXITIME coordinate and the width of the gaussian-like PSF along the BEX coordinate), respectively. Contrary to the method of the global search, this background region considers the fluctuation of background events not only along the MAXITIME coordinate but also the BEX one. We apply the width of the foreground region as 0.75 times the PSF. With these regions the null hypothesis probability under assumption of Poisson-distributed background is re-calculated by same equation of the global search. To search the precise position associated with the trial, we iterate this re-calculation around the roughly determined position. The region of the iteration is 5.25 times larger than the PSF in each coordinate. Finally, we determine a precise position which has minimum null hypothesis probability in the region of the iteration. With the refined position, we evaluate foreground and background fluxes in Section 4.5. A relation between the estimated null hypothesis probability Chapter 4. Analysis 50 and the count rates of a source and background is shown in Figure 4.6. On the right lower corner, where the source count rate is brighter and the background is fainter, the probability −100 Pnull is smallest < 10 . This plot clearly supports the validity for the localization process.

4.4.2 Detection threshold

A histogram of null hypothesis probability obtained by the localization process for all trials is shown in Figure 4.7. Contrary to the global search, the histogram has two components; the power-law like distribution in the high probability range, and another component in −12 Pnull . 10 . While the former is interpreted as the accidental detections due to ﬂuctuation of the Poisson-distributed X-ray background, the latter must be contribution of real X-ray transients. Considering completeness of the contribution, we determined the threshold of signiﬁcant −9 detection by the localization as Pnull < 10 . Based on the threshold and assumption of the Poisson-distributed X-ray background, number of false detection should be ∼ 1 in all analyzed data, which include 105 trials/camera/day cameras days ∼ 109 trials.

4.4.3 Estimation of position error

In this section we estimate an error in position determined by the aforementioned localization process. We performed the same processes of transient search to three known brighter and persistent X-ray sources; Cyg X-2, Perseus Cluster and X Per. Since the X-ray events associated with these sources are excluded by the data selection as mentioned in Section 4.1.4, we especially make MAXITIME-BEX two-dimensional histograms which include their X-ray events. Then we apply the same processes (global search, grouping and position determination) to the special histograms and obtained plausible positions as summarized in Figure 4.8. For brighter source Cyg X-2, the error with 90% conﬁdence level is ∼ 0.5◦. On the other hand, the errors of the fainter sources (Perseus Cluster and X Per) are ∼ 1.0◦. We adopt a typical error of the position determination as 1.0◦ conservatively through this paper. Chapter 4. Analysis 51

40 4 3.5 30 3 BEX (mm) 2.5 2 20 1.5 1 10 0.5 0 69700 69800 69900 MAXITIME - 326765000.0 (s)

40 0 -10 30 -20 BEX (mm) -30 -40 20 -50 -60 10 -70 -80 69700 69800 69900 MAXITIME - 326765000.0 (s)

Figure 4.5: Upper: An event map in the MAXITIME-BEX coordinate and boxes to estimate number of foreground and background events. The events in the solid box and dashed frame are estimated as the foreground and background events, respectively. Lower: The distribution of null hypothesis probability. To facilitate visibility, the color of the box and frame in the upper panel are changed from blue to red. The black dash-dot box is a region where the program performed the iteration of position determination. Chapter 4. Analysis 52

) 0.2 0 -1

-20 mm -1 0.15 -40

-60 0.1 -80

-100 0.05

-120

Background count rate (count s 0 0 0.1 0.2 0.3 0.4 Source count rate (count s-1 mm-1)

Figure 4.6: Contour plot of null hypothesis probability estimated in the localization process. The horizontal axis represents source count rate which is a diﬀerence of those in the foreground region and background region. The vertical axis is count rate in the background region.

103

102 102

10 10 Number of Events Number of Events

1 1 -150 -100 -50 0 -40 -30 -20 -10 0

Log Pnull of localization Log Pnull of localization

Figure 4.7: Histogram of null hypothesis probability Pnull calculated in the localization. The right panel is an enlarged view around −100 < log Pnull < 0. Chapter 4. Analysis 53

40 20 1 1 18 16 0.8 0.8 39 14 12 0.6 0.6 10 38 8 0.4 0.4 6 Declination (degree)

4 Cumulative frequency 37 Cumulative frequency 0.2 0.2 2 0 0 325 326 327 328 0 37 38 39 40 Right ascension (degree) 325 326 327 328 Right ascension (degree) Declination (degree)

4 1 1 43 3.5 3 0.8 0.8 42 2.5 0.6 0.6 2 41 1.5 0.4 0.4 1 Declination (degree) Cumulative frequency Cumulative frequency 0.2 40 0.5 0.2 0 0 48 49 50 51 0 40 41 42 43 Right ascension (degree) 48 49 50 51 Right ascension (degree) Declination (degree)

33 4 1 1 3.5 32 3 0.8 0.8 2.5 0.6 0.6 31 2 1.5 0.4 0.4 30 1 Declination (degree) Cumulative frequency Cumulative frequency 0.2 0.5 0.2 0 0 57 58 59 60 0 30 31 32 33 Right ascension (degree) 57 58 59 60 Right ascension (degree) Declination (degree)

Figure 4.8: Result of error estimation of position determination with Cyg X-2 (upper panel), Perseus Cluster (middle) and X Per (lower). Right: A distribution of the positions determined from 46-days 7-cameras data set. Center and Left: Cumulative histograms of the distribution along R.A. and Dec. The three vertical lines from left to right deﬁne cumulative frequency of 5%, 50% and 95%, respectively.

4.5 Calculation of X-ray ﬂux

After the position determination we calculate X-ray flux of transient candidates. The energy bands of the flux are 2–20, 2–8, 8–20 keV and 4–10 keV. The source flux FX is represented as

N − N F = fore back . (4.3) X AT Chapter 4. Analysis 54

) 1.5 ) -1 -1

s s 0.3 -2 -2

1 0.2

0.5 0.1 Flux 4-10 keV (count cm Flux 4-10 keV (count cm

0 0 -150 -100 -50 0 -20 -15 -10 -5 log null hypothesis probability log null hypothesis probability

Figure 4.9: Scatter plot of null hypothesis probability Pnull calculated in the localization and source ﬂux calculated with Equation 4.3. The right panel is an enlarged view around −20 < log Pnull < 0. The plots do not include trials which corresponds to possible sun leak event

Here, Nfore and Nback are number of X-ray events in the foreground and background regions which are located by the aforementioned method of the localization analysis in the MAXITIME- BEX coordinate. An effective area, A, associated with a position of a candidate is obtained from functions in MAXITOOL and MAXIUTIL, which are libraries to hundle MAXI’s observational data probided by the MAXI science team. A scan-transit time T is calculated from a rotation angular velocity of the ISS and an angle corresponding to the width of the foreground region in the celestial coordinate. The correlation between the source flux and the null hypothesis probability which is calculated in the localization process is shown in Figure 4.9. The figure provides that the flux is proportional to the probability and the minimum detected flux is ∼ 0.06 count cm−2 s−1.

As a result of the calculation, we obtain the flux in unit of count cm−2 s−1. A conversion from the unit of “count” to that of “photon” is generally cumbersome because the energy response of GSC has to be considered. However, the flux with the unit of “count” in the 4–10 keV band may be directly assumed to be that in the “photon” unit without much error since the detection efficiency of GSC is almost 100% in this energy band (Matsuoka et al. [47]). Chapter 4. Analysis 55

4.6 Check candidates

Although we checked for clustering of extracted trials, multiple detections may remain in the localization process. In order to omit such duplicated detections, each candidate with the reﬁned position is checked for whether it is multiply detected with the tolerance of 2◦ and 1000 sec. This time tolerance is chosen to extract multiple detections within a orbital period of the ISS (∼ 92 min).

In order to avoid false detections we check the detected candidates for the following criteria.

1. Association with “sun leak” events 2. Background-enhanced region due to solar X-ray 3. Lack of eﬀective area in the localization process 4. In |BEX| ≤ 20mm and ≥ 100mm 5. Association with outbursts of a known X-ray source

4.6.1 Candidate associated with the solar X-ray leak

As describe in Section 3.2.4.4, due to illegal path to penetrate solar X-rays into the GSC proportional counter, the “sun leak events” were reported at the times of large solar ﬂares. Since the geometry of the GSC camera deﬁnes the path, we can predict the unique loci of these events in the scan and detector coordinates with the quadratic function shown in Figure 3.12. To ignore such sun leak events from the detected transient candidates, we calculate a scan angle

θ and a detector angle φ of each candidate. Next, we check an angle of θleak which is calculated from the function with the φ value. If the θ of the candidate is included within a tolerance of ◦ the θleak, 1.5 , we exclude the candidate.

4.6.2 Exclusion of events in background-enhanced region due to solar X-ray

When the solar ﬂare occurs, X-ray background of GSC temporally increases. Figure 4.10 shows the distribution of background count rate which is obtained in the global search process. The distribution has a cluster in the lower count rate and the others out of the region. These outliers will be associated with the count excesses caused by solar ﬂares. Thus we ignore such trials with high background count rate to prevent false detections caused by temporal increase of background. Chapter 4. Analysis 56

) 0.5 -1 mm

-1 0.4

0.3

0.2

0.1

Background 2 count rate (count s 0 0.1 0.2 0.3 0.4 0.5 Background 1 count rate (count s-1 mm-1)

Figure 4.10: Scatter plot of count rate in the background regions. Background 1 and 2 correspond to before and after the foreground region. The unit of count s−1 mm−1 means number of event counts in a second and a mm in the BEX coordinate. The plot does not −9 include trials which corresponds to possible sun leak event and have Pnull > 10 .

4.6.3 Exclusion of events with lack of eﬀective area in the localization process

In the data selection (Section 4.1) we ignore events near the galactic plane and the several persistent X-ray sources. As the result, trials of the global search and the localization often have partial lack of the effective area. Because number of background X-ray counts in a test region is several or about ten at most, the lack will affect to estimation of null hypothesis probability. Thus, we accept only trials with no or small lack of effective area of foreground and background. The threshold of acceptance is decided as 10% of the foreground and background area whose widths along the MAXITIME and BEX coordinates are determined from the PSF of GSC. In other wards, the following study ignores the trial with the < 90% area of the testing region. Chapter 4. Analysis 57

102

10 Number of Events

-100 0 100 BEX (mm)

Figure 4.11: Distribution of detected candidates in the BEX coordinate. The plot does not −9 include trials which corresponds to possible sun leak event and have Pnull > 10 .

4.6.4 Exclusion of events in the center and both sides of BEX

Geometry of the GSC proportional counter makes the X-ray sources in centain BEX coordinate ranges to be undetectable. The ﬁrst one is dead region under the shadow of the grid structure which supports the window of the proportional counter perpendicular to the anode wires. In the BEX coordinate the region corresponds to |BEX| < 20 mm (see also Figure 4.2). We ignore trials located in the region from the detected candidates. Another undetectable region is the both side of the anode wires. Although we ignored X-ray event in |BEX| > 120 mm in the ﬁrst data screening process, a large number of detections are clearly seen in Figure 4.11. This may be originated from additional background due to the charged particles and γ-ray photons mounted on the Soyuz. In order to obtain more plausible candidates we additionally neglect these trials in the region of |BEX| > 100 mm. Chapter 4. Analysis 58

4.6.5 Exclusion of candidates associated with outbursts of a known X-ray source

The detected candidates include an X-ray source which is not a short transient event, MAXI J1659−152. MAXI J1659−152 is a “transient” which has continuously emitted bright X-rays in 2010 (Kalamkar et al. [32]). Since this is located at the galactic latitude b = +16.5◦ and has the relatively fainter averaged ﬂux of 8.4 × 10−11 erg cm−2 s−1 ∼ 7mCrab (Hiroi et al. [28]), we do not screen out X-ray events near the source in the data selection process (Section 4.1). However, the peak ﬂux of the source was very high (∼ 1 Crab) during the outburst. Thus the trials associated with MAXI J1659−152 are detected by the procedure of our analysis. Since such trials are not short transients which are targets of our survey, we additionally ignore the trials which are detected near MAXI J1659−152 within 3◦.

4.6.6 Manually exclusion of obviously false detection

Checking event maps and light curves of all detections, a candidate in the data obtained on February 21 2011 emerges as a suspicious detection. The event map of the candidate shows an extended distribution of X-ray events which is apparently larger than width of the PFS of GSC as shown in Figure 4.12. Since the candidate was detected on the edge of the GSC camera, incresing of charged particles is a sutable origin to this extended emissions. The candidate passes all criteria of exclusion as itemized in this section, however, we assume it as a false detection and will ignore it from the following analysis.

4.6.7 Eﬀect of the exclusions to our analysis

We performed the data selections in the global search and exclusions to prevent from false detections as described in previous sections. Here, we evaluate the eﬀect of these treatments to our analysis. Figure 4.13 shows a distribution of number of bins in the analyzed MAXITIME- BEX histograms. The “original” data with no screening and exclusion typically have 8 × 106 bins in the histograms. After the data selection as mentioned in Section 4.1 and the exclusions described in this section, we obtain “cleaned” and “available” event data which consists of 75% and 25% of the “original” data. Two of the most eﬀective exclusion from the “original” to “available” are masking of the events detected at BEX of |BEX| < 100 mm and > 20 mm, and neglecting of trials which have small covering area due to the lack of exposure (see Section 4.6.3 Chapter 4. Analysis 59

2.0

-80 1.5 -90

1.0 -100 BEX (mm)

-110 0.5

-120 0.0 351598000 351598200 351598400 351598600 MAXITIME (sec)

Figure 4.12: Event map of a candidate of short X-ray transient detected on February 21 2011. The black dashed box and blue dashed frame are foreground and background regions which are used to estimate ﬂux and Pnull in the localization process. The excess emission is distributed in the center of the map and extended from the black dashed region whose width along the BEX coordinate is 0.75 time of the PSF of GSC.

100

50 50 Number of days Number of days

× 3 × 3 0 10 0 10 0 2000 4000 6000 8000 10000 0 2000 4000 6000 8000 10000 Number of data in one day Number of data in one day

Figure 4.13: The black, red and blue histograms demonstrate distribution of available event in the “original”, “cleaned” and “available” data, respectively. The plots in the left and right panels are obtained with the GSC 4 and 5 cameras. The horizontal axis means number of available bins of the MAXITIME-BEX histogram which includes X-ray events obtained in one day. The vertical axis is number of the event data. and Section 4.6.4). The former criterion reduces one-third of the X-ray counts of the “original” data. The later one neglects a number of trials around the lack of the exposure due to the shut on/oﬀ of the HV, screening of the Galactic plane and neighbors of the bright sources. In addition, the neglecting will aﬀect detections of GRBs, which will be described in Section 5.1.1.2. Chapter 5

Discussion

As a result of our analysis, we obtain 146 short X-ray transients (SXTs) with the null hypothesis −9 probability of Pnull < 10 . Here, we perform cross-matching of the candidates with selected catalogs and discuss the statistical features and the origins of the X-ray sources.

5.1 Catalog Matching

We perform source identiﬁcation with catalogs of GRBs and non-GRBs. The latter contains persistent X-ray and gamma-ray sources that possibly produce short X-ray transient (SXT) events. The estimated accuracy of our transient search reported in Section 4.4.3 is 2◦ for events near the detection threshold. Since it is much larger than the errors in the non-GRB source catalogs, which are sub-degree at a maximum, we accept the 2◦ tolerance in the cross matching. On the other hand, the positional error in the Fermi-GBM catalog is larger than that of our SXTs, hence we chose larger tolerance for it. The GRB and non-GRB catalogs used in this cross matching are summarized in Table 5.1 and Table 5.2, respectively.

All of the results are listed in Table C.1. The detected time, position (Right Ascension and

Declination) and null hypothesis probability Pnull are determined by the localization method described in the previous chapter. The ﬂux in the 4–10 keV is estimated by the method described in Section 4.5. If a candidate is associated with a GRB, name of the GRB, time lag (the plus or minus signs mean whether the SXT was detected after or before the GRB) and angular distance between them are shown in the counterpart entry. In the following subsections we will describe the characteristics of the associated sources and possible origins of the detected SXTs.

60 Chapter 5. Discussion 61

5.1.1 SXTs associated with GRB sources

5.1.1.1 GRB source Catalogs

We check associations of the detected SXTs in this method with the known GRBs that triggered MAXI-GSC, Swift-BAT, and Fermi-GBM based on both positional and temporal coincidence. The compared catalogs are listed in Table 5.1. The MAXI-GSC GRB catalog (Serino [68]) includes 35 GRBs, of 2 GRBs were discovered in August and September 2009 when are not included in our present analysis. They reported that only one-third of them were confirmed with other satellites and the spectral characteristic is similar to that of X-ray flashes (XRFs). The GRB-SXT association is also checked with GRBs detected with other X-ray and γ-ray satellites, namely, Swift and Fermi. For Swift GRBs we use the second Swift-BAT catalog (Sakamoto et al. [66]), which contains only GRBs detected in 2004–2009. We therefore obtained a catalog of additional GRBs detected since 2010 from the Swift scientific team (Sakamoto 2014, private communication). The Fermi GBM Burst catalog (Paciesas et al. [56]) contains the GRBs detected by the Gamma-ray Burst Monitor (GBM) of theFermi γ-ray Space Telescope, which has been monitoring GRBs since 2008 with in the 8 keV – 40 MeV energy band.

Since the positional accuracy of the Swift is an order of arcmin, signiﬁcantly smaller than that of MAXI-GSC in our analysis, we adopted a position tolerance of source matching for them as 2◦, our positional accuracy. On the other hand, we adopted the tolerance for Fermi-GBM events as 5◦, their typical accuracy (Ukwatta et al. [79]). The tolerance of the temporal coincidence is chosen to be one day in order to include not only the prompt emission, but also possible precursors and afterglows.

5.1.1.2 Result of cross matching

The cross matching of the SXTs in the present analysis with the previously reported GRBs are listed in Table C.1. Here, we summarize associations of the SXTs with the previously reported GRBs with MAXI-GSC in Table A.2. Both of them are based on the same observations, but the detection algorithm is diﬀerent. Our transient search analysis successfully detects 11 of the 33 GRBs which have been reported since October 2009 until April 2013. However the remaining 22 GRBs are not conﬁrmed in our analysis. Chapter 5. Discussion 62

One reason is simple; some of these occurred in the temporal or detector region filtered out in our analysis. In the global search process, we set background windows excluding time intervals near the shut down or turn on of the GSC HV (high voltage). Thus, this configuration neglects SXTs that occurred near these times. For example, GRB 091120, one of the brightest GRBs detected with MAXI-GSC, occurred 20 sec before the shut off of the HV to prevent from exposure to the Sun and is not detected in our transient search procedure. Another problem is the exclusion of the transient candidates based on several parameters as described in Section 4.6. These criteria resulted in ignoring nen GRBs due to the position near the detector end, lack of effective exposure, or a high background rate.

Other missing detections are not simple to explain. According to the result shown in Figure 4.9, we roughly obtained the effective detection limit with our analysis as 0.1 counts cm−2 s−1 ∼ 100 mCrab. The GRBs which are not detected by our method has the flux of a several times 10−9 erg cm−2 s−1. The flux is comparable with the estimated detection limit. We note that the limit depends on fluctuation of non-X-ray backgrounds (NXBs). In the environment of MAXI, the NXB temporally varies and affects the detection limit. The uniform choice of background used in the present analysis may not be optimal in these events.

The cross matching of the SXTs with the GRB catalogs suggests one suspicious and another confirmed detections of GRB afterglows. The suspicious one is SXT 100907 detected about 1500 sec after the trigger of GRB 100907 by Fermi-GBM with angular distance of 14◦. The temporal and positional coincidence possibly indicated the association, however the detected position is near one of YSOs, YZ CMi. From this YSO, an X-ray flare SXT 110308 has already been detected and reported as Serino et al. [69]. Thus, a flare of the YSO is more likely origin of the SXT rather than a GRB afterglow. On the other hand, SXT 121027 is temporally (∼ 2400 sec after the GRB trigger) and positionally (within 0.31◦) consistent with GRB 121027A. The observation of the Swift-XRT revealed that this GRB exhibited a special light curve including a bump of X-ray flux from 10−11 erg cm−2 s−1 to 10−8 erg cm−2 s−1 in the duration 103–104 sec after the GRB trigger (Wu et al. [83]). It is the only SXT in our analysis that is clearly identified with a GRB afterglow. Chapter 5. Discussion 63

Table 5.1: GRB catalogs used for source identiﬁcation ∗ Name Objects Nass Reference MAXI-GSC GRB 33 11 [68] Swift-BAT GRB 15+298 2 [66]† Fermi-GBM burst 819 2 [56] ∗ Number of GRBs detected in the term of our study. † private communication.

5.1.2 SXTs associated with non-GRB sources

5.1.2.1 Non-GRB source Catalogs

The detected SXTs are tested for matching with some X-ray/γ-ray source catalogs; MAXI- GSC 37-month catalog, Swift-BAT source catalogs, Fermi-LAT 2nd catalog, and RXTE slew survey catalog. The MAXI-GSC source catalog (Hiroi et al. [28]) includes high Galactic-latitude (|b| > 10◦) 500 X-ray sources detected with GSC in the first 37 month. The dominant categories of the identified sources in the catalog are galaxy clusters and Seyfert galaxies. In addition, active stars and X-ray binaries including newly discovered ones are also listed. The Swift-BAT 70-month catalog (Baumgartner et al. [4]) and the Swift-BAT Hard X-ray Transient Monitor catalog1 (Krimm et al. [41]) list a large number of hard X-ray sources including those that possibly show X-ray flares and outbursts (i.e., AGNs, X-ray binaries and flare stars); the former includes sources detected in the first 70 months of the data between 2004 December and 2010 September, and the later frequently updated by the Swift science team when a new active source is reported.

Fermi-LAT catalog (Nolan et al. [54]) includes γ-ray sources detected in the first 24 month period of the observation by the Large Area Telescope (LAT) in the 100 MeV to 100 GeV energy range. The most dominant contributors are blazars which occupy more than half of the cataloged sources. In addition, minor groups are pulsars, supernova remnants and other various type sources. While significant fraction of the sources in the catalogue are not identified with known sources, many of them are likely to be pulsars or blazars that are faint in other energy bands.

