HTRS 2011
Fast X-ray timing and spectroscopy at extreme count rates
7-11 February 2011, Champ´ery, Switzerland
Abstract Book
List of papers
I Invited contributions 11 1 The High Time Resolution Spectrometer aboard the International X-ray Observatory Didier Barret ...... 13
2 Multiwavelength high-time resolution observations Piergiorgio Casella ...... 14
3 Relativistic jets from Galactic accreting binary systems St´ephaneCorbel ...... 15
4 M/R constraints from NS Iron line Maria Diaz Trigo ...... 16
5 What Thermonuclear X-ray Bursts can tell us about Neutron Stars Maurizio Falanga ...... 17
6 General properties of magnetars and observational developments at high energies Wim Hermsen ...... 18
7 Fitting a self-consistent physical model to the power spectral density of XTE 1550-564 Adam Ingram ...... 19
8 Present and Future of Magnetar bursting/outbursting studies GianLuca Israel ...... 20
3 9 Status of IXO and the ESA Cosmic Visions Pro- gramme David Lumb ...... 21
10 New results on the X-ray spectra of thermonu- clear bursts Cole Miller ...... 22
11 Relativistic Disk Reflection in Stellar-Mass Black Holes J. M. Miller ...... 23
12 M/R constraints from accreting Millisecond Pul- sars and cooling stage of X-ray bursts Juri Poutanen ...... 24
13 Testing the No-Hair Theorem with Observations of Astrophysical Black Holes Dimitrios Psaltis ...... 25
14 The Properties of High Frequency QPOs and their Connection to the Physical Properties of Black Holes Ron Remillard ...... 26
15 Black Hole Spin Measurements via X-ray Contin- uum Spectroscopy James Steiner ...... 27
16 Lense-Thirring Precession Luigi Stella ...... 28
17 Timing of XRBs in the HTRS era Phil Uttley ...... 29
18 X-ray variability of black holes on all mass scales Simon Vaughan ...... 30
19 Cooling of accretion-heated neutron stars Rudy Wijnands ...... 31
20 HTRS Performance Simulations
4 J¨ornWilms ...... 32
II Oral contributions 33 1 Timing black-hole binaries: status and prospects Tomaso Belloni ...... 35
2 Undetectable, intermittent, and detectable mil- lisecond X-ray oscillations of accreting neutron stars in LMXBs Stratos Boutloukos ...... 36
3 Epicyclic frequencies and QPOs Michal Bursa ...... 37
4 Time lags as a constrain on the accretion disc ge- ometry in BHXRBs Pablo Cassatella ...... 38
5 Can a ”propelling” disc stay trapped near co- rotation? Caroline D’Angelo ...... 39
6 Living in a LOFT Marco Feroci ...... 40
7 Systematic Uncertainties in the Spectroscopic Mea- surements of Neutron-Star Masses and Radii from Thermonuclear X-ray Bursts Tolga Guver ...... 41
8 Signatures of superfluid neutron star dynamics Brynmor Haskell ...... 42
9 Broad iron emission lines in the intermediate state of black-hole transients Beike Hiemstra ...... 43
5 10 Detectors for fast X-ray spectroscopy, timing and imaging Peter Lechner ...... 44
11 Spectral and timing features in the ultra-compact X-ray binary 4U 0614+091 Oliwia Madej ...... 45
12 Kilohertz Quasi-Periodic Oscillations as a tool to probe the neutron-star interior and gravitational field Mariano Mendez ...... 46
13 Fast variability as a tracer of states and state- transitions in black holes Teo Mu˜noz-Darias ...... 47
14 GRAVITAS: General Relativistic Astrophysics Via Timing and Spectroscopy Kirpal Nandra ...... 48
15 Oscillation-Phase-Resolved Spectroscopy in the IXO Era Joey Neilsen ...... 49
16 A newly discovered accreting pulsar in Terzan 5 Alessandro Papitto ...... 50
17 Constraining the mass and moment of inertia of neutron stars from quasi-periodic oscillations in X-ray binaries J´erˆomeP´etri ...... 51
18 The pertinence of Jet Emitting Discs in Micro- quasars Pierre-Olivier Petrucci ...... 52
19 The Advanced X-ray Timing Array (AXTAR) Paul Ray ...... 53
20 A non-magnetar Soft Gamma Repeater
6 Nanda Rea ...... 54
21 Theoretical disk spectra and BH spin determina- tion Agata Rozanska ...... 55
22 Kilohertz quasi-periodic oscillations and broad iron emission lines as a probe of strong-field gravity Andrea Sanna ...... 56
23 HIFI - A High Framerate Imager for GRAVITAS Alexander Stefanescu ...... 57
24 All-Sky Monitoring of Variable Sources with Fermi GBM Colleen Wilson-Hodge ...... 58
25 The complete sample of type-I X-ray bursts in the low-mass X-ray binary 4U 1636-53 with RXTE Guo-Bao Zhang ...... 59
III Poster contributions 61 1 Preliminary concept of modern X-ray timing mis- sion Vadim Arefiev ...... 63
2 The 2008 outburst of the new X-ray transient XTE J1719-291 Montserrat Armas Padilla ...... 64
3 How the fast X-ray timing at extreme count rates will constrain the equation of state of the dense matter from the burst oscillations of the neutron stars Romain Artigue ...... 65
7 4 kHz QPO studies with IXO - Testing the Moving Hotspots Model Matteo Bachetti ...... 66
5 Re Analysis of Timing Parameters of OAO 1657- 415 Altan Baykal ...... 67
6 Time lags in 4U 1608-52 Marcio Guilherme Bronzato de Avellar ...... 68
7 Flame spreading velocity in Type I X-ray bursts Yuri Cavecchi ...... 69
8 The COSPIX mission : focusing on the energetic and obscured Universe Philippe Ferrando ...... 70
9 Phase lags and cyclotron lines in 4U 0115+63 Carlo Ferrigno ...... 71
10 Supergiant Fast X-ray Transients and other wind- fed accretors - testing with the Corbet diagram Sergei Grebenev ...... 72
11 Aperiodic X-ray variability studies of the black hole candidate MAXI J1659-152 Maithili Kalamkar ...... 73
12 Pulse profile variations in GX 1+4 Peter Kretschmar ...... 74
13 Dipping and Absorption in the stellar wind in GX 301-2 Ingo Kreykenbohm ...... 75
14 Pulse-phase spectroscopy as a tool for the study of X-ray pulsars Alexander Lutovinov ...... 76
15 On the nature of low frequency quasi periodic os- cillations: a variability study of the BHC GX 339-4
8 Sara Motta ...... 77
16 X-ray bursts and burst oscillations from the slowly spinning X-ray pulsar IGR J17480-2446 Sara Motta ...... 78
17 Spectroscopy of the Pulsar-like White Dwarf AE Aquarii from Chandra and Swift-XRT Data Bosco Oruru ...... 79
18 On the time evolution of the lower kHz QPO in 4U 1608-52 St´ephanePaltani ...... 80
19 A filter wheel for the HTRS St´ephanePaltani ...... 81
20 Central Engines of GRBs Tsvi Piran ...... 82
21 INTEGRAL and the unified model of AGN Claudio Ricci ...... 83
22 Type I and type II X-ray bursts of the Rapid Burster as seen by Swift Gloria Sala ...... 84
23 A Catalogue of X-ray bursters detected by JEM- X on board INTEGRAL Celia Sanchez-Fernandez ...... 85
24 Simulations of the performance of the HTRS on IXO Christian Schmid ...... 86
25 Measuring strong magnetic fields of neutron stars with the next-generation of X-ray instruments Gabriele Sch¨onherr ...... 87
26 Investigating the disc-jet coupling in accreting com- pact objects by studying the “radio quiet” sources Paolo Soleri ...... 88
9 27 XTE J1752-223: A broad band X-ray spectral in- vestigation combined with timing studies Holger Stiele ...... 89
28 X-ray bursting neutron star atmosphere models Valery Suleimanov ...... 90
29 Magnetized neutron star atmospheres: beyond the cold plasma approximation Valery Suleimanov ...... 91
30 Extreme-value analysis of the X-ray emission of Cygnus X-1 Maria S¨uveges ...... 92
31 Synchrotron self-Compton model for optical/X- ray correlation in black hole X-ray binaries Alexandra Veledina ...... 93
32 Fast variability of gamma-ray emission from su- permassive black hole binary OJ 287 Ievgen Vovk ...... 94
33 The Digital Data Processing Unit for the HTRS on board IXO Henning Wende ...... 95
34 LATE POSTER: Are the two peaks of the Cathedral QPO real har- monics? J´erˆomeRodriguez ...... 96
10 I Invited contributions
11
1 The High Time Resolution Spectrometer aboard the International X-ray Observatory
Didier Barret Ravera, Laurent; Bodin, Pierre; Amoros, Carine; Boutelier, Martin; Glorian, Jean-Michel; Godet, Olivier; Orttner, Guillaume; Lacombe, Karine; Pons, Roger; Rambaud, Damien; Ramon, Pascale; Ramchoun, Souad; Biffi, Jean- Marc; Belasic, Marielle; Cl´edassou,Rodolphe; Faye, Delphine; Pouilloux, Benjamin; Motch, Christian; Michel, Laurent; Lechner, Peter H.; Niculae, Adrian; Strueder, Lothar W.; Distratis, Giuseppe; Kendziorra, Eckhard; Santangelo, Andr´ea;Tenzer, Christoph; Wende, Henning; Wilms, Joern; Kreykenbohm, Ingo; Schmid, Christian; Paltani, St´ephane;Cadoux, Franck; Fiorini, Carlo; Bombelli, Luca; M´endez,Mariano; Mereghetti, Sandro
CESR/IRAP, Toulouse The High Time Resolution Spectrometer (HTRS) is one of the six focal plane instruments of the International X-ray Observatory (IXO). The HTRS is the only instrument matching the top level mission requirement of handling a one Crab X-ray source with an efficiency greater than 10%. It will provide IXO with the capability of observing the brightest X-ray sources of the sky, with sub-millisecond time resolution, low deadtime, low pile-up (less than 2% at 1 Crab), and CCD type energy resolution (goal of 150 eV FWHM at 6 keV). The HTRS is a non-imaging instrument, based on a monolithic array of Silicon Drift Detectors (SDDs) with 31 cells in a circular envelope and a X-ray sensitive volume of 4.5 cm2 x 450 µm. As part of the assessment study carried out by ESA on IXO, the HTRS was studied by a consortium led by CNES and CESR. In this paper, we will describe the expected per- formance of the HTRS, as assessed through Monte Carlo simulations. We will illustrate the instrument capabilities.
13 2 Multiwavelength high-time resolution observa- tions
Piergiorgio Casella School of Physics & Astronomy, University of Southampton, UK The study of X-ray variability in X-ray binaries has represented - and still is - one of the most powerful tools to study the accretion flow in the vicin- ity of compact objects. The advent of new instrumentation during the last decade has made possible to extend high time resolution astronomy to op- tical and infrared wavelengths. This has opened new exciting windows, as the novel technique of echo-tomography, which allows us to map the binary geometry, and the study of the fast-variable emission from relativistic jets, which provides unprecedented insights on the geometry and the Physics of the jets themselves. This field is at its dawn, but it is already showing its large potential. I will quickly review the state of the art, and show how having an X-ray timing satellite flying in the near future will be crucial to fully harness the great potential of these techniques.