RXTE slew survey catalog (Revnivtsev et al. [61]) provided 294 source from six-year long observation of the RXTE satellite in the 3–20 keV band. The sources are associated with

1http://swift.gsfc.nasa.gov/results/transients/ Chapter 5. Discussion 64

Table 5.2: Source catalogs used for source identiﬁcation ∗ Name Objects Nass Nramdom Reference Swift-BAT 70 month 887 (1210) 30 33.3 ± 2.0 [4] Swift-BAT Monitor 571 (982) 34 20.7 ± 1.5 [41] CAB 347 (409) 19 13.0 ± 1.4 [13] MAXI-GSC 37 month 500 (500) 35 24.4 ± 1.6 [28] Fermi-LAT 1366 (1873) 50 48.4 ± 1.9 [54] Globular Cluster 157 (157) 8 2.5 ± 0.6 [25] RXTE slew 321 (321) 28 12.3 ± 1.2 [61] VERON 1364 (1374) 49 33.9 ± 1.8 [81] SACY 1645 (1791) 41 41.8 ± 2.4 [77] NYMG 73 (73) 6 2.4 ± 0.6 [44] ∗ Number of sources outside of galactic plane region. Value in parentheses is total number of cataloged sources including galactic plane.

AGNs, galaxy clusters and known Galactic X-ray binaries. The position accuracy of the slew survey, less than 1◦, is comparable to that of GSC in our analysis.

In addition to these general X-ray/γ-ray source catalogs, we should also test catalogs for classes of possible SXT sources; AGNs, active stars and globular clusters. The VERON catalog (Véron- Cetty & Véron[81]) contains numerous quasars and blazars. Since the positional errors of the SXTs in our analysis is too large for reliable cross matching with the quasars, we only used 1374 BL Lac sources in this catalog in our discussion. Active stars are potential sources of X-ray flares, for which we consulted three catalogs: Chromospherically Active Binary catalog (CAB; Eker et al. [13]), Search for associations containing young stars catalog (SACY; Torres et al. [77]), and Nearest Young Moving Groups catalog (NYMG; López-Santiago et al. [44]). The CAB catalog includes binary stars with a strong chromosphere and coronal activity. The SACY catalog is based on the optical spectroscopy survey to search for young stars among the ROSAT all-sky survey X-ray sources. The NYMG catalog lists possible members of YSOs close to the Sun. Note that the positional distribution of source in the SACY catalog is dense in the southern hemisphere, especially in regions around the Large Magellanic Cloud. We check possible associations of the SXTs with globular clusters, which often host X-ray binaries, using the 2010 edition of milky way globular clusters catalog (Harris [25] with updates on the web site2). This catalog contains well-known 157 globular clusters in our galaxy.

2http://physwww.mcmaster.ca/∼harris/Databases.html Chapter 5. Discussion 65

5.1.2.2 Results of the cross matching

The result of the cross matching with non-GRB source catalogs is shown in Table C.1. Before discussing each SXT and its origin, we check contribution of false associations with the sources in the catalogs. For each catalog, we estimated number of accidental associations, Nrandom, with Monte Carlo method. In the method, 1000 test sources are randomly produced in the all sky, and then tested for positional matching with the sources in the catalogs with 2◦ positional tolerance. We performed one hundred trials with the test sources and obtained averages and standard deviations of accidental associations as the raw of Nrandom in the Table C.1. We ﬁnd that the number of accidental associations is comparable to that of the detected counterparts in most catalogs owing to the large positional error. On the other hand, the matching results of the Swift-BAT monitor catalog, MAXI-GSC catalog and RXTE slew catalog seem to show excesses of Nass over the accidental associations.

On the basis of these possible non-accidental associations, we discuss the nature of SXTs associated with some categories of possible SXT progenitors. As mentioned in Section 2.1.3, SXTs associated with the globular clusters are likely be X-ray bursts from neutron stars in LMXBs. Four SXTs (SXT 091105, 100813, 110110 and 120805) are associated with LMXB 4U 0513−40 located in the globular cluster NGC 1851. They have the ﬂux of 0.1–0.5 counts s−1 ∼ a few times 10−9 erg cm−2 s−1 and the hardness ratio of 0.2–0.4. Such detections are consistent with the large amplitude variability of the source in ∼ 10−11–10−10 erg cm−2 s−1 and the maximum of 10−9 erg cm−2 s−1 (Fiocchi et al. [15], Maccarone et al. [45]). The SXT 120804 is associated with MAXI J1647−227 which is discovered with MAXI-GSC (Negoro et al. [53]) in June 2012 and identiﬁed as a binary with a neutron star because of the detection of the type I X-ray burst Kennea et al. [34]. The detected SXT is most likely another X-ray burst from MAXI J1647−227.

X-ray flares of RS CVn and dMe stars are also major constituents of the detected SXTs. SXT 100123, 111229s, 120310s and 130226 can be identified as such flares from a RS CVn, HR 1099. Some of them are successively triggered in more than two consecutive scan transits of GSC. The dMe star, AT Mic, is the probable progenitor of SXTs 110221, 110902 and 120419. The hardness ratios of the flares, 0.03–0.2, are relatively smaller than that of X-ray bursts of neutron star binaries since spectrum of stellar flare is thermal emission of optically thin plasma with relatively low temperature. Although young stellar objects are also possible SXT sources, Chapter 5. Discussion 66 accidental association is not negligible as mentioned above. We only suggest a likely association of SXT 100907 as YZ CMi.

Third promising origin of SXT is X-ray ﬂares from blazars, especially BL Lac objects. STXs associated with some BL Lacs, Mrk 205 and PKS 2149−306, and a few Seyfert galaxies are detected, however relatively large null hypothesis probabilities, 10−10–10−9, call for careful conﬁrmation. Moreover, similar to the case of active stars, the large number of catalogued sources make accidental association more likely.

5.2 Hardness of the detected SXTs

Hardness ratio, which is a fraction between fluxes in two different energy bands, is an useful indicator of spectral features of X-ray sources. In the case of stellar flares, since the emission is thermal radiation form optically thin plasma on the photosphere, the hardness ratio is smaller, i.e., its spectrum is softer. In other classes of X-ray sources whose spectra are represented with power-law function, such as GRBs, flares of AGNs have various hardness values depending of the spectral indices. X-ray bursts of LMXBs have harder spectra than stellar flares. They are described with blackbody radiation with temperatures of a few keV.

In our analysis the hardness ratios of the detected SXTs are calculated in the 2–8 keV and 8–20 keV energy bands and listed in Table C.1. Their distribution and relation with the X- ray flux are shown in Figure 5.1. A dominant population is located around the hardness of 0.3–0.7 regardless of the significance (null hypothesis probability) of events. In contrast, the hardness ratios of the detected GRBs are widely distributed in 0.1–0.7. Comparing the hardness distribution shown in Figure 5.2 (Serino [68]), the difference between the populations are more obvious. The distribution of the sources that do not include the GSC-triggered GRBs is similar to that of X-ray bursts rather than that of GRBs and stellar flares. This implies that the SXTs detected with our analysis are not stellar flares but more likely X-ray bursts of neutron star binaries or other class of events whose spectra have a hard component in the 8–20 keV band.

Assuming the spectra of the SXTs as power-law like non-thermal emission or thermal blackbody radiation, the expected relation between the hardness and the parameters of these spectral functions are shown in Figure 5.3. The power-law function is deﬁned as photon ﬂux density dN/dE = E−Γ, with the photon index of Γ. The spectral parameter of the blackbody function is its temperature kT in unit of keV. Note that we ignore interstellar absorption component Chapter 5. Discussion 67

0.5 ) ) -1 -1 s s

-2 -2 0.4 1

0.3

0.5 0.2

0.1 Flux 4-10 keV (count cm Flux 4-10 keV (count cm

0 0 10-2 10-1 1 10-2 10-1 1 Hardness 8-20 keV / 2-8 keV Hardness 8-20 keV / 2-8 keV

10 Number of Events

0 10-2 10-1 1 Hardness 8-20 keV / 2-8 keV

Figure 5.1: Hardness-flux diagram (upper) and histogram of hardness (lower). The right upper panel shows an enlarged view of the left one. The candidates which do not include the −9 GSC-known GRBs and has the Pnull < 10 . are represented as the open circles and blank −10 histogram. The black-filled circles and gray-filled histogram are that with the Pnull < 10 . The GSC-known GRBs are shown as the triangle marks and hatched histogram. from the estimation since the effect is very small in our sample taken off the galactic plane and measured above 2 keV. From Figure 5.3, spectra of the SXTs with the hardness ratio of ∼ 0.5 have the flat power-law index Γ ∼ 1.0 or the relatively high blackbody temperature kT & 2.0 keV. The photon indices are consistent with that of GRB spectra in the soft X-ray/γ-ray band (Sakamoto et al. [64]). Moreover, the temperatures are also comparable to those of X-ray bursts from LMXBs (Galloway et al. [18]). From the aspect of distribution of hardness ratio, the dominant origins of the detected SXTs are GRBs with hard X-ray spectra and/or XRBs from LMXBs. Chapter 5. Discussion 68

10 GRB

0 10 XRB

number 5 flare

0 0.01 0.1 1 hardness ratio Figure 5.2: Histogram of the hardness ratio of the GRBs, X-ray bursts (XRBs), and stellar ﬂares discovered with GSC (Serino [68]). In the upper panel the hatched histogram means the GRB detected with only MAXI (without any other observatory).

2 and kT Γ

0 0 0.5 1 Hardness ratio Figure 5.3: Correspondence of hardness ratio to photon index Γ (ﬁlled circle) and temperature of blackbody kT (box). The deﬁnition of Γ and kT is shown in the text. Chapter 5. Discussion 69

Table 5.3: Statistical test of isotropy −9 −10 Test Pnull < 10 Pnull < 10 Expected hcos θi −0.01 ± 0.14 0.04 ± 0.16 0.020 hsin2 b − 1/3i 0.11 ± 0.06 0.11 ± 0.06 0.066

5.3 Spatial distribution

The spatial distribution of the detected transients is shown in Figure 5.4 in the Galactic coordinates and the equatorial coordinates. The distribution looks roughly uniform over the sky. We verify the impression by calculating dipole and quadruple statistics which are described in Section 2.3.1. If the exposure and eﬀective area of the observation are even, an uniform distribution over the sky implies a isotropy of the source distribution. However, the exposure of GSC strongly depends on an angle toward the rotation axis of the ISS orbit. In the veriﬁcation of spatial distribution of SXTs detected with GSC we have to consider not only the distribution of the SXTs but also the anisotropy of the exposure map.

Figure 5.5 is the spatial distribution of the GSC exposure in the 43 month. The unit of the exposure is product of time in second and effective area in cm2. The belt-like blank region and the blank circles correspond to the Galactic plane and the bright sources excluded from our analysis, respectively. The effective exposure is slightly larger at the high declination. Note that the lighter regions at the equatorial poles results from the blind areas in the GSC whose field of view across the direction of motion is not 180◦ but 160◦.

In order to investigate an isotropic distribution of the detected sources, we make histograms of the dipole and quadruple statistics for the SXT candidates and the distribution of the effective exposure, as shown in Figure 5.6. The resulting statistics are calculated from these histograms and summarized in Table 5.3. The errors are standard deviations. We conclude that those quantities obtained from the data and the exposure map are consistent with the isotropic distribution of the detected SXTs. Chapter 5. Discussion 70

+60°

+30°

0°

-30°

-60°

+60°

+30°

0°

-30°

-60°

Figure 5.4: Map of detected X-ray transients in the Galactic coordinate (upper) and the equatorial coordinate (lower). The radii of the circles are proportional to the logarithm of the X-ray ﬂuxes in the 4–10 keV band. Chapter 5. Discussion 71

+60 ◦

+30 ◦

180 ◦ 120 ◦ 60 ◦ 0 ◦ 300 ◦ 240 ◦ 180 ◦

0 1

+60 ◦

+30 ◦

180 ◦ 120 ◦ 60 ◦ 0 ◦ 300 ◦ 240 ◦ 180 ◦

0 1

Figure 5.5: Normalized exposure map of our analysis in the Galactic coordinate (upper) and the equatorial coordinate (lower). The normalization factor is 5993982 cm2 s. Chapter 5. Discussion 72

0.06

15 0.05

10 0.04

Number of events 5 0.03 normalized comulative exposure

-1 -0.5 0 0.5 1 -1 -0.5 0 0.5 1 cos θ cos θ

0.1

0.05 5 Number of events normalized comulative exposure 0 0 -0.5 0 0.5 1 -0.5 0 0.5 1 sin2b - 1/3 sin2b - 1/3

Figure 5.6: Distribution of cos θ and sin2 b−1/3 obtained from the detected SXTs (left panels) and the exposure map (right panels). The hatched histograms in the left panels are provided −10 from the SXTs with Pnull < 10 . Chapter 5. Discussion 73

40 80

30 60

20 40 Number of events Number of events 20 10

0 0 0.5 1 0 0.5 1

V / Vmax V / Vmax

−9 −10 Figure 5.7: Histogram of V/Vmax of the SXTs with Pnull < 10 (blank) and < 10 (hatched). The right panel is an enlarged view of the left histogram.

5.4 Study of spatial distribution

5.4.1 V/Vmax test

Supposing the specially uniform distribution of SXTs in the Euclidean space, the V/Vmax test will provide the average of 0.5. In the test we calculate each V/Vmax value from the foreground and background X-ray counts which are evaluated in the localization process for each SXT. The minimum detection limit Clim is obtained as a solution of the inverse function of the Poisson- distribution survival function with the mean of Nb (Equation 4.2), number of the background counts which is normalized with the area ratio of the fore and background regions, and the threshold upper tail probability of 10−9.

The histogram of the V/Vmax is shown in Figure 5.7. From the distribution of the SXTs −9 −10 with the null hypothesis probability of 10 and 10 , we obtained the average of V/Vmax, 3 hV/Vmaxi = 0.829±0.024 and 0.739±0.037, respectively . These values are significantly different from the expected value 0.5 for the uniform spatial distribution. Moreover, the distribution as shown in Figure 5.7 is dominant in the V/Vmax = 0.9–1.0. This strongly suggests that the SXTs with the lower flux contain false detections whose foreground counts accidentally (or more likely systematically) include a few additional counts thereby exceeding the detection threshold.

3The errors are estimated from (12n)−1/2, where n is the number of the SXTs Chapter 5. Discussion 74

5.4.2 LogN-log S plot

We made log N-log S plot to investigate the spatial distribution of the detected SXTs and compare it with the results from other satellites. Figure 5.8 shows the plot with each threshold −9 −10 −15 −50 of null hypothesis probabilities; Pnull < 10 , 10 , 10 and 10 . Supposing no temporal variability of an X-ray flux during the scan transit of GSC, we obtain the fluence integrated the count flux reported in Table C.1 over 30.0 sec which is the half width of the typical scan transit time as described in Section 3.2.4.1.

−2 −9 In the bright ﬂuence region (S & 8 photon cm ) of the plot with Pnull < 10 , the plot will represented with the S−3/2 distribution, however it may not be consistent with results of other surveys which has the S−1 function. The abnormal feature will be acceptable with considering the ﬂuctuation of the small number of SXTs. In fact, our study unfortunately does not detect some of the brightest GRBs as listed in Table A.2. If they contributed to the log N-log S plot, it would provide more reasonable result in our analysis.

−9 In contrast, the histogram in the fainter region is complex, because that with Pnull < 10 shows much steeper power-law function with the index of & 3. That is inconsistent to the traditional short X-ray transient surveys. To explain such unreasonable distribution, we will have to accept −9 −10 that the SXTs with Pnull < 10 –10 include false detections, as mentioned in Section 5.4.1. −10 −2 The histogram with the Pnull < 10 in the fainter fluence region (S . 8 photon cm ) has the N ∝ S−3/2 distribution and is consistent with the expectation of uniformly distributed transient phenomena. According to the past studies of GRBs, however, the slope of the plot in the low fluence region should be typically flatten owing to a cosmological effect. The different feature of the log N-log S plots in our analysis and traditional surveys implies that our result includes close (low-redshift) and intrinsically dim SXTs. The possible origins are X-ray activities of active stars and X-ray binaries in our galaxy, and XRFs (X-ray flashes), which are probably occur in closer regions than hard (classical) GRBs (Sakamoto et al. [62]). They are expected to show a log N-log S curve with slopes steeper than that of GRBs. Note that even though we −10 carefully searched SXTs in the MAXI-GSC data, the detections with Pnull < 10 may still include additional false detections.

Figure 5.9 shows a log N-log S plot that is corrected to be consistent with the uniform distribution in space for the detected SXTs, so that an average of V/Vmax becomes close to 0.5.

First we calculate average number of SXTs in the V/Vmax histogram using the bins below 0.5 in

Figure 5.7. Next we discounted weight for the SXTs with V/Vmax > 0.5 per bin width of 0.1, so Chapter 5. Discussion 75

that the V/Vmax bins above 0.5 have counts equal to those below 0.5. With this discount weight we plot the log N-log S histogram as shown in Figure 5.9. As a result the contribution of the

V/Vmax > 0.5 SXTs to the log N-log S plot decreases. The distribution in Figure 5.9 shows no excess over the −3/2 slope and becomes shallower in the region of the faint ﬂuence.

−9 When we choose the detection threshold of Pnull = 10 in Section 4.4.2, number of false (accidental) detection is estimated as ∼ 1 for the analyzed data set consisting of ∼ 109 trials. −9 −10 Conversely, the 84 SXTs with the Pnull = 10 –10 seem to include much more false detections than the expectation. This inconsistency casts doubt on the basic assumption that the fluctuation of the GSC background is the Poisson noise. The environment of MAXI possesses various origins of background for the X-ray detectors; not just NXB and CXB, but solar X-ray emission, its scattering and reflection to structures of the ISS and the Cs137 source mounted on the Soyuz spacecraft. Such background sources may generate unexpected (and maybe systematic) false detections which are difficult to discard in our analysis.

Our transient search provide a few bright SXTs which has the ﬂuence of > 10 counts cm−2 in the log N-log S plot. From this result we estimate the number of SXTs per year for the entire sky. Here, we should consider the following correction factors: eﬀective time duration of the

GSC operation fope, field of view in the one-scan transit fFoV and data rejection factor applied in our transient search process fts. Since the GSC is turned off in the South Atlantic Anomaly (SAA) and high-latitude regions of the orbit, the operating efficiency (ratio of the observing time to the total real time in orbit) , the fope is estimated as ∼ 40% in a day. The fraction of ◦ ◦ the field of view with respect to the entire sikh, fFoV, is 1.48%, as derived from two 160 × 3 field of view of the GSC-H and GSC-Z. The efficiency of the analyzed data has been already mentioned in Section 4.6.7 and estimated to be ∼ 25% of the original data observed with GSC. As a result, we obtained the number of the SXTs per a year for the whole sky as the order of 103 with the correction factor of 0.4 × 0.015 × 0.25 = 1.5 × 10−3. According to the log N-log S distribution of Arefiev et al. [2] as shown in Figure 2.10, an expected number of brighter SXTs which last X-ray events with the flux of 1 Crab with a duration of 60 sec (comparable to the GSC one-scan transit) is estimated to be ≈ 103. This is roughly comparable to the expected number of SXTs in our analysis. Chapter 5. Discussion 76

-9 Pnull < 10

-10 Pnull < 10 2 10 -15 Pnull < 10

-50 Pnull < 10 N (> S)

1 10 102 -2 S4-10 keV (count cm )

Figure 5.8: The log N-log S plot of the 146 SXTs. The colors mean diﬀerence of detection thresholds. The dotted line is power-law function with the index of −3/2. Chapter 5. Discussion 77

-9 Pnull < 10

-10 Pnull < 10 2 10 -15 Pnull < 10

-50 Pnull < 10 N (> S)

1 10 102 -2 S4-10 keV (count cm )

Figure 5.9: The log N-log S plot corrected to consider uniform spatial distribution. Chapter 5. Discussion 78

5.5 Origin of the low-ﬂux SXTs

In Section 5.4.1 and Section 5.4.2, we found that the detected SXTs with the lower ﬂux provide

V/Vmax and log N-log S plots that are astrophysically unreasonable. Here, we examine distributions of the SXTs with respect to their detected time, detector and position on earth in order to investigate the origin of such results.

First, we focus on temporal and/or detector-based bias in detection of the SXTs. If false detections originate from solar ﬂares, the SXTs may have temporally-biased with the solar activity. On the other hand, NXB (charged particles and γ-ray photons) may induce false faint (large V/Vmax) SXTs localized in particular locations in detectors. Figure 5.10 shows histograms of the 146 SXTs to the detected time, GSC camera and position on the detector coordinate. These histograms indicate the detections of the SXTs are uniformly distributed to time and detector coordinate without considering the V/Vmax values. Thus, solar X-rays, external charged particles and/or high-energy photons are probably not suitable to the origin of excess detections with larger V/Vmax.

In Figure 5.10, the numbers of SXTs are not even across the GSC cameras. It is generally possible since the rejection of X-ray events in the galactic plane may aﬀect this inhomogeneous distribution, However, cameras 1 and 7 should have approximately equal number of SXTs since they share the same monitoring ﬁeld in the sky (see Figure 3.3). Nevertheless the number of the SXTs detected with the GSC 7 is about three times larger than that detected with GSC 1. It is noteworthy that this unevenness is particularly noticeable in SXTs with large V/Vmax values.

The number of the V/Vmax > 0.9 SXTs detected with the GSC 7 is twice as large as that with the GSC 1. This fact suggests that noise of the detectors and/or their processing circuits may produce such “suspicious” SXTs.