14 3 Relativistic jets from Galactic accreting binary systems
St´ephaneCorbel Univ. Paris Diderot & CEA Saclay In the past decade, several considerable achievements have been reached in the field of Galactic accreting binary systems, especially in light of the extreme variability of their relativistic jets. These jets are now known to exist in at least three different favours: the self absorbed compact jets in the hard state, the transient and discrete ejection events associated with the state transitions, and the emission associated with the interaction of the jets with the interstellar medium. Althought their phenomenology is now rather well established, their emission and contribution to the total energy budget of microquasars is still the subject of active debates. In this review, I will present the most relevant observations concerning our understanding of relativistic jets in accreting systems, discussing in particular some open issues.
15 4 M/R constraints from NS Iron line
Maria Diaz Trigo C. Ng, M. Cadolle Bel, S. Migliari
ESO, ESA The recent claim of broad skewed iron lines from neutron star low-mass X-ray binaries has opened the exciting possibility of determining an upper limit to the radius of the neutron star, the most difficult parameter to obtain in order to constrain their equation of state. With the aim of studying further this possibility, we performed a sys- tematic analysis of XMM-Newton archival observations of bright neutron star low-mass X-ray binaries (LMXBs). We performed a detailed data analysis taking into account the systematic effects that arise as a consequence of the high count rates present in the observations of these sources. The properties of the iron lines differed from previous published analyses due to either incorrect pile-up corrections or different continuum parameterization. Most important, we could fit the Fe line with a simple Gaussian component for all the sources. The lines did not show the asymmetric profiles that were interpreted as an indication of relativistic effects in previous analyses of these LMXBs. I will present the results of the outlined study and discuss the implications of these results for the origin of broad Fe lines in LMXBs.
16 5 What Thermonuclear X-ray Bursts can tell us about Neutron Stars
Maurizio Falanga ISSI, Bern Some of the most extreme conditions of the universe exist in neutron stars, as these stars are very dense, have magnetic fields, and sometimes spin very rapidly and emit intense radiation. Many aspects of extreme physics can be studied only by observing and understanding neutron stars, as these prob- lems cannot be addressed by doing experiments in laboratories. One such problem is the lack of knowledge of the nature of dense cold matter at the neutron star cores, and only the accurate measurements of the mass, ra- dius, and spin period of a neutron star can resolve this. A promising way to measure these stellar properties is to study type I X-ray bursts, which are produced by thermonuclear burning of matter accumulated on the sur- faces of accreting neutron stars. This is because, these intense bursts, which sometimes exhibit timing features (such as bursts oscillations from the stellar surfaces) and may show surface spectral features, contain detailed informa- tion about the neutron stars. Moreover, X-ray bursts can be helpful for constraining the stellar atmospheric parameters, and for understanding the thermonuclear flame spreading under extreme physical conditions that exist on neutron star surfaces. In this talk, I will review our current knowledge of type I X-ray bursts and address questions on the observational future.
17 6 General properties of magnetars and observa- tional developments at high energies
Wim Hermsen SRON Netherlands Institute for Space Research, Utrecht The magnetar model has been proposed by Thompson and Duncan first to explain the enigmatic behavior of Soft Gamma-ray Repeaters (SGRs) and later also that of Anomalous X-ray Pulsars (AXPs). Many of the (out)bursting, timing and spectral characteristics of these objects can be explained with this model proposing rotating neutron stars with extremely strong magnetic fields (in excess of 1014 Gauss). This strong magnetic field being the source of the energy required to explain the observed high lumi- nosities. Even persistent emission is found to be 1-3 orders of magnitude more luminous at X-ray energies below 10 keV, as well as for an apparently different component above 10 keV, than can be explained with rotational energy loss. However, there is no agreed model scenario for the production of the high-energy emission in the magnetospheres of magnetars. In this re- view I will concentrate on the observational characteristics at high-energies including new constraints on the production of the non-thermal emission above 10 keV.
18 7 Fitting a self-consistent physical model to the power spectral density of XTE 1550-564
Adam Ingram Chris Done
Durham University Physics Department The variability properties of Black Hole Binaries (BHBs) have been stud- ied for well over 20 years and a very detailed phenomenological picture has been developed, particularly of the properties of the Power Spectral Den- sity (PSD). However, the underlying physical processes that generate the variability are very poorly understood, especially low frequency Quasi Peri- odic Oscillations (QPOs). I describe a model that asociates the QPO with Lense-Thirring Precession of the hot inner flow and show how this process is affected by fluctuations in mass accretion rate which themselves generate broadband variability power. This causal connection between physical pro- cesses allows me to define a full, self-consistent model of the PSD which I fit to data from the 1998 outburst of XTE 1550-564. This is the first ever attempt to fit a physical model of the PSD to data.
19 8 Present and Future of Magnetar bursting/out- bursting studies
GianLuca Israel Osservatorio di Roma, Monte Porzio Catone In the latest few years the study of X/Gamma-ray flashes, long bursts, out- bursts and giant flares from mangetars has opened new horizons in the field, through the study of the neutron star structure, magnetic field properties and the physical mechanism(s) responsible for their emission. The detection of QPOs during giant flares impacted on the NS equation of states and on the possibility of constraining them (in the future). The study of intermedi- ate flares gave us an important tool for the study of a magnetically trapped fireball and on the different species of photons (E-mode and O-mode polar- ized ones) responsible for the timing and spectral properties of bursts. Its application resulted in an independent measurement of the distances and the magnetic field strengths of bursting magnetars. The long-term monitoring of outbursts from magnetars allowed, for the first time, to test new spectral models obtained through a detailed calculation (Montecarlo) of the photon radiative transfer into a ”twisted” magnetosphere. All these studies are ex- pected to be boosted in the future thanks to the capabilities of a number of proposed high energy missions, such as IXO, NHXM, LOFT, etc.
20 9 Status of IXO and the ESA Cosmic Visions Pro- gramme
David Lumb SRE-SA ESTEC, Noordwijk The overall status of the ESA CV programme is described.The progress of IXO towards the down selection milestone in the Cosmic Visions programme is reviewed. Lessons learned in the industrial assessment study and technical reviews are presented. Continuation studies at ESA into high effective area telescopes suitable for high time resolution X-ray astronomy missions are briefly reviewed.
21 10 New results on the X-ray spectra of thermonu- clear bursts
Cole Miller Stratos Boutloukos, Frederick Lamb, Ka-Ho Lo
University of Maryland, Department of Astronomy, College Park X-ray burst spectra have long been used to estimate neutron star masses and radii. These estimates assumed that burst spectra are accurately described by the model atmosphere spectra developed over the last two decades. These model atmospheres have small scale heights and predict spectra that deviate significantly from Planck spectra. Fits of the Planck function to these spec- tra give temperatures significantly higher than the effective temperature. Using RXTE data, we measured X-ray spectra during several bursts with greater precision than previous measurements and compared these measure- ments with the spectra predicted by existing model atmospheres. We found that the spectra predicted by existing model atmospheres were strongly in- consistent with these new, high-precision measurements, whereas a simple Bose-Einstein spectrum is fully consistent with them. These results indicate that burst atmospheres are more extensive and have lower densities and more uniform temperatures than existing model atmospheres, with spectra that reflect saturated Comptonization. If the emission from burst atmospheres is efficient, the measured spectra imply fluxes ∼ 3-5 times the Eddington flux from ∼ 20-30% of the stellar surface. We discuss the implications of our results for determinations of neutron star masses and radii and the advances in modeling burst atmospheres that have occurred since we first reported these results. These results are based on research supported by NSF grant AST0708424 at Maryland and by NSF grant AST0709015 and the Fortner Chair at Illi- nois.
22 11 Relativistic Disk Reflection in Stellar-Mass Black Holes
J. M. Miller The University of Michigan The X-ray spectra of stellar-mass black holes reveal the clear signatures of relativistic disk reflection. This takes the form of a skewed iron emission line, and a characteristic reflection ”hump” at higher energy. The latter feature is an important confirmation of the interaction and dynamical effects implied by the iron line, and is particularly well observed in stellar-mass black holes. The degree to which the line and reflection hump are skewed is a sensitive diagnostic of the spin of the central black hole. At present, spins have been measured in a dozen systems using fits to relativistic disk reflection spectra, and the future for such efforts is particularly bright. This talk will review recent progress and discuss means by which a new mission with large collecting area and high thoughput can make major advances.
23 12 M/R constraints from accreting Millisecond Pul- sars and cooling stage of X-ray bursts
Juri Poutanen Valery Suleimanov, Mike Revnivtsev, Klaus Werner
Astronomy Division, Department of Physics, University of Oulu I will review the recent works on the analysis and theoretical interpretations of the pulse profiles of accreting X-ray millisecond pulsars. The emission in these pulsars is produced at hot spots in the accretion shocks near magnetic poles. The radiation spectrum can be described as a blackbody like from the neutron star surface and hard Comptonized emission from the shock. The pulse profiles can be described by the combined action of the relativistic effects such as Doppler effect, relativistic aberration and gravitational light bending. Theoretical pulse profiles can be fitted to the data and constraints on the neutron star parameters can be obtained. Thermal emission during X-ray bursts is a powerful tool to determine neutron star masses and radii, if the Eddington flux and the apparent ra- dius in the cooling tail can be measured accurately, and distances to the sources are known. We propose here an improved method of determining the basic stellar parameters using the data from the cooling phase of long, photospheric radius expansion bursts covering a large range of luminosities. For this purpose, we computed a large set of atmosphere models for burst luminosities varying by two orders of magnitude and for various chemical compositions and surface gravities. We show that the variation of the inverse square root of the apparent blackbody radius with the flux, observed during the photospheric radius expansion bursts from a number of sources at low accretion rate is entirely consistent with the theoretical expectations of the color-correction factor evolution. However, for bursts happening at higher accretion rates the observed evolution is inconsistent with theory, implying that accretion strongly disturbs the neutron star atmosphere. These find- ings have profound implications for the recent claims on determination of the neutron star radii and masses from such bursts. Our method allows us to determine both the Eddington flux and the ratio of the stellar apparent radius to the distance much more reliably. For 4U 1724-307, we find a lower limit on the neutron star radius of 13 km, independently of the chemical composition. These results suggest that the matter inside neutron stars is characterized by a stiff equation of state.
24 13 Testing the No-Hair Theorem with Observa- tions of Astrophysical Black Holes
Dimitrios Psaltis Astronomy and Physics Departments, The University of Arizona The Kerr spacetime of spinning black holes is one of the most intriguing predictions of Einstein’s theory of general relativity. The special role this spacetime plays in the theory of gravity is encapsulated in the no-hair theo- rem, which states that the Kerr metric is the only realistic black-hole solu- tion of the vacuum field equations. Recent high-energy observations of black holes have secured our understanding of their basic properties while open- ing up new opportunities for devising tests of the Kerr metric. In this talk I will show how timing and spectroscopic observations of accreting black- holes with future instruments can lead to the first direct test of the no-hair theorem.
25 14 The Properties of High Frequency QPOs and their Connection to the Physical Properties of Black Holes
Ron Remillard MIT, Cambridge The properties of high-frequency QPOs in black hole binaries are reviewed including their conditions of occurrence, frequency stability, amplitude spec- tra, and correlations with low-frequency QPOs. The circumstantial argu- ments that such QPOs may constrain the mass and spin of black holes are summarized. Updates are given for the 3:2 frequency ratio for pairs of QPOs seen in some sources, and for the relationship between QPO frequency and black hole mass. Comments are offered for the inadequacies of current con- cepts for the oscillation mechanism and for prospects to make advances with new X-ray instrumentation.