Second, in order to distinguish false detections originated by NXB, here mainly considering charged particles, we make map of detected positions of the SXTs on earth as shown in Fig- ure 5.11. If charged particles will accidentally trigger SXT-like events, the positional distribution of the SXTs should be localized in the high latitude region and around the SAA, where the particles are more dense. In contrary to this anticipation, the map has uniform distribution to the

SXTs. Thus, we imply that the SXTs with the large V/Vmax are not originated with the excess of the charged particles. Chapter 5. Discussion 79

Third, another benchmark to investigate contribution of the NXB to the “false” detections using the VETO signal counts. The VETO anode wires are located under photon signal anodes wires and both sides of a GSC camera (see Section 3.2.2), and they are sensitive mostly to charged particles penetrating the detector walls or electrons produced in the detector material. A sudden increase of NXB may cause a false SXT, but that should be also noticeable in the increase of VETO counts. In Appendix B, we show maps of X-ray events in GSC, light curves (count rate time history), and plots of the VETO temporal variation for the detected 146 SXTs. One of the SXTs, SXT 120416 may be associated with a short term variation of the VETO counts as shown in Figure B.115. However, looking at the VETO plots, almost all SXTs are detected with the stable and relatively law (100–300 counts) and stable VETO counts. This result also supports that the SXTs are not accidentally triggered by temporal variation of the density of the charged particles. Chapter 5. Discussion 80

2 Number of Events

0 350000000 400000000 MAXITIME (sec)

2 Number of Events

0 -100 0 100 BEX (mm)

Number of Events 10

0 0 1 2 3 4 5 6 7 8 CAMERA ID

Figure 5.10: Histogram of the SXTs to detected time, detector coordinate and GSC camera. The SXTs with V/Vmax > 0.9 represented with the red ones in the upper and middle panels and the hatched one in the lower panel. Chapter 5. Discussion 81

+60°

+30°

0°

-30°

-60°

Figure 5.11: Positional distribution of the detected SXTs on earth. The red and blue colored points correspond to the SXTs with the V/Vmax > 0.9 and < 0.9, respectively. Chapter 6

Conclusion

We developed a new method to systematically search short X-ray transients (SXTs) from the MAXI-GSC data obtained from October 2009 to May 2013. This method employs a simple detection algorithm in which the null hypothesis probability for the observed X-ray counts in a test region is evaluated assuming the Poisson fluctuation of the background. First, we screened out specific events on the Galactic plane, in the regions near known bright X-ray sources, and those detected in area with particularly high background. Next, we extracted candidates of short X-ray transients with the low detection threshold assuming a Poisson-distributed background. Third, these candidates were examined for their accurate positions, detection significances and X-ray fluxes. In the analysis, we only considered number of counts in a test region and did not perform any fitting to background models and the point spread function of the GSC counter. Finally, we obtained 147 SXTs with the low null hypothesis probability 10−9 based on the Poisson-fluctuated background.

These SXTs were checked for positional and temporal coincidence with known GRB catalogs and optical/X-ray/γ-ray source catalogs which include possible sources of short X-ray activity. We conﬁrmed 11 SXTs as the prompt emissions of known GRBs and one as the afterglow of the untypical GRB. The cross matching with the non-GRB catalogs revealed some SXTs are positionally associated with X-ray binaries consisting of neutron stars or active stars such as RS CVn. However, the large position error provided by our analysis prevent from association of other SXTs with GRBs or other classes of sources such as blazars. As regard to hardness ratio, the detected SXTs mostly have hard X-ray spectra which are consistent with X-ray bursts of neutron-star binaries or prompt emission of hard GRBs rather than stellar ﬂares.

82 Chapter 6. Conclusion 83

The positionally isotropic distribution of the detected SXTs was confirmed with the hcos θi and hsin2 b − 1/3i statistics, since these quantities were consistent with the expectations from the exposure map of GSC. However the V/Vmax tests indicate that the spatial distribution of the SXTs is not uniform. This is probably caused by the false detections triggered by fluctuation of the background of GSC or some other unknown reasons. The log N-log S plot showed similar −10 results. The Pnull < 10 SXTs provided the power-law like distribution with the slope of −9 −10 ∼ −3/2. On the other hand, the SXTs with Pnull = 10 –10 showed steeper distributions with indices larger than −3/2 in the < 0.5 counts cm−2 lower fluence region.

The exceeding population in the log N-log S plot formally indicates that the dim and distant phenomena are abundantly detected by our analysis; however, such phenomena are not astrophysically plausible. We conclude that our analysis suﬀers from false detections that have X-ray count slightly larger than the threshold. To investigate the origin of the detections, we examined correlations with the detected time, position on earth and increasing of background. Finally we found that the false detections are related to the noise of a detector or readout electronics. To perform advance study, we need to consider such noise in addition to the Poisson-ﬂuctuation of the background. Appendix A

List of Detected Transients

In our transient search analysis, we obtain 146 short X-ray transient candidates. Information of these candidates, detected time in the Universal Time, position, null hypothesis probability and ﬂuxes, are summarized in Table C.1. Possible counterparts of the detected SXT candidates with the source catalogs listed in Table 5.1 and Table 5.2 are also shown. The counterparts of the GSC-known GRBs are shown other than the GRB itself in the table. Table A.2 summarizes information of the GRBs reported in Serino [68]. In addition, the table notes that whether our analysis detected these GRBs or not.

84 Appendix A. List of Detected Transients 85 § J052038.3-394517(9) Counterpart ‡ HR 20 − † 8 f 8 − † 2 f 10 − † 4 f ∗ null P List of detected short X-ray transients and their possible counterparts. ) ◦ ( ) Table A.1: ◦ ( R.A. Dec. (UT) Time Name 091012 2009-10-12091014 10:26:08 2009-10-14091105 181.38 14:08:14 2009-11-05 +63.08 134.00 19:46:46091115 +46.60 -15.78 78.13 2009-11-15091119 01:51:19 0.211 -9.10 -40.56 2009-11-19091202 107.68 00:15:30 0.262 0.092 -11.06 2009-12-02091202 05:41:18 -71.00 98.82 0.109 0.093 2009-12-02091204 0.123 162.85 13:50:24 +68.45 -0.001 0.415 -9.10 2009-12-04 0.200091205 +54.45 23:41:35 59.48 -0.007 SDSS J11558+6135(8), 2009-12-05 -9.25091212 0.115 091012 314.40 0.054 10:16:12 (C:+0.00204, 0.71) NGC -9.01 -57.39 2712(1), S4 2009-12-12 0.150091218 0.114 313.96 0859+47(5) 12:46:17 0.268 -22.50 0.139 -0.005 2009-12-18 0.126091227 17:44:08 CXO -9.05 +1.29 J051406.4-400238(1), 47.01 4U 0.088 -0.031 0513-40(2, -9.38 2009-12-27091229 7), 234.29 0.048 12:39:05 NGC1851(4), 0.108 -11.29 -72.12 0.073 2009-12-29100108 0.114 209.61 13:13:23 0.111 0.381 -3.86 0.149 2010-01-08 0.111 0.827100120 +52.37 20:59:02 -9.33 47.36 0.009 0.119 1ES -10.35 2010-01-20100123 338.61 0.033 1044+549(8) 16:23:29 0.120 +14.06 0.084 -9.80 0.063 2010-01-23100207 0.125 05:18:35 0.298 0.143 48.44 J041155.1-580154(9) -10.41 -7.45 0.139 2010-02-07 0.171 0.532100217 09:48:24 J210417.6-221011(9) +12.87 54.20 0.039 0.132 0.136 1RXS 2010-02-17100220 235.52 0.044 J205528.2-002123(5), 19:32:04 -9.11 SDSS J20549+0015(8) 0.274 0.110 +0.35 0.004 2010-02-20 -9.09100302 +19.39 110.19 06:51:17 0.260 0.170 ESO 031- 0.079 2010-03-02 -16.13 0.028 G100315 +26.90 0.133 179.08 06:10:59 008(1, RXC 0.179 J1540.1-0318(4) 4), -9.12 J032736.1-725044(9) SDSS 2010-03-15 0.717 0.134100327 +61.43 0.243 146.49 J14067+5308(8) 19:24:43 0.072 -9.57 0.134 2010-03-27 0.395100415 +34.03 159.32 0.131 17:08:43 0.399 -9.24 0.057 0.127 2010-04-15100418 +32.70 345.93 0.094 03:45:16 -11.27 0.977 PKS 0.150 0.122 2227-088(1, 0.145 2010-04-18 5),100419 +42.93 06:21:02 QSO 0.239 0.168 B2227-0848(2) -9.96 0.115 7.50 0.001 2010-04-19 2.559100420 18:12:35 HR 0.112 70.30 1099(2, -9.25 3, 0.148 0.062 4, 3C 2010-04-20 0.009 -16.40100425 261.20 7), 323.1(1) 19:11:19 SDSS 0.091 J03440-0000(8) 0.111 -24.86 0.133 V340 2010-04-25 0.540100509 +24.15 310.70 Gem(3), 05:18:17 -28.40 FIRST J07247+2621(8) 0.809 0.285 0.115 SBS 2010-05-09 256.83 1200+608(5), 15:58:48 -9.33 0.328 SDSS +5.76 J11558+6135(8) SDSS -9.31 J09476+3453(8) 0.039 1.031 +24.41 185.88 0.577 0.141 0.151 SDSS -9.42 0.138 J10421+3254(8) 0.155 -44.37 0.194 -9.14 0.169 PSR 0.117 J2302+4442(5), 100327 0.056 (C:+0.00032, -10.51 0.337 0.149 0.08) 0.010 0.106 J002606.8-154217(9), 0.362 0.194 0.097 100415A 0.057 (C:+0.00022, 0.056 0.83) PMN 0.099 J0434-2342(5), 0.045 RXS J04402-2459(8) 0.530 0.034 0.234 4U 0.348 1700+24(1, 2, 4, 7) 2FGL J1214.1-4410(5), PKS 1224-443(8) Appendix A. List of Detected Transients 86 § 100510A (C:+0.00038, 0.30) RXS J02375-3603(8), J023126.4-375831(9) J052038.3-394517(9) 100823A (A:+0.00046, 0.19), 100823A (C:+0.00046, 0.19) J225725.9-564544(9) S5 2023+760(5), S5 2023+76(8) J052038.3-394517(9), HD35650(10) Counterpart ‡ HR (Continued) 20 − † 8 f 8 − † 2 Table A.1: f 10 − † 4 f ∗ null P ) ◦ ( ) ◦ ( R.A. Dec. (UT) Time Name 100510 2010-05-10 19:27:42100516 355.43 2010-05-16100524 08:05:33 -35.65 2010-05-24100611 10:18:05 -84.07 2.88 2010-06-11100611 106.34 06:24:27 0.729 2010-06-11 -50.80 +33.66 352.98 08:41:50 1.271100708 +40.95 -12.16 37.97 -9.86 0.674 2010-07-08100712 02:17:45 0.137 -9.84 -36.66 0.135 0.530 2010-07-12100720 227.19 20:38:10 0.117 0.077 0.124 J234154.4-355838(9), 2010-07-20100808 +28.34 100510810 179.63 16:17:56 -9.74 (B:+0.00042, 0.047 1.29), 0.109 0.094 2010-08-08100813 +74.33 19:50:49 0.089 77.43 -9.07 0.403 0.040 1.220 2010-08-13 329.96 04:18:46 0.105 -10.05 -15.70 0.133 1RXS 0.369100823 +29.24 J002159.2-514028(5), AM 0.112 78.50 0.043 1(6), 0.157 RXS B3 2010-08-23 J00141-5022(8) 2322+396(2, 17:26:15 -9.39 0.106 8), 1ES 0.404 -9.41 -39.92 2321+419(5) 0.053100829 0.131 20.51 0.035 IC 0.148 1816(1), -37.62 2010-08-29 0.341 PKS100907 18:30:01 0227-369(2, 0.116 5), 0.329 0.134 +5.82 RBS 2010-09-07101019 0.313 1467(5), 18:25:49 RXS 10.36 0.016 J15087+2709(8) Mrk 205(1, 0.074 2010-10-19 0.492 -58.37101020 4, 161.17 16:16:12 7), +11.26 AS 0.135 Dra(3) 2010-10-20 0.555 0.639 337.38 0.166 16:06:02 -33.97 -9.04 1.399101025 346.18 0.337 -12.13 -11.17 2010-10-25101030 0.172 0.300 05:09:27 CXO -58.22 J051406.4-400238(1), 4U 0.180 0513-40(2, -9.92 2010-10-30 0.205101102 7), 13:55:11 NGC1851(4), 0.215 36.82 -10.12 0.306 2010-11-02101108 0.140 0.089 16:30:41 HD 29.19 8357(2), UV 0.115 0.041 Psc(3), 2010-11-08 0.144 -4.07 NGC 21:25:07 0.433 6934(6), 0.119 -35.68 4.14 0.134101111 308.06 0.053 BK Psc(3), -9.19 GB6 0.072 J104230.3-334014(9) J0045+1217(5), 2010-11-11 -56.24101208 +74.62 Ko 07:19:06 -9.64 0.365 1(6), J004401.5+093302(9) 0.113 0.602 2010-12-08101220 278.15 15:50:38 Mrk 0.105 915(1, -9.02 0.155 -9.24 7), PKS 2MASX 2010-12-20110106 +22.83 2223-114(8) J23013626-5913210(1), 332.63 13:39:46 0.087 2MASX J23013626-5913210(4), 0.097 0.125 CP 0.013 2011-01-06 Tuc(7), 110110 18:08:11 -15.17 -62.74 78.38 0.041 0.111 0.058 0.085 2011-01-10 09:18:45 +73.65 0.228 56.77 -10.87 0.474 0.057 0.107 0.404 79.11 ESO 0.131 354- 0.511 -9.37 1.834 G -1.84 004(1), J020118.8-370457(9) 0.092 0.124 -40.22 PMN 4C 0.131 J0019-5641(5), +74.26(1, NGC 4, 1261(6) 7), -9.67 0.227 0.069 VW -14.03 0.113 Cep(2), 0.139 0.559 0.217 0.047 0.096 J222312.0-623555(9) 0.362 0.413 0.087 0.106 GB6 J0516+7350(5) 0.907 0.294 QSO B0336-0156(2), CXO PKS J051406.4-400238(1), 0336-01(5), 4U SDSS 0513-40(2, J03440-0000(8), 7), HIP17695(10) NGC1851(4), Appendix A. List of Detected Transients 87 § J163635.0-753603(9) Counterpart ‡ HR (Continued) 20 − † 8 f 8 − † 2 Table A.1: f 10 − † 4 f ∗ null P ) ◦ ( ) ◦ ( R.A. Dec. (UT) Time Name 110113 2011-01-13110115 06:41:51 2011-01-15110116 353.27 01:56:43 2011-01-16110117 +19.81 20:09:21 90.36 2011-01-17110122 342.06 22:03:34 -19.20 -54.07 2011-01-22110123 +24.53 313.04 23:07:10 0.247 2011-01-23110209 09:48:28 -9.00 0.378 -14.73 -9.20 6.64 2011-02-09110215 312.31 19:48:48 0.128 0.116 +29.18 0.141 -9.74 2011-02-15110216 226.36 02:20:33 0.085 -67.36 0.308 0.135 2011-02-16110219 0.095 192.91 11:28:47 -9.72 -25.52 0.133 EQ Peg(2, 0.058 -9.06 2011-02-19 0.104 3) 110221 106.52 23:09:31 -16.81 0.156 1.568 -9.27 2011-02-21 0.427110225 0.085 +31.85 131.32 0.010 20:10:58 0.085 TY -12.65 Pic(3) BZB 2011-02-25 0.063110308 0.101 +44.48 310.43 J2255+2410(5) 04:20:19 0.093 -10.78 0.100 0.136 2011-03-08 0.099110313 195.97 0.075 21:32:15 -31.90 0.208 0.505 -9.22 1.172 2011-03-13110406 +67.50 116.50 0.023 10:41:25 1.184 0.183 RXC -15.35 0.118 J0020.6+2840(4), 0.026 2011-04-06 RBS110415 20:22:43 0042(5), 0.234 +3.41 23.20 RXS 0.054 -9.06 J00184+2947(8) 0.089 0.204 0.052 2011-04-15110423 241.91 23:46:39 2FGL J1513.5-2546(5) 0.410 -16.63 0.296 0.134 0.049 RXC 2011-04-23 -2.58110426 +15.54 J1257.1-1724(4), 182.73 07:17:25 A1644(7) 0.095 0.287 0.038 2011-04-26 0.550110505 122.17 15:08:55 -10.15 -9.16 1.055 -0.18 0.090 US 0.094 2011-05-05110525 221.02 1889(8) 12:22:07 0.103 -1.09 0.100 0.127 AT 2011-05-25 0.956 110525 Mic(2, 169.39 14:25:31 7), -9.01 0.116 -10.46 2MAXI 0.107 J2037-306(4), J204151.2-322604(9) 2011-05-25 0.120110528 306.83 16:06:43 -9.13 0.095 0.031 -12.19 -3.73 0.064 YZ 2011-05-28110607 227.97 CMi(2) 13:59:14 0.142 -14.89 0.127 0.266 0.186 -10.53 0.600 2011-06-07110618 196.76 17:47:13 0.149 +9.47 0.061 2MAXI 0.384 J1602+161(4), 2MASX -9.09 2011-06-18 0.148 2FGL J01344565-0430134(1), 260.37 06:26:57 J1612.0+1403(5), FIRST -18.26 0.001 A2147(7), J01271-0151(8) 0.430 -10.07 PKS 0.061 1604+159(8) 0.235110620 0.116 +12.43 242.17 PKS 0.006 0.116 1215-002(8) -9.57 0.158 2011-06-20 0.140110624 0.076 12:13:49 -76.54 2MASX -9.38 0.118 J08045299-0108476(1), 110426629 2011-06-24 PKS110625 0.107 (B:+0.00172, 0.020 00:32:52 B0802-010(5) 0.324 3.35), 73.74 110426A 0.098 (C:+0.00022, 0.055 -9.65 2011-06-25 0.092 0.36) 110704 01:08:42 0.142 11.35 0.114 -58.04 2011-07-04 0.464110726 0.104 0.073 17:58:34 J202817.1-134829(9) 17.66 -36.85 0.038 2FGL 2011-07-26 0.135110805 200.71 J1506.9+1052(5) 21:56:59 -9.04 0.791 -29.00 2011-08-05 0.336 +84.41 0.024 17:46:09 -9.25 0.175 87.21 QSO 154.00 B1722+119(2), -9.05 0.176 0.189 -11.24 0.124 PKS +83.64 1725+123(5), H 1722+119(8) RXC 0.152 -21.45 0.116 0.111 J1601.7-7544(4), 0.061 PKS -10.24 1610-77(5), CIZA 0.100 J1601.7-7544(7), 0.094 0.061 0.322 0.130 -9.25 0.055 0.044 RBS 0.398 0.100 594(1), 2MAXI 0.141 J0451-585(4), J050046.6-571511(9) PKS 0.549 0.469 0042-373(5), 0.047 0.231 Pyxis(6) NGC S5 6715(6), 1322+83(5) -0.020 0.468 2QZ J011332-2956(8), J011240.6-291041(9) -0.085 S5 0454+84(8) Appendix A. List of Detected Transients 88 1919B(10) § J075407.7-654137(9) 111024A (C:+0.00049, 0.04) PKS 0336-01(5), SDSS J03440-0000(8) 1H 1914-194(5), TEX 1914-193(8) Counterpart ‡ HR (Continued) 20 − † 8 f 8 − † 2 Table A.1: f 10 − † 4 f ∗ null P ) ◦ ( ) ◦ ( R.A. Dec. (UT) Time Name 110826 2011-08-26110826 07:12:20 2011-08-26 21:10:41 61.35110828 114.25 -20.02 2011-08-28110831 21:30:37 -66.51 2011-08-31110902 167.43 08:35:42 -9.39 -10.34 2011-09-02110907 17:38:14 0.121 -5.44 0.173 4.59 2011-09-07110924 312.43 12:04:43 0.150 0.173 -10.92 2011-09-24 -61.26111005 122.89 21:07:42 -32.14 0.046 0.096 2011-10-05111010 0.176 287.18 15:17:34 +8.53 -9.22 -14.15 0.303 0.552 2011-10-10 0.089111015 236.35 00:27:13 -70.15 0.131 0.203 EXO 2011-10-15 -9.85111021 0748-676(1, 0.159 09:44:53 2, -35.21 7), 0.140 0.281 PMN 7.09 -9.34 2011-10-21 J0730-6602(5), 111024 0.142 03:55:46 1.782 14.16 -13.34 +18.81 0.089 0.096 2011-10-24 0.116 0.142 323.98 07:22:09 2MAXI J1108-056(4), HD96064(10) -15.21 0.164 0.635 0.341 0.120111106 +24.36 221.97 0.059 -9.43 0.168 J002406.7-621100(9) AT 2011-11-06111113 +25.87 Mic(2, 0.085 22:02:15 7), -9.18 0.508 0.150 2MAXI -9.47 J2043-324(4), 0.098 J204151.2-322604(9) 2011-11-13 -144.57111121 14:44:43 1WGA 0.707 0.205 0.109 28.19 J0814.3+0858(8), BD+07 0.121 0.580 2011-11-21111204 1.197 213.05 05:03:45 2FGL 0.156 J1902.7-7053(5) 0.048 0.098 -64.26 J154513.6-341733(9) 2011-12-04 2.713111212 10:54:07 -0.003 -10.71 0.232 4.54 0.028 -11.08 2011-12-12111212 156.90 0.441 -0.020 01:50:04 -10.21 2011-12-12 0.283 -38.46111229 0.153 +60.35 05:08:08 0.163 46.59 0.100 2FGL 2011-12-29 0.132111229 359.22 J2132.5+2605(5), 07:04:08 2MASX HIP106231(10) +30.11 J14530794+2554327(1), -9.50 0.092 -9.43 PKS 1441+25(5), 2011-12-29 +28.91 0.082 TEX 08:36:34 1451+270(8), 54.71 0.120 0.095 0.043 -9.47111230 0.619 -10.23 54.10 0.089 0.116 +0.44 0.472 2011-12-30120122 0.105 09:23:16 J020212.0-623532(9) 0.138 0.086 0.036 2MASX 2012-01-22 -22.67 0.090 -0.21120131 J14165001-1158577(1), 22:45:02 1RXS 0.343 38.68 J141650.6-115845(7) 0.967 2012-01-31 0.314120202 0.299 232.79 0.040 05:04:41 -10.75 0.005 -43.41 PKS S4 2012-02-02 0.516120207 +67.47 0010-401(5), 163.32 1030+61(2, 14:20:05 0.450 NGC 0.155 5), 1851(6), FG 0.015 PKS Uma(3), -16.00 2012-02-07 0010-401(8) TEX120208 0.127 09:40:02 1015+594(8) 0.230 -10.23 +4.09 95.39 2MASX J00032742+2739173(1), 2012-02-08 0.160 II 223.57 06:10:24 Peg(2, 0.246 +51.56 0.129 0.035 3, 7), 2MAXI 0.329 -9.94120213 +65.16 291.13 J2354+286(4) HR 0.131 1099(2, 0.150 3, 4, 2012-02-13 -9.23 0.098 0.026 7), 10:08:17 -18.39 SDSS 0.050 HR J03440-0000(8) -9.07 1099(2, 3, 0.116 0.161 4, 0.078 7), 33.57 -10.87 0.379 0.134 0.117 0.049 CC Eri(2, 3), 0.133 0.148 RXC -8.75 J0232.2-4420(4), 0.097 RBS 0.424 0318(5), 0.182 J023423.0-434739(9) 0.041 RXS 0.827 J10561+0252(8) -9.66 0.060 0.305 0.148 EK 0.330 Dra(2), RXS J14514+6354(8), 0.178 HD129333(10) SWIFT J1922.7-1716(1, 2, 4), 0.051 0.286 ARP 318(1), PKS 0214-085(5), SDSS J02195-0750(8) Appendix A. List of Detected Transients 89 § PKS 0336-01(5), SDSS J03359-0024(8) J013500.7-295430(9), 120424A (C:+0.00050, 0.38) J164527.9-250319(9) J052038.3-394517(9) A3814(7), 2QZ J215200-2823(8), J214023.7-280433(9) Counterpart ‡ HR (Continued) 20 − † 8 f 8 − † 2 Table A.1: f 10 − † 4 f ∗ null P ) ◦ ( ) ◦ ( R.A. Dec. (UT) Time Name 120307 2012-03-07120310 12:47:31 2012-03-10 184.68 06:50:06120310 -22.37 53.95 2012-03-10120310 08:22:24 -9.29 2012-03-10 -0.09120325 09:54:49 54.19 2012-03-25120416 0.105 11:20:52 -14.61 54.61 +0.21 2012-04-16 0.082120419 330.84 19:33:52 0.208 +0.49 2012-04-19 -11.95120424 0.055 17:38:06 0.248 +1.16 98.65 2012-04-24 -13.48 0.194 310.33 16:47:57 0.669 +65.67 0.064 -9.35 0.429120517 0.223 UV -30.93 Crv(3), 24.42 0.257 J121814.7-205028(9) 2012-05-17 -9.69 0.408120529 0.151 0.079 16:25:54 HR -29.86 1099(2, -9.87 2012-05-29 3, 0.181120604 0.138 253.68 0.004 00:31:35 4, 0.185 7), -20.93 2012-06-04 0.162120608 0.142 122.53 0.025 08:27:22 HR 0.010 1099(2, -7.76 3, 4, 2012-06-08 0.214120619 0.271 215.12 7), 0.027 07:00:19 HR 0.138 -63.98 SDSS 1099(2, J03440-0000(8) 3, 4, -11.24 2012-06-19 0.523120622 +15.20 352.71 0.004 7), 08:59:40 PMN 0.168 SDSS J2206-0031(5), J03440-0000(8) J220028.6+011259(9) -9.02 2012-06-22120701 0.215 +38.13 207.17 0.066 03:22:34 VII 0.018 Zw -9.95 073(1) 2012-07-01 0.308120702 0.173 +40.13 206.24 03:12:07 AT 0.127 -10.02 Mic(2, 7), 0.198 2MAXI 2012-07-02 0.165120702 J2037-306(4), 0.021 00:35:09 BB J204151.2-322604(9) 0.169 Scl(3), -9.69 2QZ 0.167 -1.95 2.37 J014542-3031(8), 2012-07-02120804 355.77 0.103 02:15:48 0.070 0.137 0.174 0.091 -11.31 2012-08-04 -25.41 +17.72 04:15:03 2FGL 0.625 J1656.4-0738(5) 11.20 0.035 0.213 0.542 120805 0.154 251.61 2MAXI -10.18 -14.12 J0827-642(4), 0.257 2FGL -62.98 0.053 J0807.0-6511(5), 2012-08-05 0.361 J082141.7-634045(9) 02:30:31 -23.31 0.152 0.156 V455 And(2), 0.250120810 NVSS 0.043 J232914+375414(5) -9.69 0.174 0.500 78.55 -13.02 NGC 2012-08-10120927 5290(1), 11:38:46 0.119 0.110 RGB 0.065 0.006 J1341+399(2), -40.25 0.191 RXS 2012-09-27 J13410+3959(8) 121011 319.60 17:36:52 SDSS 0.068 J13406-0148(8), 0.374 0.275 0.011 120622A (C:+0.00056, -47.65 2012-10-11121014 0.84) 350.92 11:11:04 -11.50 0.061 RBS 0016(5), 0.064 2012-10-14 NGC121027 0.533 16:27:29 288(6), +3.96 RXS 28.89 -10.25 0.894 J00085-2339(8) 0.231 2012-10-27 0.859121104 124.91 08:13:07 +36.53 2FGL 0.132 J0048.8-6347(5), MAXI 2012-11-04 J003452.2-615506(9) -9.51 +73.55 J1647-227(2), 0.220 03:26:42 2MAXI 63.38 J1647-229(4), 0.126 2FGL J1645.7-2148c(5), -9.07121218 0.134 326.99 0.256 -9.32 -58.54 0.023 2012-12-18 0.086 0.207 09:08:15 CXO -28.72 J051406.4-400238(1), 4U 0.081 0513-40(2, -15.11 0.179 0.211 7), 171.77 0.051 NGC1851(4), 0.112 NVSS -9.41 0.193 J212035-125443(5), 0.056 0.590 RXS -12.25 J21231-1036(8), J211909.5-124642(9) 0.024 0.364 0.165 NGC 0.265 7679(1) -9.74 0.159 0.216 0.224 XX Tri(3), RXC J0152.7+3609(4) 2MAXI 0.135 0.066 J0825+733(4), 0.614 Z Cam(7) 0.039 1H 0.416 0419-577(7), J041155.1-580154(9), 121027A (A:+0.02822, 0.087 PKS 0.31) 2149-306(1, 4, 5), QSO 2.235 B2149-306(2), 2MASX J11315154-1231587(1) Appendix A. List of Detected Transients 90 -GBM Fermi -BAT GRB, (B) Swift § Counterpart ‡ HR (Continued) 20 − † 8 f 8 − † 2 Table A.1: f 10 − † 4 f ∗ null P ) ◦ ( ) ◦ ( R.A. Dec. in 4–10 keV, 2–8 keV and 8–20 keV 1 − s 2 − (UT) Time Logarithim of null hypothesis probability Unit of count cm Hardness ratio between the ﬂuxes in the 8–20 keV and 2–8 keV energy bands. Possible counterparts. Value in parentheses means catalogGSC name 37-month, including (5) the Fermi-LAT source; 2nd, (1) (6) Swift-BAT Globular 70-month,GRB, (2) Cluster, (C) Swift-BAT (7) transient GSC RXTE, monitor GRB. list, (8) (3) Two VERON CAB, (blazar valuesGRB (4) only), following with (9) the the SACY alphabetical unit , index of (10) of day NYMG, the (A) and GRB degree, catalog respectively. are temporally and positionally diﬀerence between the transient and the Name 130102 2013-01-02130106 04:42:29 2013-01-06130123 309.07 12:13:12 2013-01-23130206 324.29 07:34:58 -72.36 2013-02-06130226 05:22:27 -77.84 -57.77 8.70 2013-02-26130310 307.94 02:07:12 -14.57 0.439 2013-03-10 -32.12130320 12:55:36 -56.42 54.03 0.780 0.160 2013-03-20130325 179.80 01:27:56 -10.10 0.195 +1.06 0.228 -9.52 2013-03-25130407 13:35:51 -77.99 0.132 95.29 0.109 0.292 2013-04-07 -12.53130418 0.217 252.94 23:37:24 0.111 -19.94 2MAXI -9.40 0.557 2013-04-18 0.218130520 +65.63 0.152 J2043-714(4), 248.23 00:51:11 2FGL 0.028 J2042.8-7317(5), 130102B∗ J205919.2-781048(9) 2013-05-20 (C:+0.00054, 0.279 0.157 +10.18 325.78 0.069 14:59:38 0.16) -9.25 -10.18 0.251† 0.203 163.57 0.019 0.315 -14.75 -21.09 0.126 0.130 J003308.1-320117(9) ‡ -0.020 2FGL 0.091 0.068 0.104 -25.53 0.216 J2039.8-5620(5), J203313.7-580625(9) -0.098§ -9.06 HR 0.311 0.074 1099(2, 0.041 3, J115941.8-760130(9) 4, -9.08 0.131 7), SDSS 0.098 J03359-0024(8) 0.815 0.394 0.103 0.126 PKS 0.315 0627-199(5, 0.087 8), 0.055 130407A (C:+0.00027, 0.35) 0.113 0.535 RBS 1.295 1769(5), RXS J21388-2053(8) NGC 3393(1), SHBL J10468-2535(8), J105604.3-265309(9) Appendix A. List of Detected Transients 91