26 15 Black Hole Spin Measurements via X-ray Con- tinuum Spectroscopy
James Steiner Harvard-Smithsonian Center for Astrophysics, Cambridge A remarkable property of an astrophysical black hole is that it is completely described by just its mass and spin. Our research group is dedicated to mea- suring these two fundamental properties of the black hole primaries found in X-ray binary systems. We measure spin by modeling the thermal X-ray continuum spectrum that originates in the innermost regions of the accre- tion disk. We fit for the disk’s inner radius, which we identify with the innermost stable circular orbit (ISCO) predicted by GR. By measuring this radius in concert with optical estimates for the black hole mass, and the orbital inclination angle and distance of the system, we uniquely determine a black hole’s spin. I will present an overview of our spin measurements and discuss progress which would be enabled with a large collecting area HTRS.
27 16 Lense-Thirring Precession
Luigi Stella INAF - Osservayorio Astronomico di Roma The effects of Lense-Thirring precession and, more generally, frame dragging are expected to be important in the motion of matter close to spinning compact objects. I will concentrate on phenomena that can probe in situ the very strong gravitational field and single out Lense-Thirring precession in the close vicinity of accreting neutron stars and black holes: these are the fast quasi periodic oscillations in the X-ray flux of these objects. The expected magnitude of Lense-Thirring/frame dragging effects in the regions where these signals originate are large and thus their detection does not pose a challenge; rather it is the interpretation of these phenomena that needs to be corroborated through deeper studies and observations with very large effective area X-ray instruments.
28 17 Timing of XRBs in the HTRS era
Phil Uttley School of Physics and Astronomy, University of Southampton I will review the future potential of the combination of high throughput, time-resolution and spectral-resolution which is offered by future instru- ments like the HTRS. I will start by highlighting what is already possible with the fast-timing modes and soft response of current CCD detectors. In particular, XMM-Newton’s EPIC-pn is opening up new capabilities in com- bined spectral-timing measurements, allowing us to measure the causal links between the power-law continuum and the disc thermal and reflection com- ponents in X-ray binaries. The much greater throughput offered by future missions will push these measurements deep into the regime of light-crossing times for the innermost radii in these systems, enabling reverberation map- ping and even Doppler tomography of the inner disc. These observations will provide direct measurements of disc radii in km, not just relative units such as Gravitational radii. Together with the ability to probe the coherence times of the highest-frequency
29 18 X-ray variability of black holes on all mass scales
Simon Vaughan University of Leicester We will review the similarities in X-ray variability properties of the differ- ent types of luminous accreting black holes, namely those in X-ray bina- ries (XRBs), Active Galactic Nuclei (AGN), and also Ultra-luminous X-ray sources (ULXs). If strong gravity dominates the dynamics of the inner accretion flows around black holes then an elementary consequence is scale invariance: many important aspects of accretion onto supermassive black holes (M > 106 Msun) in AGN should be fundamentally the same as for stellar mass black holes (M ∼ 10 Msun) in XRBs. (And similarly for ULXs.) Over the past few years we have revealed many remarkable similarities in the X-ray variability of nearby AGN and BH XRBs, supporting the idea of ’black hole unification.’ In parallel with these X-ray advances, studies of transient radio jets from XRBs have led to the development of a scheme that unifies jet pro- duction with accretion state. Relativistic jets must be launched from the region of strong-field gravity, and so scale invariance implies this accretion state/jet unification scheme should extend to AGN. From these results a new paradigm is emerging in which accretion mode, X-ray spectrum, high- frequency timing properties and jet production for both XRB ’states’ and AGN ’types’ may be unified into a single framework for the activity cycles of accreting black holes. We will discuss the present state of comparative work on X-ray behaviour of luminous accreting black holes (from the ,XMM-Chandra-Suzaku era), with particular emphasis on the results of a recent ∼600 ks XMM-Newton observation of the prototypical low-mass Seyfert 1 galaxy NGC 4051. Time permitting, we will attempt to outline some of the possibilities for exploring and exploiting the differences and similarities between the BH types using future, high throughput and fast timing X-ray missions.
30 19 Cooling of accretion-heated neutron stars
Rudy Wijnands Astronomical Institute “Anton Pannekoek”, University of Ams- terdam Neutron stars in X-ray binaries are heated due to the accretion of matter onto their surfaces. When the accretion stops the deposited heat is partly radiated away from the surface as thermal radiation. By studying this ra- diation and how it various over time (i.e., how the surfaces cool down) new insights in the properties of the crust and the core of neutron stars have been obtained in the last decade. I will summarize what we have learned so far about the behavior of neuron star matter, what the uncertainties still are in our understanding, and how we can make progress in the future using more optimized observing strategies and new upcoming and planned X-ray satellites.
31 20 HTRS Performance Simulations
J¨ornWilms Remeis-Sternwarte and ECAP, University of Erlangen-Nuremberg The very high count rates that will be obtained with future X-ray satellites pose a significant challenge for the prediction of their performance. Over the past years we have developed a set of software tools for such performance predictions that is easily adaptable for most pixel-based X-ray detectors planned. These simulations take into account all relevant phenomena dis- torting the measured signals. Amongst others, these phenomena include energy pile up, i.e., the detection of more than one photon in the same de- tector pixel during one read-out cycle, and pattern pile up, i.e., detection of two photons in adjacent pixels in a pattern that resembles that caused by charge splitting of a single higher energetic photon. Here, we present results of simulations of such observations for the IXO-HTRS and -WFI as well as other planned future instruments and discuss the challenges that the analysis of data from these instruments will present.
32 II Oral contributions
33
1 Timing black-hole binaries: status and prospects
Tomaso Belloni INAF-OAB The low-frequency (0.1-10 Hz) variability of black-hole binaries is now well sampled and its basic characteristics are known. Interpretations which take into account also the spectral distribution are being put forward and need to be compared with existing observations in order to plan future ones. I will present a summary of the current status of our knowledge of source states and related timing features, and discuss the future prospects in view of future planned and proposed missions. I will also examine the current status on HFQPOs and their possible connection with kHz QPOs in neutron-star systems.
35 2 Undetectable, intermittent, and detectable mil- lisecond X-ray oscillations of accreting neutron stars in LMXBs
Stratos Boutloukos Frederick Lamb, Ka Ho Lo, Coleman Miller
University of Tuebingen, Theoretical Astrophysics Accretion-powered X-ray oscillations have been detected in only 14 of about 130 accreting neutron stars in LMXBs. Three such stars produce detectable oscillations only intermittently. We show that the absence of detectable oscillations from most LMXBs, and the intermittent detection of oscillations in a few, can both be explained if the emitting regions of accreting neutron stars with millisecond spin periods typically have small inclinations relative to the spin axis and move in response to changes in the accretion flow. This nearly-aligned moving spot model is consistent with the small observed amplitudes and nearly sinusoidal waveforms of most accreting millisecond X-ray pulsars, and the large, rapid phase variations of several. These results are based on research supported by NASA grant NAG 5- 12030, NSF grant AST0709015, and funds of the Fortner Endowed Chair at Illinois, and by NSF grant AST0708424 at Maryland.
36 3 Epicyclic frequencies and QPOs
Michal Bursa Astronomical Institute, Czech Academy of Sciences The idea that observed HFQPOs arise as a manifestation of some sort of orbital motion in the accretion flow has a number of supporting arguments. The omnipresent signature of a special 3:2 ratio in HFQPO frequencies led to the idea of a resonance between two modes of disk epicyclic frequencies and still today that idea represents one of the most favourite explanations of the phenomenon. The talk will review some observational facts and results that have been obtained within the epicyclic resonance theory and draw down some ideas about directions of the future research with respect to high precision timing measurements.
37 4 Time lags as a constrain on the accretion disc geometry in BHXRBs
Pablo Cassatella P. Uttley, J. Wilms, J. Poutanen
University of Southampton Timing analysis in the last few decades has proven successful to give an insight into the accretion geometry and emission mechanisms in black hole X-ray binaries. In particular, time lags show that the hard X-ray emis- sion usually lags the soft emission, and papers like Kotov et al (2001) have addressed this issue. While there is evidence that the variability originates in the disc, thanks to the propagation of fluctuations throughout the disc and to a central corona, it has also been observed that the central X-ray emission is in- tercepted by the disc in its outer regions, possibly in a flared geometry, and this can contribute to the observed lags vs frequency. Here we show a reflection model that can be used within XSPEC or ISIS to concurrently fit both lags vs frequency and mean spectrum, and constrain the geometry of the reflecting disc. We will show a few examples using fast mode observations of black hole binaries with XMM-Newton.
38 5 Can a ”propelling” disc stay trapped near co- rotation?
Caroline D’Angelo Henk Spruit
Anton Pannekoek Institute/MPA Garching A strong stellar magnetic field significantly alters the behaviour of an ac- cretion disc around the star. In particular, at low accretion rates it is often assumed that the interaction between the field and the disc will expel the gas out of the disc in an outflow rather than allow it to accrete onto the star – a state known as the ”propeller regime”. However, recent work has suggested that if the disc is truncated close to the co-rotation radius (where the star’s spin frequency equals the Keplerian frequency of the disc) the disc-field interaction will mostly confine the gas, preventing it from escaping from the disc. The interaction changes the density profile of the disc, and can completely halt accretion onto the star without producing an outflow. In turn, this forces the inner edge of the disc to stay trapped close to the co-rotation radius even when there is no net mass flow through the disc, so that the disc never moves into the true propeller regime. In my talk I will explain how a trapped disc can form and show how this can sometimes result in bursts of accretion onto the star, such as have been observed as low-frequency QPOs in X-ray pulsars SAX J1808.4-3658 and NGC 6440 X- 2. Finally, I will discuss how trapped discs could also be responsible for the weak recurrent outbursts seen in NGC 6440 X-2 and IGR J00291+5934.
39 6 Living in a LOFT
Marco Feroci Luigi Stella (INAF/OAR Rome) on behalf of the LOFT consortium
INAF/IASF, Rome LOFT (Large area Observatory For x-ray Timing) is an innovative mission concept for the next generation of X-ray timing experiments, being submit- ted to the ESA Call for Medium size missions. Recent developments in the field of Silicon detectors allowed us to design a realistic observatory devoted to X-ray timing studies with an effective area above 10 m2, in the energy range 2-30 keV, with an energy resolution between 200 and 500 eV. Such an exceedingly large area (20 times RXTE/PCA), with a time resolution bet- ter than 10 µs, will enable unprecedently fast and accurate time variability studies related to accreting collapsed objects (e.g. fast coherent pulsations and QPOs). The scientific payload is complemented by a coded-mask wide field monitor based on similar detectors. In this paper we will present the mission concept, the payload design and the expected performance.
40 7 Systematic Uncertainties in the Spectroscopic Measurements of Neutron-Star Masses and Radii from Thermonuclear X-ray Bursts
Tolga Guver Dimitrios Psaltis, Feryal Ozel
University of Arizona Low mass X-ray binaries that show thermonuclear X-ray bursts are excel- lent targets for constraining the equation of state of neutron star matter. We make use of time resolved, high count rate X-ray spectra to measure the Eddington limit and the apparent radii of neutron stars, which we then use to determine the masses and radii. In an effort to asses the system- atic uncertainties in these measurements, we have analyzed archival RXTE observations of 447 X-ray bursts observed from 12 X-ray sources. I will present the results of this analysis and discuss the potential uncertainties introduced by systematic effects on the mass and radius measurements. I will also discuss the strategies for understanding and reducing systematic errors using future X-ray missions that combine good timing and spectral resolution with the ability to observe high count-rate sources.