Table A.2: List of GRBs detected by MAXI

Name Time R.A. Dec. Flux∗ Detected ? (UT) (◦) (◦) 091012 2009-10-12 10:23:12 182.82 +63.37 0.38 ± 0.04 Y 091120 2009-11-20 04:34:40 226.81 −21.79 7.69 ± 0.23 N† 091201 2009-12-01 21:48:36 118.6 +16.6 0.47 ± 0.06 N‡ 091230 2009-12-30 06:27:30 132.91 −53.88 0.27 ± 0.04 Nk 100315A 2010-03-15 17:13:40 74.95 −6.63 0.17 ± 0.03 N(−6.05) 100327 2010-03-27 17:08:15 346.03 +42.90 0.26 ± 0.03 Y 100415A 2010-04-15 03:44:57 7.48 −15.57 0.96 ± 0.11 Y 100510A 2010-05-10 19:27:09 355.8 −35.6 2.57 ± 0.19 Y 100616A 2010-06-16 01:42:13 50.95 −40.62 0.37 ± 0.05 N† 100701A 2010-07-01 06:54:28 188.86 −34.26 0.88 ± 0.09 N†§ 100823A 2010-08-23 17:25:35 20.70 +5.84 1.85 ± 0.10 Y 100911 2010-09-11 14:58:20 103.41 −70.43 0.30 ± 0.03 N# 101117A 2010-11-17 07:32:59 89.63 −2.30 0.84 ± 0.09 Nk 101210 2010-12-10 03:38:27 61.66 −5.36 0.22 ± 0.03 N(−4.88) 110213B 2011-02-13 14:31:48 41.76 +1.15 7.53 ± 0.32 N† 110402 2010-04-02 02:34:07 62.52 −3.00 0.44 ± 0.05 N† 110426A 2011-04-26 15:08:36 221.18 −10.78 0.51 ± 0.06 Y 110916 2011-09-16 20:33:01 171.68 −17.77 0.27 ± 0.03 N† 111024A 2011-10-24 07:21:27 221.93 +25.87 2.76 ± 0.17 Y 120424A 2012-04-24 16:47:14 23.985 −29.87 0.47 ± 0.05 Y 120510A 2012-05-10 08:47:44 44.285 +72.850 1.52 ± 0.12 N† 120528B 2012-05-28 18:12:08 77.59 −37.80 1.28 ± 0.10 N§ 120528C 2012-05-28 21:21:58 12.93 −0.95 0.32 ± 0.05 N(−3.53) Appendix A. List of Detected Transients 92

Table A.2: (Continued)

Name Time R.A. Dec. Flux∗ Detected ? (UT) (◦) (◦) 120614A 2012-06-14 05:49:10 312.73 +65.16 0.96 ± 0.07 N# 120622A 2012-06-22 03:21:46 205.43 −1.71 0.45 ± 0.06 Y 120626B 2012-06-26 13:38:14 175.77 +68.50 0.31 ± 0.04 N(−5.83) 120711A 2012-07-11 02:45:12 94.703 −71.001 0.89 ± 0.09 Nk 120908A 2012-09-08 22:35:13 230.64 −25.79 0.18 ± 0.03 N(−8.91) 121025A 2012-10-25 07:46:30 248.75 +27.73 0.24 ± 0.04 N† 121209A 2012-12-09 21:59:11 327.02 −7.69 0.52 ± 0.06 N§ 121229A 2012-12-29 05:00:21 190.10 −50.59 0.39 ± 0.05 N# 130102B 2013-01-02 04:41:42 309.58 −72.38 1.36 ± 0.11 Y 130407A 2013-04-07 23:37:01 248.10 +10.51 0.41 ± 0.05 Y ∗ X-ray flux cited from Serino [68] in the unit of 10−8 erg s−1 cm−2 in 2–20 keV. † Occurred immediately after/before the HV on/off. ‡ Detected by GSC 3. § Detected at |BEX| < 20 or > 100 mm. k Detected near galactic plane or bright sources. # Ignored because of lack of effective background area. Appendix B

Event map, light curve and VETO curve

This appendix shows event maps, light curves and VETO count curves of all detected SXTs. The event map includes X-ray events around the SXT and regions to estimate the ﬂux and

Pnull in our analysis. The black dashed box and blued frame are conrrespond to the region of foreground and background. The light curve is made from a projection of the event map whose width along the BEX coordinate is limited within the width of the foreground region. The time variability of the VETO counts is also shown. This count increases near the high background regions, i.e., at the high latitude and the South Atlantic Anomaly. If a SXT is accidentally triggered by the temporal ﬂuctuaion of the background (charged paticles), the VETO count will consistently increase.

93 Appendix B. Event map, light curve and VETO curve 94

2.0 1.0 40 40 0.8 1.5 30 30 0.6 1.0 20 0.4 BEX (mm) BEX (mm) 20 0.5 0.2 10

0.0 10 0.0 308658200 308658300 308658400 308844400 308844500 308844600 MAXITIME (sec) MAXITIME (sec)

3 2.0

1.5 2

1.0 Count Count

1 0.5

0 0.0 308658200 308658300 308658400 308844400 308844500 308844600 MAXITIME (sec) MAXITIME (sec)

400 200 300

200 100 VETO count VETO count 100

0 0 308658200 308658300 308658400 308844400 308844500 308844600 MAXITIME (sec) MAXITIME (sec)

Figure B.1: Event map, light curve Figure B.2: Event map, light curve and VETO count time variability of and VETO count time variability of SXT 091012 with log Pnull = −15.8. SXT 091014 with log Pnull = −9.1.

-20 3 2.0 100

1.5 -30 90 2

-40 80 1.0

BEX (mm) 1 BEX (mm) 70 -50 0.5

60 0 0.0 310765400 310765600 310765800 311565000 311565100 MAXITIME (sec) MAXITIME (sec)

6 3

4 2 Count Count

2 1

0 0 310765400 310765600 310765800 311565000 311565100 MAXITIME (sec) MAXITIME (sec)

300

200 200

VETO count VETO count 100 100

0 0 310765400 310765600 310765800 311565000 311565100 MAXITIME (sec) MAXITIME (sec)

Figure B.3: Event map, light curve Figure B.4: Event map, light curve and VETO count time variability of and VETO count time variability of SXT 091105 with log Pnull = −11.1. SXT 091115 with log Pnull = −9.1. Appendix B. Event map, light curve and VETO curve 95

-50 2.0 1.0

0.8 -60 1.5 100

0.6 -70 1.0 0.4 BEX (mm) BEX (mm) 80 -80 0.5 0.2 -90 0.0 60 0.0 311904600 311904800 311905000 311905200 313076900 313077000 313077100 MAXITIME (sec) MAXITIME (sec)

2.0 2.0

1.5 1.5

1.0 1.0 Count Count

0.5 0.5

0.0 0.0 311904600 311904800 311905000 311905200 313076900 313077000 313077100 MAXITIME (sec) MAXITIME (sec)

200 300

150 200 100 VETO count VETO count 100 50

0 0 311904600 311904800 311905000 311905200 313076900 313077000 313077100 MAXITIME (sec) MAXITIME (sec)

Figure B.5: Event map, light curve Figure B.6: Event map, light curve and VETO count time variability of and VETO count time variability of SXT 091119 with log Pnull = −9.2. SXT 091202 with log Pnull = −9.1.

2.0 1.0 120 -60 0.8 1.5

100 -70 0.6 1.0 -80 0.4 BEX (mm) BEX (mm) 80 0.5 -90 0.2

0.0 0.0 313047600 313047700 313285200 313285300 313285400 MAXITIME (sec) MAXITIME (sec)

2.0 2.0

1.5 1.5

1.0 1.0 Count Count

0.5 0.5

0.0 0.0 313047600 313047700 313285200 313285300 313285400 MAXITIME (sec) MAXITIME (sec)

400 200 300

200

VETO count VETO count 100

100

0 0 313047600 313047700 313285200 313285300 313285400 MAXITIME (sec) MAXITIME (sec)

Figure B.7: Event map, light curve Figure B.8: Event map, light curve and VETO count time variability of and VETO count time variability of SXT 091202 with log Pnull = −9.0. SXT 091204 with log Pnull = −9.4. Appendix B. Event map, light curve and VETO curve 96

2.0 1.0 -70

0.8 1.5 -80

100 0.6 -90 1.0 0.4 BEX (mm) BEX (mm) -100 0.5 80 0.2 -110

0.0 0.0 313323300 313323400 313937100 313937200 MAXITIME (sec) MAXITIME (sec)

2.0 2.0

1.5 1.5

1.0 1.0 Count Count

0.5 0.5

0.0 0.0 313323300 313323400 313937100 313937200 MAXITIME (sec) MAXITIME (sec)

200 300

150 200 100 VETO count VETO count 100 50

0 0 313323300 313323400 313937100 313937200 MAXITIME (sec) MAXITIME (sec)

Figure B.9: Event map, light curve Figure B.10: Event map, light curve and VETO count time variability of and VETO count time variability of SXT 091205 with log Pnull = −11.3. SXT 091212 with log Pnull = −9.3.

-40 1.0 2.0

0.8 -70 -50 1.5

0.6 -60 -80 1.0 0.4 BEX (mm) BEX (mm) -70 -90 0.5 0.2 -80 -100 0.0 0.0 314473300 314473400 314473500 315232600 315232700 315232800 MAXITIME (sec) MAXITIME (sec)

2.0 4

1.5 3

1.0 2 Count Count

0.5 1

0.0 0 314473300 314473400 314473500 315232600 315232700 315232800 MAXITIME (sec) MAXITIME (sec)

300 400

300 200

200

VETO count 100 VETO count 100

0 0 314473300 314473400 314473500 315232600 315232700 315232800 MAXITIME (sec) MAXITIME (sec)

Figure B.11: Event map, light curve Figure B.12: Event map, light curve and VETO count time variability of and VETO count time variability of SXT 091218 with log Pnull = −10.4. SXT 091227 with log Pnull = −9.8. Appendix B. Event map, light curve and VETO curve 97

2.0 110 1.0 -10

0.8 1.5 100 -20 90 0.6 1.0 -30 0.4 BEX (mm) BEX (mm) 80 0.5 -40 70 0.2

0.0 0.0 315407500 315407600 315407700 316299400 316299500 316299600 MAXITIME (sec) MAXITIME (sec)

4 2 Count Count 2 1

0 0 315407500 315407600 315407700 316299400 316299500 316299600 MAXITIME (sec) MAXITIME (sec)

300

200

100 VETO count VETO count 100

0 0 315407500 315407600 315407700 316299400 316299500 316299600 MAXITIME (sec) MAXITIME (sec)

Figure B.13: Event map, light curve Figure B.14: Event map, light curve and VETO count time variability of and VETO count time variability of SXT 091229 with log Pnull = −10.4. SXT 100108 with log Pnull = −9.1.

2.0 1.0 -40 90

0.8 1.5 -50 80 0.6 -60 1.0 70 0.4 BEX (mm) BEX (mm) 60 -70 0.5 0.2 50 -80 0.0 0.0 317319700 317319800 317319900 317539000 317539100 317539200 MAXITIME (sec) MAXITIME (sec)

3 3

2 2 Count Count

1 1

0 0 317319700 317319800 317319900 317539000 317539100 317539200 MAXITIME (sec) MAXITIME (sec)

300 200 200

VETO count VETO count 100 100

0 0 317319700 317319800 317319900 317539000 317539100 317539200 MAXITIME (sec) MAXITIME (sec)

Figure B.15: Event map, light curve Figure B.16: Event map, light curve and VETO count time variability of and VETO count time variability of SXT 100120 with log Pnull = −9.1. SXT 100123 with log Pnull = −16.1. Appendix B. Event map, light curve and VETO curve 98

-70 2.0 2.0 -50

-80 1.5 -60 1.5

-90 1.0 -70 1.0

BEX (mm) -100 BEX (mm) -80 0.5 0.5 -110 -90 0.0 0.0 318851200 318851300 318851400 319750200 319750300 319750400 MAXITIME (sec) MAXITIME (sec)

3 4

3 2

2 Count Count

1 1

0 0 318851200 318851300 318851400 319750200 319750300 319750400 MAXITIME (sec) MAXITIME (sec)

300 200

200 150

100 VETO count 100 VETO count 50

0 0 318851200 318851300 318851400 319750200 319750300 319750400 MAXITIME (sec) MAXITIME (sec)

Figure B.17: Event map, light curve Figure B.18: Event map, light curve and VETO count time variability of and VETO count time variability of SXT 100207 with log Pnull = −9.1. SXT 100217 with log Pnull = −9.6.

2.0 2.0 50 90

1.5 1.5 40 80

1.0 70 1.0 30 BEX (mm) BEX (mm) 60 0.5 0.5 20 50 0.0 0.0 319963800 319963900 319964000 320825300 320825400 320825500 MAXITIME (sec) MAXITIME (sec)

2.0 2.0

1.5 1.5

1.0 1.0 Count Count

0.5 0.5

0.0 0.0 319963800 319963900 319964000 320825300 320825400 320825500 MAXITIME (sec) MAXITIME (sec)

200 200

100 100 VETO count VETO count

0 0 319963800 319963900 319964000 320825300 320825400 320825500 MAXITIME (sec) MAXITIME (sec)

Figure B.19: Event map, light curve Figure B.20: Event map, light curve and VETO count time variability of and VETO count time variability of SXT 100220 with log Pnull = −9.2. SXT 100302 with log Pnull = −11.3. Appendix B. Event map, light curve and VETO curve 99

60 1.0 2.0

0.8 50 30 1.5

40 0.6 20 1.0 0.4 BEX (mm) 30 BEX (mm) 0.5 0.2 10 20

0.0 0.0 321996200 321996300 321996400 323024800 323024900 323025000 MAXITIME (sec) MAXITIME (sec)

4 3

2 Count Count 2

0 0 321996200 321996300 321996400 323024800 323024900 323025000 MAXITIME (sec) MAXITIME (sec)

300 300

200 200

VETO count 100 VETO count 100

0 0 321996200 321996300 321996400 323024800 323024900 323025000 MAXITIME (sec) MAXITIME (sec)

Figure B.21: Event map, light curve Figure B.22: Event map, light curve and VETO count time variability of and VETO count time variability of SXT 100315 with log Pnull = −10.0. SXT 100327 with log Pnull = −9.3.

2.0 1.0

50 -50 0.8 1.5

40 0.6 -60 1.0 0.4

BEX (mm) 30 BEX (mm) -70 0.5 20 0.2 -80 0.0 0.0 324618200 324618300 324618400 324886700 324886800 324886900 MAXITIME (sec) MAXITIME (sec)

6 3

4 2 Count Count

2 1

0 0 324618200 324618300 324618400 324886700 324886800 324886900 MAXITIME (sec) MAXITIME (sec)

200 200 150 150

100 100 VETO count VETO count

50 50

0 0 324618200 324618300 324618400 324886700 324886800 324886900 MAXITIME (sec) MAXITIME (sec)

Figure B.23: Event map, light curve Figure B.24: Event map, light curve and VETO count time variability of and VETO count time variability of SXT 100415 with log Pnull = −28.4. SXT 100418 with log Pnull = −9.3. Appendix B. Event map, light curve and VETO curve 100

2.0 2.0 120 -50 110 1.5 1.5 -60 100 1.0 1.0 -70 BEX (mm) BEX (mm) 90 0.5 0.5 -80 80 0.0 0.0 325015900 325016000 325105800 325105900 325106000 MAXITIME (sec) MAXITIME (sec)

4 2.0

3 1.5

2 1.0 Count Count

1 0.5

0 0.0 325015900 325016000 325105800 325105900 325106000 MAXITIME (sec) MAXITIME (sec)

300

200 200

100 VETO count VETO count 100

0 0 325015900 325016000 325105800 325105900 325106000 MAXITIME (sec) MAXITIME (sec)

Figure B.25: Event map, light curve Figure B.26: Event map, light curve and VETO count time variability of and VETO count time variability of SXT 100419 with log Pnull = −9.3. SXT 100420 with log Pnull = −9.4.

2.0 2.0

100 -70 1.5 1.5 90 -80

1.0 1.0 80 -90 BEX (mm) BEX (mm)

0.5 0.5 70 -100

60 0.0 -110 0.0 325487800 325487900 325488000 326735600 326735800 326736000 326736200 MAXITIME (sec) MAXITIME (sec)

3 3

2 2 Count Count

1 1

0 0 325487800 325487900 325488000 326735600 326735800 326736000 326736200 MAXITIME (sec) MAXITIME (sec)

400 400

300 300

200 200 VETO count VETO count

100 100

0 0 325487800 325487900 325488000 326735600 326735800 326736000 326736200 MAXITIME (sec) MAXITIME (sec)

Figure B.27: Event map, light curve Figure B.28: Event map, light curve and VETO count time variability of and VETO count time variability of SXT 100425 with log Pnull = −9.1. SXT 100509 with log Pnull = −10.5. Appendix B. Event map, light curve and VETO curve 101

5 90 1.0 40

4 80 0.8 30 3 70 0.6

20 2 0.4 BEX (mm) BEX (mm) 60

1 0.2 10 50

0 0.0 326834700 326834800 326834900 327312200 327312300 327312400 MAXITIME (sec) MAXITIME (sec)

2.0 15

1.5 10 1.0 Count Count

5 0.5

0 0.0 326834700 326834800 326834900 327312200 327312300 327312400 MAXITIME (sec) MAXITIME (sec)

300 400

300 200

200 VETO count VETO count 100 100

0 0 326834700 326834800 326834900 327312200 327312300 327312400 MAXITIME (sec) MAXITIME (sec)

Figure B.29: Event map, light curve Figure B.30: Event map, light curve and VETO count time variability of and VETO count time variability of SXT 100510 with log Pnull = −84.1. SXT 100516 with log Pnull = −9.9.

1.0 2.0 70 -20 0.8 1.5 60 0.6 -30 1.0 50 0.4 BEX (mm) BEX (mm) -40 0.5 40 0.2 -50 0.0 0.0 328011400 328011500 328011600 329560700 329560800 329560900 329561000 329561100 MAXITIME (sec) MAXITIME (sec)

2.0 3

1.5 2

1.0 Count Count

1 0.5

0.0 0 328011400 328011500 328011600 329560700 329560800 329560900 329561000 329561100 MAXITIME (sec) MAXITIME (sec)

300

200 200

VETO count 100 VETO count 100

0 0 328011400 328011500 328011600 329560700 329560800 329560900 329561000 329561100 MAXITIME (sec) MAXITIME (sec)

Figure B.31: Event map, light curve Figure B.32: Event map, light curve and VETO count time variability of and VETO count time variability of SXT 100524 with log Pnull = −12.2. SXT 100611 with log Pnull = −9.7. Appendix B. Event map, light curve and VETO curve 102

1.0 1.0 60

0.8 90 0.8 50 0.6 80 0.6

40 0.4 0.4 BEX (mm) BEX (mm) 70

30 0.2 0.2 60

0.0 0.0 329552500 329552600 329552700 331870600 331870700 MAXITIME (sec) MAXITIME (sec)

3 3

2 2 Count Count

1 1

0 0 329552500 329552600 329552700 331870600 331870700 MAXITIME (sec) MAXITIME (sec)

300

200

100 VETO count 100 VETO count

0 0 329552500 329552600 329552700 331870600 331870700 MAXITIME (sec) MAXITIME (sec)

Figure B.33: Event map, light curve Figure B.34: Event map, light curve and VETO count time variability of and VETO count time variability of SXT 100611 with log Pnull = −9.8. SXT 100708 with log Pnull = −9.1.

-40 1.0 -50 2.0

0.8 -50 -60 1.5

0.6 -60 -70 1.0 0.4 BEX (mm) BEX (mm) -80 -70 0.5 0.2 -90 -80 0.0 0.0 332282200 332282300 332282400 332957800 332957900 MAXITIME (sec) MAXITIME (sec)

2.0 3

1.5 2

1.0 Count Count

1 0.5

0.0 0 332282200 332282300 332282400 332957800 332957900 MAXITIME (sec) MAXITIME (sec)

200 200 150

100

VETO count VETO count 100

0 0 332282200 332282300 332282400 332957800 332957900 MAXITIME (sec) MAXITIME (sec)

Figure B.35: Event map, light curve Figure B.36: Event map, light curve and VETO count time variability of and VETO count time variability of SXT 100712 with log Pnull = −10.1. SXT 100720 with log Pnull = −9.4. Appendix B. Event map, light curve and VETO curve 103

1.0 3 50 0.8 -20

40 2 0.6 -30

30 0.4 BEX (mm) BEX (mm) -40 1 20 0.2 -50 10 0.0 0 334612100 334612200 334612300 334988100 334988200 334988300 334988400 334988500 MAXITIME (sec) MAXITIME (sec)

3 10

Count Count 5 1

0 0 334612100 334612200 334612300 334988100 334988200 334988300 334988400 334988500 MAXITIME (sec) MAXITIME (sec)

300 200

200

VETO count 100 VETO count 100

0 0 334612100 334612200 334612300 334988100 334988200 334988300 334988400 334988500 MAXITIME (sec) MAXITIME (sec)

Figure B.37: Event map, light curve Figure B.38: Event map, light curve and VETO count time variability of and VETO count time variability of SXT 100808 with log Pnull = −9.4. SXT 100813 with log Pnull = −37.6.

4 2.0 -70

3 1.5 -80 100

1.0 -90 2 BEX (mm) BEX (mm) 80 -100 1 0.5

-110 0 0.0 335899500 335899600 336421700 336421800 336421900 MAXITIME (sec) MAXITIME (sec)

3 10

Count 5 Count 1

0 0 335899500 335899600 336421700 336421800 336421900 MAXITIME (sec) MAXITIME (sec)

300 200

200

100 VETO count VETO count 100

0 0 335899500 335899600 336421700 336421800 336421900 MAXITIME (sec) MAXITIME (sec)

Figure B.39: Event map, light curve Figure B.40: Event map, light curve and VETO count time variability of and VETO count time variability of SXT 100823 with log Pnull = −58.4. SXT 100829 with log Pnull = −9.0. Appendix B. Event map, light curve and VETO curve 104

2.0 2.0

-30 -60 1.5 1.5

-40 -70 1.0 1.0 -80

BEX (mm) -50 BEX (mm) 0.5 0.5 -90 -60

0.0 0.0 337199000 337199100 337199200 340820100 340820200 MAXITIME (sec) MAXITIME (sec)

4 3

3 2

2 Count Count

1 1

0 0 337199000 337199100 337199200 340820100 340820200 MAXITIME (sec) MAXITIME (sec)

300

200 200

VETO count VETO count 100 100

0 0 337199000 337199100 337199200 340820100 340820200 MAXITIME (sec) MAXITIME (sec)

Figure B.41: Event map, light curve Figure B.42: Event map, light curve and VETO count time variability of and VETO count time variability of SXT 100907 with log Pnull = −11.2. SXT 101019 with log Pnull = −9.9.

2.0 2.0 60 60

1.5 1.5 50 50

1.0 40 1.0 40 BEX (mm) BEX (mm)

0.5 30 0.5 30

0.0 0.0 340905800 340905900 340906000 340906100 341298400 341298500 341298600 341298700 MAXITIME (sec) MAXITIME (sec)

3 3

2 2 Count Count

1 1

0 0 340905800 340905900 340906000 340906100 341298400 341298500 341298600 341298700 MAXITIME (sec) MAXITIME (sec)

400 400 300 300

200 200 VETO count VETO count

100 100

0 0 340905800 340905900 340906000 340906100 341298400 341298500 341298600 341298700 MAXITIME (sec) MAXITIME (sec)

Figure B.43: Event map, light curve Figure B.44: Event map, light curve and VETO count time variability of and VETO count time variability of SXT 101020 with log Pnull = −10.1. SXT 101025 with log Pnull = −9.2. Appendix B. Event map, light curve and VETO curve 105

2.0 2.0 -30 100

-40 1.5 90 1.5

-50 1.0 80 1.0 BEX (mm) BEX (mm) 70 -60 0.5 0.5

60 -70 0.0 0.0 341762000 341762200 342030500 342030600 342030700 MAXITIME (sec) MAXITIME (sec)

2.0 2.0

1.5 1.5

1.0 1.0 Count Count

0.5 0.5

0.0 0.0 341762000 341762200 342030500 342030600 342030700 MAXITIME (sec) MAXITIME (sec)

300

200 200

VETO count 100 VETO count 100

0 0 341762000 341762200 342030500 342030600 342030700 MAXITIME (sec) MAXITIME (sec)

Figure B.45: Event map, light curve Figure B.46: Event map, light curve and VETO count time variability of and VETO count time variability of SXT 101030 with log Pnull = −9.6. SXT 101102 with log Pnull = −9.0.

2.0 2.0 -40 120

1.5 1.5 110 -50

1.0 100 1.0 -60 BEX (mm) BEX (mm) 90 0.5 0.5 -70 80 0.0 0.0 342566600 342566700 342566800 342775000 342775100 342775200 MAXITIME (sec) MAXITIME (sec)

2.0 4

1.5 3

1.0 2 Count Count

0.5 1

0.0 0 342566600 342566700 342566800 342775000 342775100 342775200 MAXITIME (sec) MAXITIME (sec)

300

200 200

VETO count 100 VETO count 100

0 0 342566600 342566700 342566800 342775000 342775100 342775200 MAXITIME (sec) MAXITIME (sec)

Figure B.47: Event map, light curve Figure B.48: Event map, light curve and VETO count time variability of and VETO count time variability of SXT 101108 with log Pnull = −9.2. SXT 101111 with log Pnull = −15.2. Appendix B. Event map, light curve and VETO curve 106

2.0 2.0 60 -40 1.5 1.5 50 -50 1.0 1.0 40 BEX (mm) BEX (mm) -60 0.5 0.5 30 -70 0.0 0.0 345138500 345138600 345138700 346167500 346167600 346167700 MAXITIME (sec) MAXITIME (sec)

2.0 3

1.5 2

1.0 Count Count

1 0.5

0.0 0 345138500 345138600 345138700 346167500 346167600 346167700 MAXITIME (sec) MAXITIME (sec)

800

200 600

400 100 VETO count VETO count

200

0 0 345138500 345138600 345138700 346167500 346167600 346167700 MAXITIME (sec) MAXITIME (sec)

Figure B.49: Event map, light curve Figure B.50: Event map, light curve and VETO count time variability of and VETO count time variability of SXT 101208 with log Pnull = −10.9. SXT 101220 with log Pnull = −9.4.

2.0 -30 2.0

-60 1.5 -40 1.5

-70 1.0 -50 1.0

BEX (mm) -80 BEX (mm)

0.5 -60 0.5 -90

-70 0.0 0.0 347652400 347652500 347652600 347966200 347966300 347966400 MAXITIME (sec) MAXITIME (sec)

4 2 Count Count 2 1

0 0 347652400 347652500 347652600 347966200 347966300 347966400 MAXITIME (sec) MAXITIME (sec)

400

200 300 150 200 100 VETO count VETO count 100 50

0 0 347652400 347652500 347652600 347966200 347966300 347966400 MAXITIME (sec) MAXITIME (sec)

Figure B.51: Event map, light curve Figure B.52: Event map, light curve and VETO count time variability of and VETO count time variability of SXT 110106 with log Pnull = −9.7. SXT 110110 with log Pnull = −14.0. Appendix B. Event map, light curve and VETO curve 107

2.0 2.0

40 1.5 -30 1.5

30 1.0 -40 1.0 BEX (mm) BEX (mm) 20 0.5 -50 0.5

10 0.0 -60 0.0 348216000 348216100 348216200 348371700 348371800 348371900 MAXITIME (sec) MAXITIME (sec)

4 2 Count Count 2 1

0 0 348216000 348216100 348216200 348371700 348371800 348371900 MAXITIME (sec) MAXITIME (sec)

400

300 300

200 200 VETO count VETO count 100 100

0 0 348216000 348216100 348216200 348371700 348371800 348371900 MAXITIME (sec) MAXITIME (sec)

Figure B.53: Event map, light curve Figure B.54: Event map, light curve and VETO count time variability of and VETO count time variability of SXT 110113 with log Pnull = −19.2. SXT 110115 with log Pnull = −9.0.

70 1.0 1.0

0.8 100 0.8 60

0.6 90 0.6 50 0.4 0.4

BEX (mm) BEX (mm) 80 40 0.2 70 0.2 30 0.0 0.0 348523600 348523700 348523800 348616900 348617000 348617100 MAXITIME (sec) MAXITIME (sec)

2.0 4

1.5 3

1.0 2 Count Count

0.5 1

0.0 0 348523600 348523700 348523800 348616900 348617000 348617100 MAXITIME (sec) MAXITIME (sec)

300 400

300 200

200

VETO count VETO count 100 100

0 0 348523600 348523700 348523800 348616900 348617000 348617100 MAXITIME (sec) MAXITIME (sec)

Figure B.55: Event map, light curve Figure B.56: Event map, light curve and VETO count time variability of and VETO count time variability of SXT 110116 with log Pnull = −9.2. SXT 110117 with log Pnull = −9.7. Appendix B. Event map, light curve and VETO curve 108

2.0 2.0 80 70

1.5 60 1.5

60 1.0 50 1.0 BEX (mm) BEX (mm)

0.5 40 0.5 40

0.0 30 0.0 349052700 349052800 349052900 349091200 349091300 349091400 MAXITIME (sec) MAXITIME (sec)

3 2.0

1.5 2

1.0 Count Count

1 0.5

0 0.0 349052700 349052800 349052900 349091200 349091300 349091400 MAXITIME (sec) MAXITIME (sec)

200 300 150 200 100 VETO count VETO count 100 50

0 0 349052700 349052800 349052900 349091200 349091300 349091400 MAXITIME (sec) MAXITIME (sec)

Figure B.57: Event map, light curve Figure B.58: Event map, light curve and VETO count time variability of and VETO count time variability of SXT 110122 with log Pnull = −9.7. SXT 110123 with log Pnull = −9.1.

2.0 2.0 -20 -20 1.5 1.5 -30 -30 1.0 1.0 -40 BEX (mm) BEX (mm) -40 0.5 0.5 -50 -50 0.0 0.0 350595900 350596000 350596100 350596200 350596300 351051400 351051500 351051600 351051700 351051800 MAXITIME (sec) MAXITIME (sec)

2.0 4

1.5 3

1.0 2 Count Count

0.5 1

0.0 0 350595900 350596000 350596100 350596200 350596300 351051400 351051500 351051600 351051700 351051800 MAXITIME (sec) MAXITIME (sec)

300

200

VETO count 100 VETO count 100

0 0 350595900 350596000 350596100 350596200 350596300 351051400 351051500 351051600 351051700 351051800 MAXITIME (sec) MAXITIME (sec)

Figure B.59: Event map, light curve Figure B.60: Event map, light curve and VETO count time variability of and VETO count time variability of SXT 110209 with log Pnull = −9.3. SXT 110215 with log Pnull = −12.7. Appendix B. Event map, light curve and VETO curve 109

2.0 50 2.0

1.5 40 1.5 100 1.0 1.0 30 BEX (mm) BEX (mm)

0.5 0.5 80 20

0.0 0.0 351170800 351170900 351171000 351472100 351472200 MAXITIME (sec) MAXITIME (sec)

4 4

3 3

2 2 Count Count

1 1

0 0 351170800 351170900 351171000 351472100 351472200 MAXITIME (sec) MAXITIME (sec)

300 400

300 200

200 VETO count VETO count 100 100

0 0 351170800 351170900 351171000 351472100 351472200 MAXITIME (sec) MAXITIME (sec)

Figure B.61: Event map, light curve Figure B.62: Event map, light curve and VETO count time variability of and VETO count time variability of SXT 110216 with log Pnull = −10.8. SXT 110219 with log Pnull = −9.2.

2.0 2.0 -70 -30

1.5 -80 1.5 -40 1.0 -90 1.0

BEX (mm) -50 BEX (mm) -100 0.5 0.5

-60 -110 0.0 0.0 351634100 351634200 351634300 351922700 351922800 351922900 MAXITIME (sec) MAXITIME (sec)

4 2.0

3 1.5

2 1.0 Count Count

1 0.5

0 0.0 351634100 351634200 351634300 351922700 351922800 351922900 MAXITIME (sec) MAXITIME (sec)

300 300

200 200 VETO count VETO count 100 100

0 0 351634100 351634200 351634300 351922700 351922800 351922900 MAXITIME (sec) MAXITIME (sec)

Figure B.63: Event map, light curve Figure B.64: Event map, light curve and VETO count time variability of and VETO count time variability of SXT 110221 with log Pnull = −15.4. SXT 110225 with log Pnull = −9.1. Appendix B. Event map, light curve and VETO curve 110

2.0 3

100 1.5 70 2 90 60 1.0 80 BEX (mm) BEX (mm) 50 1 0.5 70 40 60 0.0 0 352935000 352935100 352935200 353328000 353328100 353328200 MAXITIME (sec) MAXITIME (sec)

6 4

3 4

2 Count Count

2 1

0 0 352935000 352935100 352935200 353328000 353328100 353328200 MAXITIME (sec) MAXITIME (sec)

300

200 200

VETO count 100 VETO count 100

0 0 352935000 352935100 352935200 353328000 353328100 353328200 MAXITIME (sec) MAXITIME (sec)

Figure B.65: Event map, light curve Figure B.66: Event map, light curve and VETO count time variability of and VETO count time variability of SXT 110308 with log Pnull = −16.6. SXT 110313 with log Pnull = −9.2.

1.0 2.0

40 0.8 40 1.5

30 0.6 30 1.0 0.4 BEX (mm) BEX (mm) 20 20 0.5 0.2

10 0.0 10 0.0 355436500 355436550 355436600 356226300 356226400 356226500 MAXITIME (sec) MAXITIME (sec)

3 3

2 2 Count Count

1 1

0 0 355436500 355436550 355436600 356226300 356226400 356226500 MAXITIME (sec) MAXITIME (sec)

300

200 200

VETO count 100 VETO count 100

0 0 355436500 355436550 355436600 356226300 356226400 356226500 MAXITIME (sec) MAXITIME (sec)

Figure B.67: Event map, light curve Figure B.68: Event map, light curve and VETO count time variability of and VETO count time variability of SXT 110406 with log Pnull = −10.2. SXT 110415 with log Pnull = −9.0. Appendix B. Event map, light curve and VETO curve 111

2.0 3

40 -60 1.5 -70 2 30 1.0 -80 BEX (mm) 20 BEX (mm) 1 0.5 -90 10 0.0 -100 0 356858100 356858200 356858300 356858400 357145600 357145700 357145800 MAXITIME (sec) MAXITIME (sec)

2.0 6

1.5 4

1.0 Count Count

2 0.5

0.0 0 356858100 356858200 356858300 356858400 357145600 357145700 357145800 MAXITIME (sec) MAXITIME (sec)

200 200

100 100 VETO count VETO count

0 0 356858100 356858200 356858300 356858400 357145600 357145700 357145800 MAXITIME (sec) MAXITIME (sec)

Figure B.69: Event map, light curve Figure B.70: Event map, light curve and VETO count time variability of and VETO count time variability of SXT 110423 with log Pnull = −9.1. SXT 110426 with log Pnull = −12.2.

2.0 -70 1.0 -60

-80 0.8 1.5 -70

-90 0.6 -80 1.0 0.4 BEX (mm) BEX (mm) -100 -90 0.5 0.2 -110 -100 0.0 0.0 357913200 357913300 357913400 359654700 359654800 359654900 MAXITIME (sec) MAXITIME (sec)

4 3

3 2

2 Count Count

1 1

0 0 357913200 357913300 357913400 359654700 359654800 359654900 MAXITIME (sec) MAXITIME (sec)

300 200

150 200

100 VETO count VETO count 100 50

0 0 357913200 357913300 357913400 359654700 359654800 359654900 MAXITIME (sec) MAXITIME (sec)

Figure B.71: Event map, light curve Figure B.72: Event map, light curve and VETO count time variability of and VETO count time variability of SXT 110505 with log Pnull = −10.5. SXT 110525 with log Pnull = −10.1. Appendix B. Event map, light curve and VETO curve 112

2.0 1.0 120 60 0.8 1.5

100 0.6 50 1.0 0.4 BEX (mm) BEX (mm) 40 80 0.5 0.2

30 0.0 0.0 359648600 359648700 359648800 359906200 359906300 359906400 MAXITIME (sec) MAXITIME (sec)

2.0 2.0

1.5 1.5

1.0 1.0 Count Count

0.5 0.5

0.0 0.0 359648600 359648700 359648800 359906200 359906300 359906400 MAXITIME (sec) MAXITIME (sec)

200 200

150 150

100 100 VETO count VETO count

50 50

0 0 359648600 359648700 359648800 359906200 359906300 359906400 MAXITIME (sec) MAXITIME (sec)

Figure B.73: Event map, light curve Figure B.74: Event map, light curve and VETO count time variability of and VETO count time variability of SXT 110525 with log Pnull = −9.1. SXT 110528 with log Pnull = −9.6.