41 8 Signatures of superfluid neutron star dynamics
Brynmor Haskell Astronomical Institute ”Anton Pannekoek”, University of Ams- terdam Neutron stars offer a unique opportunity to probe a wealth of physical ef- fects that cannot be studied in terrestrial laboratories. Not only do they allow us to gain insight on the state of matter at extreme densities, but also to study the dynamics of large scale superfluid components of the star. However theoretical neutron star models are highly uncertain and disentan- gling the signature of different effects in the data can be very challenging. I will discuss recent theoretical developments in superfluid NS modeling and how the use of high precision X-ray observations, also combined with radio and possibly, in the future, with gravitational wave (GW) observations, can constrain models and shed light on different phenomena. In particular I will discuss the dynamics of accreting millisecond X-ray pulsars, which have been proposed as interesting GW sources, and how cooling observations in the X-ray, combined with precision radio data (and GW observations from next generation interferometers), can constrain glitch models and further our understanding of NS superfluidity.
42 9 Broad iron emission lines in the intermediate state of black-hole transients
Beike Hiemstra Mariano Mendez, Chris Done, Maria Diaz Trigo, Diego Altamirano, Pier- giorgio Casella
Kapteyn Astronomical Institute, University of Groningen Simultaneous XMM-Newton and RXTE observations of the new X-ray tran- sient and black-hole candidate XTE J1652-453 have revealed a very strong and broad iron emission line in its X-ray spectrum. The source was observed in the hard-intermediate state in the decline of the 2009 outburst. The line has an equivalent width of ∼450 eV, which makes it among the strongest ever detected in a black-hole candidate. The line profile is consistent with being produced by reflection off the accretion disk, and is broadened by relativistic effects close to the black hole. Assuming that the accretion disk is truncated at the innermost stable circular orbit, the black hole in XTE J1652-453 has a spin parameter of ∼0.5. The high ionization parameter and the best-fitting line energy of 6.97 keV are consistent with highly ionized iron, which in turn can explain the high equivalent width since the fluorescence probability is increased due to the lack of Auger destruction. Given that the source was observed in a spectral state where spectral and timing properties are changing on short timescales, studying the line profile and its underlying continuum in the intermediate state needs to be done in short exposures. To do so, we need an instrument like the High Time Reso- lution Spectrometer, since its large collecting area, high time resolution and moderate spectral resolution will allow us to have sufficient good statistics and data to study short-term variations in the line and continuum.
43 10 Detectors for fast X-ray spectroscopy, timing and imaging
Peter Lechner P. Majewski, A. Niculae, H. Soltau, L. Strueder, A. Stefanescu, J. Treis
PNSensor & MPI-HLL, M¨unchen For high time resolution spectroscopy and imaging in X-ray astronomy two detector concepts have been developed: multi-channel Silicon Drift Detec- tors and DePFET Active Pixel Sensors. The Silicon Drift Detector (SDD) is dedicated for timing and spectroscopy. This sensor concept combines a large sensitive area with a small output capacitance guaranteeing high energy res- olution and high count rate capability. To obtain a uniform count rate over the array the sensor is placed out of focus. The formats and areas of the SDD cells can be adapted to the telescope’s radial intensity distribution. E.g. the HTRS instrument on IXO is based on a monolithic multi-channel Silicon Drift Detector (SDD) with 31 cells in a circular envelope and a sensitive volume of 4.5cm2 x 450µm. It has been designed to operate at count rates exceeding 1 million per sec with an energy resolution of 150 eV (FWHM at 6 keV), a time resolution of 10 micro-second, and a dead time and pile-up on the percent level. The DePFET (Depleted P-channel Field Effect Transistor) structure combines the functionalities of sensor and integrated amplifier. It is the basic cell of an Active Pixel Sensors for X-ray imaging and spectroscopy with Fano-limited energy resolution and high efficiency from 100 eV to 20 keV. For a row-by-row readout of the pixel matrix dedicated control and readout ASICs have been developed. With a format of e.g. 100 x 100 pixels frame rates of more than 10.000 per second are possible. Representative prototypes with a format of 256 x 256 pixels are currently under test.
44 11 Spectral and timing features in the ultra-compact X-ray binary 4U 0614+091
Oliwia Madej P.G. Jonker
SRON, Netherlands Institute for Space Research, Utrecht We have studied the ultra-compact X-ray binary 4U 0614+091, which is a promising target for the High Time Resolution Spectrometer (HTRS). It is a bright source (L∼3x1036 erg/s) consisting of a neutron star orbiting around a carbon-oxygen or oxygen-neon-magnesium white dwarf with an orbital period of around 50 min. We recently discovered an emission feature at ∼0.7 keV in this source. We interpret this feature as a relativistically broadened reflection line of O VIII Ly alpha, caused by X-rays reflected off the accretion disc in the strong gravitational field close to the neutron star. This is the first time that a broad fluorescent O VIII Ly alpha line is seen in the X-ray binary. This source also shows strong kHz quasi-periodic oscil- lations (QPOs). By modeling the relativistically broadened oxygen line and simultaneously measuring kHz QPOs we can obtain two independent mea- surements of the inner radius of the accretion disc. Since 4U 0614+091 shows transitions from low flux to high flux, using HTRS we will be able to track the changes of the inner radius of the accretion disc obtained from modeling the oxygen line and measuring the QPOs. Hence, we can investigate the correlation between the two. In this way we will be able to test theoretical models for relativistically broadened reflection lines and kHz QPOs.