2.0 2.0 50 70

1.5 1.5 40 60

1.0 1.0 30 50 BEX (mm) BEX (mm)

0.5 0.5 20 40

0.0 0.0 360783900 360784000 360784100 361693500 361693600 361693700 MAXITIME (sec) MAXITIME (sec)

2.0 4

1.5 3

1.0 2 Count Count

0.5 1

0.0 0 360783900 360784000 360784100 361693500 361693600 361693700 MAXITIME (sec) MAXITIME (sec)

300

300 200

200

VETO count VETO count 100 100

0 0 360783900 360784000 360784100 361693500 361693600 361693700 MAXITIME (sec) MAXITIME (sec)

Figure B.75: Event map, light curve Figure B.76: Event map, light curve and VETO count time variability of and VETO count time variability of SXT 110607 with log Pnull = −9.4. SXT 110618 with log Pnull = −9.7. Appendix B. Event map, light curve and VETO curve 113

2.0 2.0 100 -10 1.5 1.5

80 1.0 -20 1.0 BEX (mm) BEX (mm)

0.5 -30 0.5 60

0.0 0.0 361887100 361887200 361887300 362190600 362190700 362190800 362190900 MAXITIME (sec) MAXITIME (sec)

4 2.0

3 1.5

2 1.0 Count Count

1 0.5

0 0.0 361887100 361887200 361887300 362190600 362190700 362190800 362190900 MAXITIME (sec) MAXITIME (sec)

200 200

100 VETO count 100 VETO count

0 0 361887100 361887200 361887300 362190600 362190700 362190800 362190900 MAXITIME (sec) MAXITIME (sec)

Figure B.77: Event map, light curve Figure B.78: Event map, light curve and VETO count time variability of and VETO count time variability of SXT 110620 with log Pnull = −9.0. SXT 110624 with log Pnull = −9.3.

2.0 -10 2.0

-40 1.5 -20 1.5

-50 1.0 1.0 -30 BEX (mm) BEX (mm)

-60 0.5 0.5 -40

-70 0.0 0.0 362279200 362279300 362279400 363117400 363117500 363117600 MAXITIME (sec) MAXITIME (sec)

3 3

2 2 Count Count

1 1

0 0 362279200 362279300 362279400 363117400 363117500 363117600 MAXITIME (sec) MAXITIME (sec)

300

200 200

VETO count 100 VETO count 100

0 0 362279200 362279300 362279400 363117400 363117500 363117600 MAXITIME (sec) MAXITIME (sec)

Figure B.79: Event map, light curve Figure B.80: Event map, light curve and VETO count time variability of and VETO count time variability of SXT 110625 with log Pnull = −9.1. SXT 110704 with log Pnull = −11.2. Appendix B. Event map, light curve and VETO curve 114

1.0 2.0 -70 80 0.8 1.5 -80 70 0.6 1.0 -90 60 0.4 BEX (mm) BEX (mm)

-100 50 0.5 0.2

-110 40 0.0 0.0 365032500 365032600 365032700 365881500 365881600 MAXITIME (sec) MAXITIME (sec)

2.0 3

1.5 2

1.0 Count Count

1 0.5

0.0 0 365032500 365032600 365032700 365881500 365881600 MAXITIME (sec) MAXITIME (sec)

300 200

200

VETO count 100 VETO count 100

0 0 365032500 365032600 365032700 365881500 365881600 MAXITIME (sec) MAXITIME (sec)

Figure B.81: Event map, light curve Figure B.82: Event map, light curve and VETO count time variability of and VETO count time variability of SXT 110726 with log Pnull = −10.2. SXT 110805 with log Pnull = −9.2.

1.0 90 2.0

-30 0.8 80 1.5

0.6 -40 70 1.0 0.4 BEX (mm) BEX (mm) 60 -50 0.5 0.2

50 -60 0.0 0.0 367657800 367657900 367658000 367708100 367708200 367708300 MAXITIME (sec) MAXITIME (sec)

4 3

3 2

2 Count Count

1 1

0 0 367657800 367657900 367658000 367708100 367708200 367708300 MAXITIME (sec) MAXITIME (sec)

400

300 300

200 200 VETO count VETO count 100 100

0 0 367657800 367657900 367658000 367708100 367708200 367708300 MAXITIME (sec) MAXITIME (sec)

Figure B.83: Event map, light curve Figure B.84: Event map, light curve and VETO count time variability of and VETO count time variability of SXT 110826 with log Pnull = −9.4. SXT 110826 with log Pnull = −10.3. Appendix B. Event map, light curve and VETO curve 115

2.0 -40 1.0 100 0.8 1.5 -50 90 0.6 -60 80 1.0 0.4 BEX (mm) BEX (mm) 70 -70 0.5 0.2

60 -80 0.0 0.0 367882100 367882200 367882300 368094800 368094900 368095000 MAXITIME (sec) MAXITIME (sec)

4 2.0

3 1.5

2 1.0 Count Count

1 0.5

0 0.0 367882100 367882200 367882300 368094800 368094900 368095000 MAXITIME (sec) MAXITIME (sec)

200 200

VETO count 100 VETO count 100

0 0 367882100 367882200 367882300 368094800 368094900 368095000 MAXITIME (sec) MAXITIME (sec)

Figure B.85: Event map, light curve Figure B.86: Event map, light curve and VETO count time variability of and VETO count time variability of SXT 110828 with log Pnull = −10.9. SXT 110831 with log Pnull = −9.2.

-30 2.0 2.0

-10 -40 1.5 1.5

-50 1.0 -20 1.0 BEX (mm) BEX (mm)

-60 0.5 -30 0.5

-70 0.0 -40 0.0 368300200 368300300 368300400 368712200 368712300 368712400 MAXITIME (sec) MAXITIME (sec)

4 4

3 3

2 2 Count Count

1 1

0 0 368300200 368300300 368300400 368712200 368712300 368712400 MAXITIME (sec) MAXITIME (sec)

400

200 300

200

VETO count VETO count 100 100

0 0 368300200 368300300 368300400 368712200 368712300 368712400 MAXITIME (sec) MAXITIME (sec)

Figure B.87: Event map, light curve Figure B.88: Event map, light curve and VETO count time variability of and VETO count time variability of SXT 110902 with log Pnull = −14.1. SXT 110907 with log Pnull = −9.8. Appendix B. Event map, light curve and VETO curve 116

1.0 1.0 70

0.8 -70 0.8 60

0.6 -80 0.6 50 0.4 0.4 BEX (mm) BEX (mm) -90 40 0.2 0.2 -100 30 0.0 0.0 370213500 370213600 370213700 371142900 371143000 371143100 MAXITIME (sec) MAXITIME (sec)

3 3

2 2 Count Count

1 1

0 0 370213500 370213600 370213700 371142900 371143000 371143100 MAXITIME (sec) MAXITIME (sec)

200 200

VETO count 100 VETO count 100

0 0 370213500 370213600 370213700 371142900 371143000 371143100 MAXITIME (sec) MAXITIME (sec)

Figure B.89: Event map, light curve Figure B.90: Event map, light curve and VETO count time variability of and VETO count time variability of SXT 110924 with log Pnull = −9.3. SXT 111005 with log Pnull = −13.3.

1.0 1.0 -80 110 0.8 0.8 -90 100 0.6 0.6 -100 90 0.4 0.4 BEX (mm) BEX (mm) -110 80 0.2 0.2 -120 70 0.0 0.0 371521500 371521600 371521700 371987000 371987100 371987200 MAXITIME (sec) MAXITIME (sec)

3 4

3 2

2 Count Count

1 1

0 0 371521500 371521600 371521700 371987000 371987100 371987200 MAXITIME (sec) MAXITIME (sec)

200 200

150

100 100 VETO count VETO count

0 0 371521500 371521600 371521700 371987000 371987100 371987200 MAXITIME (sec) MAXITIME (sec)

Figure B.91: Event map, light curve Figure B.92: Event map, light curve and VETO count time variability of and VETO count time variability of SXT 111010 with log Pnull = −9.4. SXT 111015 with log Pnull = −9.2. Appendix B. Event map, light curve and VETO curve 117

1.0 4

40 0.8 80 3

0.6 30 2 0.4 60 BEX (mm) BEX (mm) 20 1 0.2

10 40 0.0 0 372484400 372484500 372484600 372484700 372756000 372756100 372756200 MAXITIME (sec) MAXITIME (sec)

3 20

15 2

Count Count 10 1 5

0 0 372484400 372484500 372484600 372484700 372756000 372756100 372756200 MAXITIME (sec) MAXITIME (sec)

300 200

200

100 VETO count 100 VETO count

0 0 372484400 372484500 372484600 372484700 372756000 372756100 372756200 MAXITIME (sec) MAXITIME (sec)

Figure B.93: Event map, light curve Figure B.94: Event map, light curve and VETO count time variability of and VETO count time variability of SXT 111021 with log Pnull = −9.5. SXT 111024 with log Pnull = −144.6.

2.0 2.0 100 70

1.5 90 1.5 60

80 50 1.0 1.0 BEX (mm) BEX (mm) 70 40 0.5 0.5

60 30 0.0 0.0 373932000 373932100 373932200 374510600 374510700 374510800 MAXITIME (sec) MAXITIME (sec)

4 3

3 2

2 Count Count

1 1

0 0 373932000 373932100 373932200 374510600 374510700 374510800 MAXITIME (sec) MAXITIME (sec)

300

200 200

VETO count 100 VETO count 100

0 0 373932000 373932100 373932200 374510600 374510700 374510800 MAXITIME (sec) MAXITIME (sec)

Figure B.95: Event map, light curve Figure B.96: Event map, light curve and VETO count time variability of and VETO count time variability of SXT 111106 with log Pnull = −11.1. SXT 111113 with log Pnull = −10.2. Appendix B. Event map, light curve and VETO curve 118

2.0 50 1.0 -10

0.8 1.5 40 -20 0.6 1.0 30 -30 0.4 BEX (mm) BEX (mm)

0.5 20 0.2 -40 0.0 0.0 375166900 375167000 375167100 376311100 376311200 376311300 MAXITIME (sec) MAXITIME (sec)

3 2.0

1.5 2

1.0 Count Count

1 0.5

0 0.0 375166900 375167000 375167100 376311100 376311200 376311300 MAXITIME (sec) MAXITIME (sec)

200 200

100 VETO count 100 VETO count

0 0 375166900 375167000 375167100 376311100 376311200 376311300 MAXITIME (sec) MAXITIME (sec)

Figure B.97: Event map, light curve Figure B.98: Event map, light curve and VETO count time variability of and VETO count time variability of SXT 111121 with log Pnull = −9.5. SXT 111204 with log Pnull = −9.4.

2.0 80 2.0 40

1.5 70 1.5 30

1.0 60 1.0 20 BEX (mm) BEX (mm) 50 0.5 0.5 10

40 0.0 0.0 376981600 376981700 376981800 376969700 376969800 376969900 MAXITIME (sec) MAXITIME (sec)

3 2.0

1.5 2

1.0 Count Count

1 0.5

0 0.0 376981600 376981700 376981800 376969700 376969800 376969900 MAXITIME (sec) MAXITIME (sec)

300

200

VETO count VETO count 100 100

0 0 376981600 376981700 376981800 376969700 376969800 376969900 MAXITIME (sec) MAXITIME (sec)

Figure B.99: Event map, light curve Figure B.100: Event map, light curve and VETO count time variability of and VETO count time variability of SXT 111212 with log Pnull = −10.2. SXT 111212 with log Pnull = −9.5. Appendix B. Event map, light curve and VETO curve 119

2.0 2.0

-40 80 1.5 1.5 -50 70

1.0 1.0 60 -60 BEX (mm) BEX (mm)

0.5 50 0.5 -70

0.0 40 0.0 378462900 378463000 378463100 378457300 378457400 378457500 MAXITIME (sec) MAXITIME (sec)

3 4

3 2

2 Count Count

1 1

0 0 378462900 378463000 378463100 378457300 378457400 378457500 MAXITIME (sec) MAXITIME (sec)

300

200 200

VETO count 100 VETO count 100

0 0 378462900 378463000 378463100 378457300 378457400 378457500 MAXITIME (sec) MAXITIME (sec)

Figure B.101: Event map, light curve Figure B.102: Event map, light curve and VETO count time variability of and VETO count time variability of SXT 111229 with log Pnull = −10.8. SXT 111229 with log Pnull = −22.7.

2.0 2.0 -10 -20 1.5 1.5 -20 -30 1.0 1.0 -30 BEX (mm) BEX (mm) -40 0.5 0.5 -40 -50 0.0 0.0 378552000 378552100 378552200 378552300 378552400 380587400 380587500 380587600 MAXITIME (sec) MAXITIME (sec)

4 4

3 3

2 2 Count Count

1 1

0 0 378552000 378552100 378552200 378552300 378552400 380587400 380587500 380587600 MAXITIME (sec) MAXITIME (sec)

300 200

200

100 VETO count VETO count 100

0 0 378552000 378552100 378552200 378552300 378552400 380587400 380587500 380587600 MAXITIME (sec) MAXITIME (sec)

Figure B.103: Event map, light curve Figure B.104: Event map, light curve and VETO count time variability of and VETO count time variability of SXT 111230 with log Pnull = −16.0. SXT 120122 with log Pnull = −10.2. Appendix B. Event map, light curve and VETO curve 120

40 2.0 2.0

80 30 1.5 1.5

1.0 1.0 20 60 BEX (mm) BEX (mm)

0.5 0.5 10

40 0.0 0.0 381301300 381301400 381301500 381301600 381507500 381507600 381507700 MAXITIME (sec) MAXITIME (sec)

2.0 3

1.5 2

1.0 Count Count

1 0.5

0.0 0 381301300 381301400 381301500 381301600 381507500 381507600 381507700 MAXITIME (sec) MAXITIME (sec)

300 300

200 200 VETO count VETO count 100 100

0 0 381301300 381301400 381301500 381301600 381507500 381507600 381507700 MAXITIME (sec) MAXITIME (sec)

Figure B.105: Event map, light curve Figure B.106: Event map, light curve and VETO count time variability of and VETO count time variability of SXT 120131 with log Pnull = −9.9. SXT 120202 with log Pnull = −9.2.

2.0 2.0

-60 -50 1.5 1.5

-70 -60 1.0 1.0

BEX (mm) -80 BEX (mm) -70 0.5 0.5

-90 -80 0.0 0.0 381922700 381922800 381922900 381996500 381996600 381996700 MAXITIME (sec) MAXITIME (sec)

3 4

3 2

2 Count Count

1 1

0 0 381922700 381922800 381922900 381996500 381996600 381996700 MAXITIME (sec) MAXITIME (sec)

300 300

200 200 VETO count VETO count 100 100

0 0 381922700 381922800 381922900 381996500 381996600 381996700 MAXITIME (sec) MAXITIME (sec)

Figure B.107: Event map, light curve Figure B.108: Event map, light curve and VETO count time variability of and VETO count time variability of SXT 120207 with log Pnull = −9.1. SXT 120208 with log Pnull = −10.9. Appendix B. Event map, light curve and VETO curve 121

2.0 1.0 -10 -30 0.8 1.5 -20 0.6 -40 1.0 -30 0.4 BEX (mm) BEX (mm) -50 0.5 -40 0.2

-60 0.0 0.0 382442800 382442900 382443000 384439500 384439600 384439700 MAXITIME (sec) MAXITIME (sec)

4 2 Count Count 2 1

0 0 382442800 382442900 382443000 384439500 384439600 384439700 MAXITIME (sec) MAXITIME (sec)

200 200

100 VETO count VETO count 100

0 0 382442800 382442900 382443000 384439500 384439600 384439700 MAXITIME (sec) MAXITIME (sec)

Figure B.109: Event map, light curve Figure B.110: Event map, light curve and VETO count time variability of and VETO count time variability of SXT 120213 with log Pnull = −9.7. SXT 120307 with log Pnull = −9.3.

2.0 2.0

-40 80 1.5 1.5

-50 70 1.0 1.0

BEX (mm) BEX (mm) 60 -60 0.5 0.5 50 -70 0.0 0.0 384677300 384677400 384677500 384682800 384682900 384683000 MAXITIME (sec) MAXITIME (sec)

4 4

3 3

2 2 Count Count

1 1

0 0 384677300 384677400 384677500 384682800 384682900 384683000 MAXITIME (sec) MAXITIME (sec)

200 200

150

100 100 VETO count VETO count

0 0 384677300 384677400 384677500 384682800 384682900 384683000 MAXITIME (sec) MAXITIME (sec)

Figure B.111: Event map, light curve Figure B.112: Event map, light curve and VETO count time variability of and VETO count time variability of SXT 120310 with log Pnull = −14.6. SXT 120310 with log Pnull = −12.0. Appendix B. Event map, light curve and VETO curve 122

90 2.0 70 2.0

80 1.5 60 1.5

70 1.0 50 1.0 BEX (mm) BEX (mm) 60 40 0.5 0.5

50 30 0.0 0.0 384688400 384688500 384688600 385989500 385989600 385989700 MAXITIME (sec) MAXITIME (sec)

2 4 Count Count

1 2

0 0 384688400 384688500 384688600 385989500 385989600 385989700 MAXITIME (sec) MAXITIME (sec)

200

200 150

100

VETO count VETO count 100 50

0 0 384688400 384688500 384688600 385989500 385989600 385989700 MAXITIME (sec) MAXITIME (sec)

Figure B.113: Event map, light curve Figure B.114: Event map, light curve and VETO count time variability of and VETO count time variability of SXT 120310 with log Pnull = −13.5. SXT 120325 with log Pnull = −9.4.

2.0 1.0 -60

-50 0.8 1.5 -70

-60 0.6 1.0 -80 0.4 BEX (mm) -70 BEX (mm) -90 0.5 0.2 -80 -100 0.0 0.0 387919900 387920000 387920100 388172200 388172300 388172400 MAXITIME (sec) MAXITIME (sec)

2.0 3

1.5 2

1.0 Count Count

1 0.5

0.0 0 387919900 387920000 387920100 388172200 388172300 388172400 MAXITIME (sec) MAXITIME (sec)

300 200

150 200

100 VETO count VETO count 100 50

0 0 387919900 387920000 387920100 388172200 388172300 388172400 MAXITIME (sec) MAXITIME (sec)

Figure B.115: Event map, light curve Figure B.116: Event map, light curve and VETO count time variability of and VETO count time variability of SXT 120416 with log Pnull = −9.7. SXT 120419 with log Pnull = −9.9. Appendix B. Event map, light curve and VETO curve 123

2.0 2.0 110

-30 1.5 100 1.5

90 -40 1.0 1.0

BEX (mm) BEX (mm) 80 -50 0.5 0.5 70 -60 0.0 0.0 388601200 388601300 388601400 390587000 390587100 390587200 MAXITIME (sec) MAXITIME (sec)

6 4

3 4

2 Count Count

2 1

0 0 388601200 388601300 388601400 390587000 390587100 390587200 MAXITIME (sec) MAXITIME (sec)

300

200 200

VETO count 100 VETO count 100

0 0 388601200 388601300 388601400 390587000 390587100 390587200 MAXITIME (sec) MAXITIME (sec)

Figure B.117: Event map, light curve Figure B.118: Event map, light curve and VETO count time variability of and VETO count time variability of SXT 120424 with log Pnull = −20.9. SXT 120517 with log Pnull = −11.2.

2.0 110 2.0

80 100 1.5 1.5 70 90 1.0 1.0 60

BEX (mm) BEX (mm) 80

50 0.5 0.5 70

40 0.0 0.0 391566600 391566700 391566800 392113500 392113600 392113700 MAXITIME (sec) MAXITIME (sec)

3 3

2 2 Count Count

1 1

0 0 391566600 391566700 391566800 392113500 392113600 392113700 MAXITIME (sec) MAXITIME (sec)

300

200

100 VETO count VETO count 100

0 0 391566600 391566700 391566800 392113500 392113600 392113700 MAXITIME (sec) MAXITIME (sec)

Figure B.119: Event map, light curve Figure B.120: Event map, light curve and VETO count time variability of and VETO count time variability of SXT 120529 with log Pnull = −9.0. SXT 120604 with log Pnull = −10.0. Appendix B. Event map, light curve and VETO curve 124

2.0 90 2.0

100 1.5 80 1.5

70 80 1.0 1.0

BEX (mm) BEX (mm) 60 0.5 0.5 60 50

0.0 0.0 392453900 392454000 392454100 393411500 393411600 393411700 MAXITIME (sec) MAXITIME (sec)

4 2 Count Count 2 1

0 0 392453900 392454000 392454100 393411500 393411600 393411700 MAXITIME (sec) MAXITIME (sec)

300

200

100 VETO count 100 VETO count

0 0 392453900 392454000 392454100 393411500 393411600 393411700 MAXITIME (sec) MAXITIME (sec)

Figure B.121: Event map, light curve Figure B.122: Event map, light curve and VETO count time variability of and VETO count time variability of SXT 120608 with log Pnull = −10.0. SXT 120619 with log Pnull = −9.7.

2.0 2.0 80 -30 1.5 1.5 70 -40

1.0 60 1.0

BEX (mm) -50 BEX (mm)

0.5 50 0.5 -60

0.0 40 0.0 393650400 393650600 394427400 394427500 394427600 MAXITIME (sec) MAXITIME (sec)

4 2 Count Count 2 1

0 0 393650400 393650600 394427400 394427500 394427600 MAXITIME (sec) MAXITIME (sec)

300 200

200 100 VETO count VETO count 100

0 0 393650400 393650600 394427400 394427500 394427600 MAXITIME (sec) MAXITIME (sec)

Figure B.123: Event map, light curve Figure B.124: Event map, light curve and VETO count time variability of and VETO count time variability of SXT 120622 with log Pnull = −11.3. SXT 120701 with log Pnull = −10.2. Appendix B. Event map, light curve and VETO curve 125

1.0 2.0

-60 0.8 -20 1.5

-70 0.6 -30 1.0 0.4 BEX (mm) -80 BEX (mm) -40 0.5 0.2 -90

0.0 -50 0.0 394510400 394510500 394510600 394504300 394504400 394504500 394504600 394504700 MAXITIME (sec) MAXITIME (sec)

2.0 4

1.5 3

1.0 2 Count Count

0.5 1

0.0 0 394510400 394510500 394510600 394504300 394504400 394504500 394504600 394504700 MAXITIME (sec) MAXITIME (sec)

300 150

100 200 VETO count VETO count 50 100

0 0 394510400 394510500 394510600 394504300 394504400 394504500 394504600 394504700 MAXITIME (sec) MAXITIME (sec)

Figure B.125: Event map, light curve Figure B.126: Event map, light curve and VETO count time variability of and VETO count time variability of SXT 120702 with log Pnull = −9.7. SXT 120702 with log Pnull = −14.1.

-40 3 3

60 -50 2 2

-60 50

BEX (mm) 1 BEX (mm) 1 -70 40

-80 30 0 0 397368800 397368900 397369000 397448900 397449000 397449100 MAXITIME (sec) MAXITIME (sec)

8 8

6 6

Count 4 Count 4

2 2

0 0 397368800 397368900 397369000 397448900 397449000 397449100 MAXITIME (sec) MAXITIME (sec)

400

200 300

200 100 VETO count VETO count 100

0 0 397368800 397368900 397369000 397448900 397449000 397449100 MAXITIME (sec) MAXITIME (sec)

Figure B.127: Event map, light curve Figure B.128: Event map, light curve and VETO count time variability of and VETO count time variability of SXT 120804 with log Pnull = −13.0. SXT 120805 with log Pnull = −47.6. Appendix B. Event map, light curve and VETO curve 126

-50 2.0 2.0 -20 -60 1.5 1.5 -30 -70 1.0 1.0 -40 BEX (mm) -80 BEX (mm) 0.5 0.5 -50 -90 0.0 0.0 397913600 397913800 397914000 397914200 402082500 402082600 402082700 MAXITIME (sec) MAXITIME (sec)

4 2.0

3 1.5

2 1.0 Count Count

1 0.5

0 0.0 397913600 397913800 397914000 397914200 402082500 402082600 402082700 MAXITIME (sec) MAXITIME (sec)

400

200 300

200 100 VETO count VETO count 100

0 0 397913600 397913800 397914000 397914200 402082500 402082600 402082700 MAXITIME (sec) MAXITIME (sec)

Figure B.129: Event map, light curve Figure B.130: Event map, light curve and VETO count time variability of and VETO count time variability of SXT 120810 with log Pnull = −10.2. SXT 120927 with log Pnull = −9.5.

2.0 40 2.0

100 1.5 30 1.5

1.0 1.0 20 80 BEX (mm) BEX (mm)

0.5 0.5 10

60 0.0 0.0 403268900 403269000 403269100 403547000 403547100 403547200 403547300 403547400 MAXITIME (sec) MAXITIME (sec)

3 2.0

1.5 2

1.0 Count Count

1 0.5

0 0.0 403268900 403269000 403269100 403547000 403547100 403547200 403547300 403547400 MAXITIME (sec) MAXITIME (sec)

300

200 200

100 VETO count 100 VETO count

0 0 403268900 403269000 403269100 403547000 403547100 403547200 403547300 403547400 MAXITIME (sec) MAXITIME (sec)

Figure B.131: Event map, light curve Figure B.132: Event map, light curve and VETO count time variability of and VETO count time variability of SXT 121011 with log Pnull = −9.1. SXT 121014 with log Pnull = −9.3. Appendix B. Event map, light curve and VETO curve 127

2.0 2.0 60 90

1.5 1.5 50 80

1.0 70 1.0 40 BEX (mm) BEX (mm) 60 0.5 0.5 30

50 0.0 0.0 404640700 404640800 404640900 405314700 405314800 405314900 MAXITIME (sec) MAXITIME (sec)

3 3

2 2 Count Count

1 1

0 0 404640700 404640800 404640900 405314700 405314800 405314900 MAXITIME (sec) MAXITIME (sec)

300

200 200

VETO count 100 VETO count 100

0 0 404640700 404640800 404640900 405314700 405314800 405314900 MAXITIME (sec) MAXITIME (sec)

Figure B.133: Event map, light curve Figure B.134: Event map, light curve and VETO count time variability of and VETO count time variability of SXT 121027 with log Pnull = −15.1. SXT 121104 with log Pnull = −9.4.

-70 1.0 2.0 -40 0.8 -80 1.5 -50 0.6 -90 -60 1.0 0.4 BEX (mm) -100 BEX (mm) -70 0.5 0.2 -110 0.0 -80 0.0 409136800 409136900 409137000 410416800 410417000 410417200 MAXITIME (sec) MAXITIME (sec)

2 4 Count Count

1 2

0 0 409136800 409136900 409137000 410416800 410417000 410417200 MAXITIME (sec) MAXITIME (sec)

300 200

200

VETO count VETO count 100 100

0 0 409136800 409136900 409137000 410416800 410417000 410417200 MAXITIME (sec) MAXITIME (sec)

Figure B.135: Event map, light curve Figure B.136: Event map, light curve and VETO count time variability of and VETO count time variability of SXT 121218 with log Pnull = −9.7. SXT 130102 with log Pnull = −57.8. Appendix B. Event map, light curve and VETO curve 128

-10 2.0 1.0

-60 0.8 -20 1.5

-70 0.6 -30 1.0 0.4 BEX (mm) BEX (mm) -80 -40 0.5 0.2 -90 0.0 0.0 410789400 410789500 410789600 410789700 410789800 412241600 412241700 412241800 MAXITIME (sec) MAXITIME (sec)

3 3

2 2 Count Count

1 1

0 0 410789400 410789500 410789600 410789700 410789800 412241600 412241700 412241800 MAXITIME (sec) MAXITIME (sec)

200 200 150

100 100 VETO count VETO count

0 0 410789400 410789500 410789600 410789700 410789800 412241600 412241700 412241800 MAXITIME (sec) MAXITIME (sec)

Figure B.137: Event map, light curve Figure B.138: Event map, light curve and VETO count time variability of and VETO count time variability of SXT 130106 with log Pnull = −14.6. SXT 130123 with log Pnull = −10.1.

2.0 2.0 120 -10 1.5 1.5

-20 100 1.0 1.0

BEX (mm) BEX (mm) -30 0.5 0.5 80 -40 0.0 0.0 413443200 413443300 413443400 415159400 415159500 415159600 415159700 415159800 MAXITIME (sec) MAXITIME (sec)

3 4

3 2

2 Count Count

1 1

0 0 413443200 413443300 413443400 415159400 415159500 415159600 415159700 415159800 MAXITIME (sec) MAXITIME (sec)

400

300 200

200

VETO count 100 VETO count 100

0 0 413443200 413443300 413443400 415159400 415159500 415159600 415159700 415159800 MAXITIME (sec) MAXITIME (sec)

Figure B.139: Event map, light curve Figure B.140: Event map, light curve and VETO count time variability of and VETO count time variability of SXT 130206 with log Pnull = −9.5. SXT 130226 with log Pnull = −12.5. Appendix B. Event map, light curve and VETO curve 129

1.0 2.0 120 0.8 -70 1.5 110

0.6 -80 100 1.0 0.4 BEX (mm) BEX (mm) -90 90 0.5 0.2 80 -100 0.0 0.0 416235200 416235300 416235400 417058000 417058100 MAXITIME (sec) MAXITIME (sec)

3 3

2 2 Count Count

1 1

0 0 416235200 416235300 416235400 417058000 417058100 MAXITIME (sec) MAXITIME (sec)

300

200 200

100 VETO count VETO count 100

0 0 416235200 416235300 416235400 417058000 417058100 MAXITIME (sec) MAXITIME (sec)

Figure B.141: Event map, light curve Figure B.142: Event map, light curve and VETO count time variability of and VETO count time variability of SXT 130310 with log Pnull = −9.4. SXT 130320 with log Pnull = −9.3.

2.0 2.0

90 -20 1.5 1.5 80

1.0 1.0 -30 70 BEX (mm) BEX (mm)

0.5 0.5 60 -40

0.0 50 0.0 417533600 417533700 417533800 418692900 418693000 418693100 MAXITIME (sec) MAXITIME (sec)

2.0 3

1.5 2

1.0 Count Count

1 0.5

0.0 0 417533600 417533700 417533800 418692900 418693000 418693100 MAXITIME (sec) MAXITIME (sec)

300

200

150 200

100 VETO count VETO count 100 50

0 0 417533600 417533700 417533800 418692900 418693000 418693100 MAXITIME (sec) MAXITIME (sec)

Figure B.143: Event map, light curve Figure B.144: Event map, light curve and VETO count time variability of and VETO count time variability of SXT 130325 with log Pnull = −10.2. SXT 130407 with log Pnull = −14.8. Appendix B. Event map, light curve and VETO curve 130

1.0 2.0 -60 0.8 -50 1.5 -70 0.6 -60 1.0 -80 0.4 BEX (mm) BEX (mm) -70 -90 0.5 0.2 -80 -100 0.0 0.0 419561400 419561500 422377100 422377200 MAXITIME (sec) MAXITIME (sec)

2.0 2.0

1.5 1.5

1.0 1.0 Count Count

0.5 0.5

0.0 0.0 419561400 419561500 422377100 422377200 MAXITIME (sec) MAXITIME (sec)

800 300 600 200 400 VETO count VETO count 100 200

0 0 419561400 419561500 422377100 422377200 MAXITIME (sec) MAXITIME (sec)

Figure B.145: Event map, light curve Figure B.146: Event map, light curve and VETO count time variability of and VETO count time variability of SXT 130418 with log Pnull = −9.1. SXT 130520 with log Pnull = −9.1. Appendix C

List of Detected Transients

131 Appendix A. List of Detected Transients 132 § 111024A (C:+0.00049, 0.04) 100510A (C:+0.00038, 0.30) 100823A (A:+0.00046, 0.19), 100823A (C:+0.00046, 0.19) J052038.3-394517(9) J052038.3-394517(9) J013500.7-295430(9), 120424A (C:+0.00050, 0.38) PKS 0336-01(5), SDSS J03359-0024(8) Counterpart ‡ HR 20 − † 8 f 8 − † 2 f 10 − † 4 f ∗ null P List of detected short X-ray transients and their possible counterparts. ) ◦ ( ) Table C.1: ◦ ( R.A. Dec. (UT) Time Name 111024 2011-10-24 07:22:09100510 221.97 2010-05-10 +25.87 19:27:42 -144.57100823 355.43 2010-08-23 1.197 17:26:15 -35.65 2.713130102 20.51 -84.07 2013-01-02120805 0.441 04:42:29 0.729 +5.82 2012-08-05 309.07 02:30:31 0.163 1.271 -58.37100813 2MASX -72.36 78.55 J14530794+2554327(1), PKS 0.674 1441+25(5), 2010-08-13 0.639 TEX 04:18:46 1451+270(8), -57.77 -40.25 0.530 1.399100415 78.50 0.439 J234154.4-355838(9), -47.65 2010-04-15111229 100510810 0.300 03:45:16 (B:+0.00042, 1.29), 0.780 -39.92 2011-12-29120424 0.533 07:04:08 0.215 7.50 0.228 -37.62 2012-04-24 0.859 16:47:57 HD 54.71 8357(2), UV Psc(3), 0.292 -16.40110113 0.313 0.220 NGC 6934(6), 24.42 +0.44 2MAXI 2011-01-13 0.492110308 J2043-714(4), 06:41:51 -28.40 0.256 2FGL J2042.8-7317(5), -29.86 130102B 2011-03-08 (C:+0.00054, -22.67100123 353.27 0.166 21:32:15 0.16) CXO 0.328 J051406.4-400238(1), 4U 0513-40(2, -20.93 2010-01-23111230 +19.81 0.299 7), 116.50 05:18:35 0.337 0.577 NGC1851(4), 2011-12-30 0.516091012 0.271 09:23:16 CXO -19.20 +3.41 J051406.4-400238(1), 54.20 0.194 4U 0513-40(2, 2009-10-12 0.523110221 7), 0.127 10:26:08 NGC1851(4), 0.247 38.68 0.337 -16.63 +0.35 2011-02-21101111 181.38 0.066 20:10:58 0.246 0.378 J002606.8-154217(9), -43.41 0.287 100415A 2010-11-11 -16.13 (C:+0.00022,121027 +63.08 310.43 07:19:06 0.83) HR 0.127 1099(2, 0.116 1.055 3, 4, -16.00 2012-10-27130407 0.243 278.15 7), 08:13:07 BB -15.78 -31.90 SDSS Scl(3), J03440-0000(8) 0.308 2QZ 0.127 J014542-3031(8), 2013-04-07 0.395120310 0.160 +22.83 23:37:24 0.211 63.38 EQ -15.35 Peg(2, 2012-03-10 0.329 0.120 3) 248.23 0.094 06:50:06 -15.17 0.262 -58.54 0.204 YZ130106 +10.18 0.026 CMi(2) 0.239 0.228 53.95 0.109 0.410 -15.11 2013-01-06110902 12:13:12 HR 0.078 -14.75 0.404 1099(2, 0.415 3, 0.038 4, 2011-09-02 -0.09120702 0.193 324.29 7), 17:38:14 CC SDSS 0.216 Eri(2, 0.092 J03440-0000(8) SDSS 3), J11558+6135(8), RXC 0.094 2012-07-02 0.364 091012 J0232.2-4420(4), 312.43 00:35:09 (C:+0.00204, -14.61 RBS 0.311 -77.84 0.71) 0318(5), 0.227 J023423.0-434739(9) AT Mic(2, 355.77 0.224 7), -32.14 0.208 2MAXI 0.098 -14.57 J2037-306(4), J204151.2-322604(9) +17.72 0.614 0.248 -14.15 0.315 0.160 1H -14.12 0419-577(7), 0.064 130407A 0.195 J041155.1-580154(9), 0.203 (C:+0.00027, 121027A 0.35) (A:+0.02822, 0.31) 0.156 0.257 0.281 0.109 0.500 HR 1099(2, 0.096 0.557 3, 4, 7), 0.006 J205919.2-781048(9) 0.341 0.011 AT Mic(2, 7), 2MAXI J2043-324(4), J204151.2-322604(9) Appendix A. List of Detected Transients 133 § J052038.3-394517(9), HD35650(10) J164527.9-250319(9) J052038.3-394517(9) 1H 1914-194(5), TEX 1914-193(8) PKS 0336-01(5), SDSS J03440-0000(8) Counterpart ‡ HR (Continued) 20 − † 8 f 8 − † 2 Table C.1: f 10 − † 4 f ∗ null P ) ◦ ( ) ◦ ( R.A. Dec. (UT) Time Name 110110 2011-01-10 09:18:45120310 79.11 2012-03-10111005 09:54:49 -40.22 2011-10-05120804 15:17:34 54.61 -14.03 2012-08-04 236.35 04:15:03 +0.49110215 0.217 251.61 -35.21 2011-02-15 0.362 -13.48130226 02:20:33 -23.31 -13.34 2013-02-26110426 0.223 192.91 0.106 02:07:12 -13.02 0.164 2011-04-26 0.408100524 15:08:55 0.294 -16.81 54.03 0.168 0.191 2010-05-24120310 221.02 0.004 10:18:05 CXO J051406.4-400238(1), -12.65 0.275 4U +1.06 0.098 0513-40(2, 2012-03-10120622 7), 106.34 08:22:24 NGC1851(4), 0.010 -10.46 0.136 0.064 0.580 2012-06-22 -12.53091205 +33.66 03:22:34 HR 54.19 1099(2, -12.19 0.505 3, J154513.6-341733(9) 4, 0.231 2009-12-05100302 0.152 206.24 7), 10:16:12 -12.16 SDSS J03440-0000(8) 0.186 +0.21 0.026 MAXI 2010-03-02 0.279110704 J1647-227(2), 313.96 06:10:59 2MAXI 0.137 J1647-229(4), 0.384 -1.95 2FGL 0.052 2011-07-04 J1645.7-2148c(5), -11.95120517 146.49 0.019 17:58:34 0.117 +1.29 0.061 RXC -11.31 2012-05-17100907 +34.03 0.194 J1257.1-1724(4), 200.71 16:25:54 A1644(7) 0.068 0.047 -11.29 0.158 2010-09-07 0.429111106 0.154 +84.41 253.68 18:25:49 HR -11.27 1099(2, 0.403 3, 0.149 110426629 4, 2011-11-06 0.361091105 (B:+0.00172, 161.17 7), 0.079 22:02:15 -11.24 3.35), SDSS 0.168 110426A J03359-0024(8) 0.119 -7.76 (C:+0.00022, 2009-11-05 0.36) 0.043 19:46:46 0.185 0.112 -33.97 0.111 28.19 0.063 -11.24110828 HR 0.119 0.094 78.13 1099(2, 0.091 -11.17 -64.26 3, 4, 2011-08-28 0.532101208 0.215 7), 21:30:37 SDSS SDSS J13406-0148(8), 0.044 J03440-0000(8) 0.809 120622A -40.56 0.180 (C:+0.00056, 1RXS -11.08 2010-12-08 0.308120208 0.84) 167.43 J205528.2-002123(5), 15:50:38 SDSS J20549+0015(8) SDSS 0.306 0.469 J09476+3453(8) -11.06 2012-02-08 0.153 332.63 0.021 06:10:24 S5 -5.44 0.041 1322+83(5) 0.132110216 0.123 291.13 0.070 -62.74 -10.92 0.134 2011-02-16 0.200111229 0.082 11:28:47 2FGL -18.39 J1656.4-0738(5) -10.87 J104230.3-334014(9) 2011-12-29 0.176 106.52 0.054 08:36:34 0.619 -10.87 0.131 0.089110505 +31.85 J020212.0-623532(9) 0.268 54.10 0.124 0.148 2011-05-05100509 0.159 12:22:07 CXO -10.78 J051406.4-400238(1), 0.182 4U 0.069 0513-40(2, 2010-05-09 -0.21091229 7), 169.39 15:58:48 1.782 NGC1851(4), 0.208 0.060 0.559 2009-12-29091218 185.88 13:13:23 2MAXI -10.75 0.183 J1108-056(4), HD96064(10) -3.73 J222312.0-623555(9) 0.330 2009-12-18 17:44:08 -44.37 0.155 47.36 0.054 SWIFT -10.53 J1922.7-1716(1, 234.29 0.230 2, +14.06 4), -10.51 0.296 0.148 0.035 -10.41 -3.86 0.097 0.235 0.150 0.099 0.136 -10.35 0.076 HR 0.110 1099(2, 0.034 3, 0.125 4, 0.324 7), 0.079 0.348 0.171 2FGL 0.717 J1214.1-4410(5), 0.044 PKS 1224-443(8) 0.260 RXC J1540.1-0318(4) Appendix A. List of Detected Transients 134 1919B(10) § J075407.7-654137(9) J225725.9-564544(9) RXS J02375-3603(8), J023126.4-375831(9) Counterpart ‡ HR (Continued) 20 − † 8 f 8 − † 2 Table C.1: f 10 − † 4 f ∗ null P ) ◦ ( ) ◦ ( R.A. Dec. (UT) Time Name 110826 2011-08-26 21:10:41120810 114.25 2012-08-10110726 11:38:46 -66.51 2011-07-26111212 319.60 21:56:59 -10.34 2011-12-12120122 05:08:08 -11.50 87.21 0.173 2012-01-22111113 359.22 22:45:02 +83.64 -10.25 0.173 2011-11-13120701 +28.91 232.79 14:44:43 -10.24 0.132 0.096 2012-07-01130325 +67.47 213.05 03:12:07 -10.23 0.126 0.130 0.552 2013-03-25110406 13:35:51 -10.23 -10.71 0.138 0.100 2.37 0.023 EXO 2011-04-06101020 0748-676(1, 252.94 20:22:43 2, 0.343 0.129 7), -10.21 PMN 0.047 0.179 2010-10-20 J0730-6602(5), -25.41 +65.63 241.91 16:06:02 0.131 0.005 0.100 NVSS 0.468130123 +15.54 J212035-125443(5), 346.18 -10.18 -10.18 RXS 0.050 J21231-1036(8), 0.092 0.015 J211909.5-124642(9) S5 2013-01-23110525 0454+84(8) 07:34:58 -10.15 -58.22 0.152 0.130 2MASX 0.379 0.043 J00032742+2739173(1), 2011-05-25100712 II 16:06:43 Peg(2, 0.174 0.104 0.103 3, -10.12 7), 8.70 2MAXI 0.472 2010-07-12120608 227.97 J2354+286(4) 20:38:10 0.116 0.065 0.041 0.115 2MASX 2012-06-08 -32.12100315 J14165001-1158577(1), 179.63 07:00:19 1RXS +9.47 0.031 J141650.6-115845(7) 0.374 0.119 0.394 2010-03-15120604 +74.33 352.71 19:24:43 -10.10 RBS -10.07 0.266 0016(5), 0.072 2012-06-04 NGC120131 +38.13 159.32 08:27:22 288(6), -10.05 0.132 RXS 2MAXI J00085-2339(8) 0.116 J1602+161(4), 0.602 2012-01-31101019 2FGL +32.70 215.12 05:04:41 J1612.0+1403(5), 0.111 -10.02 0.112 A2147(7), 0.118 PKS 2MASX 1604+159(8) 2010-10-19120419 +15.20 J23013626-5913210(1), 163.32 16:16:12 2MASX 0.106 0.169 0.028 J23013626-5913210(4), -9.96 CP 0.055 2012-04-19 Tuc(7), 100516 337.38 17:38:06 0.137 +4.09 0.035 0.251 -9.95 0.148 2010-05-16 0.464110907 310.33 08:05:33 -12.13 0.035 J003308.1-320117(9) 0.329 0.111 0.198 2FGL 2011-09-07 -9.94100611 J1506.9+1052(5) 12:04:43 -30.93 Mrk 0.257 0.167 2.88 205(1, 0.115 -9.92 2010-06-11091227 4, 0.098 122.89 06:24:27 7), AS V455 Dra(3) And(2), 0.091 -9.87 2009-12-27 1.031 0.116 -50.80100611 0.140 NVSS 352.98 12:39:05 J232914+375414(5) +8.53 SDSS 2010-06-11 0.144 0.542 0.142 +40.95 209.61 J10421+3254(8) 0.049 08:41:50 -9.86 0.214 -9.85110117 +52.37 0.053 0.424 37.97 -9.84 0.135 2011-01-17121218 0.142 0.004 22:03:34 RXS 0.365 J10561+0252(8) 0.077 -9.80 -36.66 0.124 2012-12-18 0.116110122 313.04 09:08:15 Mrk 0.018 915(1, 7), 0.109 0.139 0.094 PKS 2011-01-22120416 2223-114(8) 171.77 0.059 23:07:10 AT -9.74 -14.73 Mic(2, 7), 0.132 2MAXI 0.040 1.220 2012-04-16 J2037-306(4), 19:33:52 J204151.2-322604(9) 0.508 -12.25 0.089 6.64 0.004 1RXS -9.74 0.369 J002159.2-514028(5), 1WGA 0.105 AM 98.65 J0814.3+0858(8), 1(6), BD+07 +29.18 RXS B3 -9.74 J00141-5022(8) 0.028 0.095 2322+396(2, 8), +65.67 0.043 1ES 2321+419(5) SDSS 0.104 0.135 J14067+5308(8) -9.72 0.404 0.039 -9.69 0.010 0.156 IC 1816(1), PKS 0.138 0.087 0227-369(2, 0.093 0.085 5), 0.162 2.235 0.100 0.027 2MASX J11315154-1231587(1) 1.172 0.168 RXC J0020.6+2840(4), RBS 0042(5), VII RXS Zw J00184+2947(8) 073(1) Appendix A. List of Detected Transients 135 § J163635.0-753603(9) A3814(7), 2QZ J215200-2823(8), J214023.7-280433(9) Counterpart ‡ HR (Continued) 20 − † 8 f 8 − † 2 Table C.1: f 10 − † 4 f ∗ null P ) ◦ ( ) ◦ ( R.A. Dec. (UT) Time Name 120619 2012-06-19120702 08:59:40 2012-07-02110106 207.17 02:15:48 2011-01-06120213 +40.13 18:08:11 11.20 2012-02-13110618 10:08:17 56.77 -9.69 -62.98 2011-06-18 06:26:57 33.57 0.174 -1.84101030 242.17 -9.69 0.213 2010-10-30 -8.75100217 13:55:11 -76.54 0.110 -9.67 0.053 2010-02-17110528 19:32:04 0.068 29.19 -9.66 0.139 -9.65 2011-05-28 0.250130206 110.19 13:59:14 0.061 0.096 -35.68 0.148 NGC 2013-02-06120927 0.104 +26.90 196.76 5290(1), 05:22:27 RGB J1341+399(2), 0.894 0.178 0.087 RXS 2012-09-27 0.135 J13410+3959(8) 111121 307.94 17:36:52 -9.64 -18.26 2FGL -9.57 J0048.8-6347(5), 0.051 0.907 2011-11-21 J003452.2-615506(9) 111021 350.92 0.024 05:03:45 -56.42 0.105 0.127 QSO -9.57 0.286 2011-10-21111212 B0336-0156(2), 03:55:46 0.176 0.087 PKS +3.96 0336-01(5), 0.150 SDSS 4.54 ARP -9.52 2011-12-12 J03440-0000(8),111010 0.107 318(1), 323.98 01:50:04 HIP17695(10) RXC PKS J1601.7-7544(4), 0.041 0214-085(5), PKS SDSS 0.001 1610-77(5), 2011-10-10 0.092 -9.51 J02195-0750(8) -38.46111204 0.217 +24.36 CIZA 00:27:13 J1601.7-7544(7), 46.59 0.474 2011-12-04 0.218 0.009100420 0.134 0.073 10:54:07 +30.11 ESO -9.50 -9.47 354- 7.09 V340 2010-04-20 G 0.086100808 156.90 0.069 Gem(3), 19:11:19 004(1), 0.791 FIRST J020118.8-370457(9) J07247+2621(8) 0.120 +18.81 0.121 2010-08-08 -9.47121104 +60.35 310.70 0.051 19:50:49 0.315 0.089 0.098 2012-11-04 0.105 329.96 03:26:42 2FGL -9.43 0.590 +5.76 J2039.8-5620(5), J203313.7-580625(9) -9.43 0.086 0.028 0.090130310 +29.24 326.99 NGC 0.150 7679(1) 0.095 0.967 2013-03-10 -9.42 0.283100720 0.040 12:55:36 0.205 -28.72 -9.41 0.116 PKS 2FGL 2010-07-20110826 0.117 0010-401(5), 179.80 J2132.5+2605(5), 16:17:56 0.450 NGC HIP106231(10) 0.048 1851(6), 0.148 PKS 0.036 -9.41 2011-08-26 0010-401(8) 0.106091204 07:12:20 -77.99 77.43 0.232 0.134 2009-12-04 0.314110607 0.165 0.056 23:41:35 61.35 -15.70 0.074 S4 -9.40 2011-06-07 0.159101220 314.40 1030+61(2, 17:47:13 0.530 5), FG -20.02 Uma(3), 2010-12-20 0.555 TEX120325 0.157 260.37 0.066 13:39:46 1015+594(8) -9.39 -22.50 2012-03-25 0.203110924 +12.43 11:20:52 -9.39 0.416 0.131 78.38 -0.020 -9.38 2011-09-24091212 330.84 21:07:42 PKS 0.116 0.121 +73.65 2149-306(1, -9.38 4, -0.098 5), 2009-12-12100418 0.114 QSO 287.18 12:46:17 0.150 B2149-306(2), +1.16 0.016 0.098 J115941.8-760130(9) 2010-04-18 0.111 -9.37121014 06:21:02 -70.15 47.01 0.046 0.135 0.114 2012-10-14 -9.35 0.131 0.033 16:27:29 70.30 0.303 -72.12 0.038 -9.34 0.113 0.151 124.91 0.298 -24.86 0.336 0.181 0.142 +73.55 0.047 J210417.6-221011(9) -9.33 QSO 0.120 B1722+119(2), 0.025 -9.33 0.413 0.120 PKS 1725+123(5), -9.32 H 1722+119(8) 0.085 GB6 0.143 0.138 0.141 J0516+7350(5) 0.081 PMN 0.707 0.155 0.039 J2206-0031(5), 0.112 J220028.6+011259(9) 2FGL J1902.7-7053(5) 0.056 0.274 0.024 ESO 0.362 031- 0.216 G 008(1, 4), PMN J032736.1-725044(9) J0434-2342(5), 2MAXI RXS J0825+733(4), J04402-2459(8) Z Cam(7) Appendix A. List of Detected Transients 136 § S5 2023+760(5), S5 2023+76(8) Counterpart ‡ HR (Continued) 20 − † 8 f 8 − † 2 Table C.1: f 10 − † 4 f ∗ null P ) ◦ ( ) ◦ ( R.A. Dec. (UT) Time Name 100419 2010-04-19120307 18:12:35 2012-03-07110209 261.20 12:47:31 2011-02-09091119 +24.15 184.68 19:48:48 2009-11-19100327 226.36 00:15:30 -22.37 -9.31 2010-03-27110624 17:08:43 -25.52 98.82 0.151 -9.29 2011-06-24110805 345.93 00:32:52 +68.45 0.169 -9.27 2011-08-05130320 0.105 +42.93 17:46:09 11.35 0.010 2013-03-20 0.082 -9.25100220 0.101 154.00 01:27:56 -9.25 -36.85 2010-02-20 0.099 0.057 101108 0.114 0.055 06:51:17 -21.45 95.29 0.133 2010-11-08 0.126 179.08 0.023 21:25:07 -9.25 0.669 0.285 -19.94 -9.25120202 +61.43 308.06 0.048 UV 0.234 0.124 Crv(3), J121814.7-205028(9) 0.039 2012-02-02110219 0.141 +74.62 14:20:05 2FGL -9.25 0.152 0.381 J1513.5-2546(5) -9.24 2011-02-19 0.231 0.138110831 23:09:31 0.126 95.39 0.061 -9.24 0.122 -0.020 PSR 2011-08-31110116 131.32 J2302+4442(5), 08:35:42 0.091 100327 +51.56 (C:+0.00032, 0.398 0.115 0.125 0.08) -0.085 2011-01-16101025 +44.48 20:09:21 0.074 PKS 0.058 0042-373(5), 4.59 0.062 2010-10-25 -9.23 Pyxis(6) 111015 342.06 05:09:27 0.815 -9.22 0.107 2011-10-15 0.540 -61.26110313 +24.53 0.161 09:44:53 36.82 PKS 0.118 0627-199(5, SBS 2011-03-13 1.834 8), 0.117100425 1200+608(5), 10:41:25 SDSS 14.16 J11558+6135(8) -9.22 -9.20 0.089 4C 2010-04-25 -4.07110423 +74.26(1, 0.097 05:18:17 4, 23.20 7), -15.21 0.131 0.141 VW 0.049 2011-04-23100207 Cep(2), 256.83 07:17:25 0.827 -9.19 0.140 0.135 2010-02-07 0.550 -2.58100108 +24.41 122.17 09:48:24 -9.18 0.113 0.089 0.058 US 2010-01-08091014 235.52 1889(8) 20:59:02 0.109 -9.16 0.155 -9.14 -1.09 0.635 2009-10-14 0.427091115 +19.39 338.61 14:08:14 0.156 0.100 0.149 0.013 J002406.7-621100(9) BZB 2009-11-15100120 134.00 J2255+2410(5) 01:51:19 -0.003 -9.13 0.107 -9.12 0.194 -7.45 0.085 2010-01-20110525 +46.60 107.68 -0.020 16:23:29 0.127 0.134 0.064 0.045 2011-05-25130520 14:25:31 -9.11 0.149 -71.00 48.44 -9.10 0.057 0.600 2013-05-20 0.234100708 306.83 14:59:38 0.170 +12.87 0.001 0.092 0.145 2MASX 4U -9.10 2010-07-08120207 J01344565-0430134(1), 163.57 1700+24(1, 02:17:45 0.179 FIRST -14.89 2, J01271-0151(8) 4, 0.006 0.093 7) 2012-02-07 -9.09 2.559121011 0.115 227.19 09:40:02 -25.53 0.072 2MASX -0.001 J08045299-0108476(1), 3C -9.09 2012-10-11 0.150 PKS110123 +28.34 0.133 223.57 323.1(1) 11:11:04 B0802-010(5) -0.007 0.399 -0.005 -9.08 2011-01-23 0.134 0.116 +65.16 09:48:28 28.89 NGC PKS -9.07 2712(1), 2227-088(1, -0.031 S4 0.140 5), 0.126 312.31 0859+47(5) 0.131 QSO +36.53 B2227-0848(2) -9.07 0.133 0.087 0.020 0.977 -67.36 0.157 0.134 -9.07 0.113 0.142 0.133 0.053 -9.06 0.207 J202817.1-134829(9) 1.295 0.041 0.341 0.211 0.085 NGC 3393(1), SHBL J10468-2535(8), RBS 0.063 0.305 J105604.3-265309(9) 1467(5), 0.056 RXS J15087+2709(8) EK 0.075 Dra(2), 0.265 RXS J14514+6354(8), HD129333(10) XX 1.184 Tri(3), RXC J0152.7+3609(4) Appendix A. List of Detected Transients 137 -GBM Fermi -BAT GRB, (B) Swift § Counterpart ‡ HR (Continued) 20 − † 8 f 8 − † 2 Table C.1: f 10 − † 4 f ∗ null P ) ◦ ( ) ◦ ( R.A. Dec. in 4–10 keV, 2–8 keV and 8–20 keV 1 − s 2 − (UT) Time Logarithim of null hypothesis probability Unit of count cm Hardness ratio between the ﬂuxes in the 8–20 keV and 2–8 keV energy bands. Possible counterparts. Value in parentheses means catalogGSC name 37-month, including (5) the Fermi-LAT source; 2nd, (1) (6) Swift-BAT Globular 70-month,GRB, (2) Cluster, (C) Swift-BAT (7) transient GSC RXTE, monitor GRB. list, (8) (3) Two VERON CAB, (blazar valuesGRB (4) only), following with (9) the the SACY alphabetical unit , index of (10) of day NYMG, the (A) and GRB degree, catalog respectively. are temporally and positionally diﬀerence between the transient and the Name 110225 2011-02-25130418 04:20:19 2013-04-18091202 195.97 00:51:11 2009-12-02110625 +67.50 325.78 13:50:24 2011-06-25100829 01:08:42 -21.09 59.48 -9.06 2010-08-29110620 18:30:01 17.66 -57.39 0.134 -9.06 2011-06-20101102 12:13:49 10.36 0.095 -29.00 2010-11-02120529 0.131 16:30:41 -9.05 +11.26 73.74 0.090 2012-05-29 0.103091202 00:31:35 -9.05 0.108 -58.04 4.14 2009-12-02 -9.04 0.956 110415 122.53 0.055 05:41:18 0.111 0.116 2011-04-15 -56.24110115 0.172 162.85 23:46:39 -9.04 0.535 0.100 -63.98 0.009∗ 2011-01-15 0.205 +54.45 182.73 01:56:43 RBS 0.175 1769(5), 0.055 -9.02 0.084 RXS† J21388-2053(8) -9.02 0.089 0.189 90.36 J041155.1-580154(9) 0.549 -9.01 -0.18 0.097‡ 0.173 0.433 0.061 NGC 0.111 -54.07 0.139 6715(6),§ 0.165 2QZ J011332-2956(8), BK -9.01 J011240.6-291041(9) Psc(3), 0.322 0.088 GB6 0.057 J0045+1217(5), 0.103 Ko -9.00 1(6), 0.095 RBS J004401.5+093302(9) 594(1), 0.073 0.511 2MAXI J0451-585(4), 0.625 0.142 0.128 J050046.6-571511(9) PMN 0.827 J0019-5641(5), 2MAXI 0.085 NGC J0827-642(4), 0.061 1261(6) 2FGL J0807.0-6511(5), 1ES J082141.7-634045(9) 1044+549(8) 0.133 0.430 PKS 1.568 1215-002(8) TY Pic(3) Bibliography

[1] Ambruster, C. W., & Wood, K. S. 1986, ApJ, 311, 258

[2] Areﬁev, V. A., Priedhorsky, W. C., & Borozdin, K. N. 2003, ApJ, 586, 1238

[3] Bartoli, B., Bernardini, P., Bi, X. J., et al. 2012, ApJ, 758, 2

[4] Baumgartner, W. H., Tueller, J., Markwardt, C. B., et al. 2013, ApJS, 207, 19

[5] Bloom, J. S., Giannios, D., Metzger, B. D., et al. 2011, Science, 333, 203

[6] Briggs, M. S., Paciesas, W. S., Pendleton, G. N., et al. 1996, ApJ, 459, 40

[7] Burrows, D. N., Kennea, J. A., Ghisellini, G., et al. 2011, Nature, 476, 421

[8] Castro-Tirado, A. J., Brandt, S., Lund, N., & Sunyaev, R. 1999, A&A, 347, 927

[9] Colgate, S. A. 1974, ApJ, 187, 333

[10] Connors, A., Serlemitsos, P. J., & Swank, J. H. 1986, ApJ, 303, 769

[11] Costa, E., Frontera, F., Heise, J., et al. 1997, Nature, 387, 783

[12] Djorgovski, S. G., Metzger, M. R., Kulkarni, S. R., et al. 1997, Nature, 387, 876

[13] Eker, Z., Ak, N. F., Bilir, S., et al. 2008, MNRAS, 389, 1722

[14] Evans, C. R., & Kochanek, C. S. 1989, ApJL, 346, L13

[15] Fiocchi, M., Bazzano, A., Natalucci, L., Landi, R., & Ubertini, P. 2011, MNRAS, 414, L41

[16] Forman, W., Jones, C., Cominsky, L., et al. 1978, ApJS, 38, 357

[17] Galama, T. J., Vreeswijk, P. M., van Paradijs, J., et al. 1998, Nature, 395, 670

[18] Galloway, D. K., Muno, M. P., Hartman, J. M., Psaltis, D., & Chakrabarty, D. 2008, ApJS, 179, 360 138 Bibliography 139

[19] Giacconi, R., Gursky, H., Paolini, F. R., & Rossi, B. B. 1962, Physical Review Letters, 9, 439

[20] Giacconi, R., Branduardi, G., Briel, U., et al. 1979, ApJ, 230, 540

[21] Gotthelf, E. V., Hamilton, T. T., & Helfand, D. J. 1996, ApJ, 466, 779

[22] Greiner, J., Hartmann, D. H., Voges, W., et al. 2000, A&A, 353, 998

[23] Hall, D. S. 1976, in Astrophysics and Space Science Library, Vol. 60, IAU Colloq. 29: Multiple Periodic Variable Stars, ed. W. S. Fitch, 287

[24] Halpern, J. P., Gezari, S., & Komossa, S. 2004, ApJ, 604, 572

[25] Harris, W. E. 1996, AJ, 112, 1487

[26] Hartmann, D., & Epstein, R. I. 1989, ApJ, 346, 960

[27] Heise, J., Zand, J. I., Kippen, R. M., & Woods, P. M. 2001, in Gamma-ray Bursts in the Afterglow Era, ed. E. Costa, F. Frontera, & J. Hjorth, 16

[28] Hiroi, K., Ueda, Y., Hayashida, M., et al. 2013, ApJS, 207, 36

[29] Hjorth, J., Sollerman, J., Møller, P., et al. 2003, Nature, 423, 847

[30] Hoﬀman, J. A., Lewin, W. H. G., Doty, J., et al. 1978, ApJL, 221, L57

[31] Isobe, N., Sugimori, K., Kawai, N., et al. 2010, PASJ, 62, L55

[32] Kalamkar, M., Homan, J., Altamirano, D., et al. 2011, ApJL, 731, L2

[33] Kataoka, J., Takahashi, T., Wagner, S. J., et al. 2001, ApJ, 560, 659

[34] Kennea, J. A., Altamirano, D., Evans, P. A., et al. 2012, The Astronomer’s Telegram, 4192, 1

[35] Kimura, M., Tsunemi, H., Tomida, H., et al. 2013, PASJ, 65, 14

[36] Klebesadel, R. W., Strong, I. B., & Olson, R. A. 1973, ApJL, 182, L85

[37] Klein, R. I., & Chevalier, R. A. 1978, ApJL, 223, L109

[38] Komossa, S., & Bade, N. 1999, A&A, 343, 775

[39] Kong, A. K. H., Homer, L., Kuulkers, E., Charles, P. A., & Smale, A. P. 2000, MNRAS, 311, 405 Bibliography 140

[40] Kouveliotou, C., Meegan, C. A., Fishman, G. J., et al. 1993, ApJL, 413, L101

[41] Krimm, H. A., Holland, S. T., Corbet, R. H. D., et al. 2013, ApJS, 209, 14

[42] Levine, A. M., Bradt, H., Cui, W., et al. 1996, ApJL, 469, L33

[43] Lodato, G., King, A. R., & Pringle, J. E. 2009, MNRAS, 392, 332

[44] López-Santiago, J., Montes, D., Crespo-Chacón,I., & Fernández-Figueroa,M. J. 2006, ApJ, 643, 1160

[45] Maccarone, T. J., Long, K. S., Knigge, C., Dieball, A., & Zurek, D. R. 2010, MNRAS, 406, 2087

[46] Maselli, A., Melandri, A., Nava, L., et al. 2013, ArXiv e-prints, arXiv:1311.5254

[47] Matsuoka, M., Kawasaki, K., Ueno, S., et al. 2009, PASJ, 61, 999

[48] Metzger, M. R., Djorgovski, S. G., Kulkarni, S. R., et al. 1997, Nature, 387, 878

[49] Mihara, T., Nakajima, M., Sugizaki, M., et al. 2011, PASJ, 63, 623

[50] Moretti, A., Vattakunnel, S., Tozzi, P., et al. 2012, A&A, 548, A87

[51] Morii, M., Sugimori, K., Kawai, N., & MAXI Team. 2011, Physica E Low-Dimensional Systems and Nanostructures, 43, 692

[52] Morii, M., Tomida, H., Kimura, M., et al. 2013, ArXiv e-prints, arXiv:1310.1175

[53] Negoro, H., Nakahira, S., Tsuboi, Y., et al. 2012, The Astronomer’s Telegram, 4175, 1

[54] Nolan, P. L., Abdo, A. A., Ackermann, M., et al. 2012, ApJS, 199, 31

[55] Paciesas, W. S., Meegan, C. A., Pendleton, G. N., et al. 1999, ApJS, 122, 465

[56] Paciesas, W. S., Meegan, C. A., von Kienlin, A., et al. 2012, ApJS, 199, 18

[57] Panaitescu, A., & Kumar, P. 2001, ApJL, 560, L49

[58] P´elangeon,A., Atteia, J.-L., Nakagawa, Y. E., et al. 2008, A&A, 491, 157

[59] Pendleton, G. N., Mallozzi, R. S., Paciesas, W. S., et al. 1996, ApJ, 464, 606

[60] Pye, J. P., & McHardy, I. M. 1983, MNRAS, 205, 875

[61] Revnivtsev, M., Sazonov, S., Jahoda, K., & Gilfanov, M. 2004, A&A, 418, 927 Bibliography 141

[62] Sakamoto, T., Lamb, D. Q., Graziani, C., et al. 2004, ApJ, 602, 875

[63] Sakamoto, T., Lamb, D. Q., Kawai, N., et al. 2005, ApJ, 629, 311

[64] Sakamoto, T., Hill, J. E., Yamazaki, R., et al. 2007, ApJ, 669, 1115

[65] Sakamoto, T., Hullinger, D., Sato, G., et al. 2008, ApJ, 679, 570

[66] Sakamoto, T., Barthelmy, S. D., Baumgartner, W. H., et al. 2011, ApJS, 195, 2

[67] Schmidt, M., Higdon, J. C., & Hueter, G. 1988, ApJL, 329, L85

[68] Serino, M. 2014, in prep.

[69] Serino, M., Uzawa, A., Tsuboi, Y., et al. 2011, The Astronomer’s Telegram, 3211, 1

[70] Setti, G., & Woltjer, L. 1989, A&A, 224, L21

[71] Smith, D. A., Levine, A., Bradt, H., et al. 2002, ApJS, 141, 415

[72] Soderberg, A. M., Berger, E., Page, K. L., et al. 2008, Nature, 453, 469

[73] Strohmayer, T., & Bildsten, L. 2006, New views of thermonuclear bursts, ed. W. H. G. Lewin & M. van der Klis, 113–156

[74] Sugizaki, M., Mihara, T., Serino, M., et al. 2011, ArXiv e-prints, arXiv:1102.0891

[75] Toizumi, T. 2011, PhD thesis, Tokyo Institute of Technology

[76] Tomida, H., Tsunemi, H., Kimura, M., et al. 2011, PASJ, 63, 397

[77] Torres, C. A. O., Quast, G. R., da Silva, L., et al. 2006, A&A, 460, 695

[78] Tsunemi, H., Tomida, H., Katayama, H., et al. 2010, PASJ, 62, 1371

[79] Ukwatta, T. N., Linnemann, J. T., Tollefson, K., et al. 2011, ArXiv e-prints, arXiv:1112.0622

[80] van Paradijs, J., Groot, P. J., Galama, T., et al. 1997, Nature, 386, 686

[81] V´eron-Cetty, M.-P., & V´eron,P. 2010, A&A, 518, A10

[82] Wijers, R. A. M. J., Rees, M. J., & Meszaros, P. 1997, MNRAS, 288, L51

[83] Wu, X.-F., Hou, S.-J., & Lei, W.-H. 2013, ApJL, 767, L36

[84] Yoshida, A., Murakami, T., Itoh, M., et al. 1989, PASJ, 41, 509

[85] Zauderer, B. A., Berger, E., Soderberg, A. M., et al. 2011, Nature, 476, 425