45 12 Kilohertz Quasi-Periodic Oscillations as a tool to probe the neutron-star interior and gravita- tional field
Mariano Mendez Kapteyn Astronomical Institute, University of Groningen The expected timescale of variability close to the surface of a neutron star is of the order of a millisecond or less. The discovery of the kilohertz quasi- periodic oscillations (kHz QPOs) in neutron-star low-mass X-ray binaries provided the first direct evidence of such variability. These QPOs were immediately recognized as a powerful tool to probe the dynamics of matter and the properties of the gravitational field in the vicinity of neutron stars, as well as the neutron-star interior. After 15 years of observations with the Rossi X-ray Timing Explorer, we have collected a wealth of data on these QPOs, and a number of patterns have emerged, including correlations of the QPO properties with properties of other types of variability and source parameters (e.g., energy spectra or neutron-star spin). Notwithstanding, the mechanism that produces the kHz QPOs remains elusive. While several models have been proposed, none of them can consistently explain the richness of this phenomenon. Perhaps the most outstanding question that remains is whether these QPOs reflect the Keplerian, or other General-Relativistic, frequency when mass orbiting in an accretion disc plunges onto the neutron-star surface. There are two promising avenues to address this question: Energy-dependent time lags and simultaneous measurements of kHz QPOs and iron lines. In this talk I will discuss these two possibilities in light of the capabilities of current and future X-ray missions.
46 13 Fast variability as a tracer of states and state- transitions in black holes
Teo Mu˜noz-Darias Motta, S., Belloni, T. M.
INAF- Osservatorio Astronomico di Brera I present a new tool to study the different accretion regimes present in black hole transients by focusing solely on the evolution of the rms-flux relation along their outburst. This technique enables an accurate identification of the various accretion states without using spectral information. I will show the results obtained by applying this method to several black hole transients. The hard, soft and the two intermediate states can be clearly identified by this simple technique, and marked transitions are observed between them. I will also show that black holes seem to follow a specific flux-variability relation when type-B oscillation are present in their power density spectra. I will finally discuss on the physical implications of this method and the potentials that it offers for future studies of black hole transients using high time resolution instruments.
47 14 GRAVITAS: General Relativistic Astrophysics Via Timing and Spectroscopy
Kirpal Nandra Max-Planck Institut f¨urextraterrestriche Physik GRAVITAS is an X-ray observatory, designed and optimised to address the ESA Cosmic Vision theme of “Matter under extreme conditions”. It is sub- mitted as a response to the call for M3 mission proposals. The concept centres around an X-ray telescope of unprecedented effective area, which will focus radiation emitted from close to the event horizon of black holes or the surface of neutron stars. To reveal the nature and behaviour of mat- ter in the most extreme astrophysical environments, GRAVITAS targets a key feature in the X-ray spectra of compact objects: the iron Kα line at ∼ 6.5 keV. The energy, profile, and variability of this emission line, and the properties of the surrounding continuum emission, shaped by GR effects, provide a unique probe of gravity in its strong field limit. Among its prime targets are hundreds of supermassive black holes in bright Active Galactic Nuclei (AGN), which form the perfect laboratory to help understand the physical processes behind black hole growth. Accretion plays a fundamen- tal role in the shaping of galaxies throughout cosmic time, via the process of feedback. Modest (∼sub-arcmin) spatial resolution will therefore deliver the necessary sensitivity to extend high quality X-ray spectroscopy of AGN to cosmologically-relevant distances. Closer to home, ultra-high count rate capabilities and sub-millisecond time resolution will enable the study of GR effects and the equation of state of dense matter in the brightest X-ray bi- naries in our own Galaxy, using multiple probes, such as the broad iron line, the shape of the disk continuum emission, quasi-periodic oscillations, rever- beration mapping, and X-ray burst oscillations. Despite its breakthrough capabilities, all enabling technologies for GRAVITAS are in a high state of readiness. It will use ultra light-weight X-ray optics and a focal plane detec- tor based on silicon technology. The baseline launcher will be a Soyuz-Fregat to place GRAVITAS into a low-inclination, low-earth orbit, providing low and relatively stable background.
48 15 Oscillation-Phase-Resolved Spectroscopy in the IXO Era
Joey Neilsen Ron Remillard, and Julia C. Lee
Center for Astrophysics, Cambridge We present the first phase-resolved analysis of a variability class in GRS 1915+105 that combines the broadband timing and spectral capabilities of RXTE with Chandra’s superb spectral resolution. We demonstrate that rapid changes in the broadband X-ray spectrum can cause significant changes in the ionization, structure, and density of accretion disk winds on timescales of 5 seconds or less. Yet recent results indicate that spectral lines may vary on even shorter timescales during quasi-periodic oscillations. With next- generation X-ray facilities, which will easily track individual QPO waves, it will be possible to determine precisely how emission and absorption features respond to quasi-periodic oscillations in the X-ray lightcurve. Therefore, using our phase-resolved spectral analysis of GRS 1915+105 as an example, we discuss the enormous potential of QPO-phase-resolved spectroscopy with next-generation X-ray instruments, including reverberation mapping and de- tailed measures of nearby plasma conditions on sub-second timescales. In this way, future observations of QPOs will provide sensitive, powerful probes of the physical processes that link accreting black holes and their environ- ments. With the capabilities of IXO, GRAVITAS, and LOFT, this physics will be routinely accessible not just in GRS 1915+105, but also in black hole X-ray binaries across the galaxy.
49 16 A newly discovered accreting pulsar in Terzan 5
Alessandro Papitto S. Motta, A. Riggio, A. D’A`ı,T. Di Salvo, R. Iaria, L. Burderi, T. Belloni
Universit`adi Cagliari The 90.5 ms accreting pulsar, IGR J17480-2446, has been recently discovered in the globular cluster Terzan 5. The analysis of the Doppler shifts induced by the orbital motion reveal how the neutron star belongs to a ∼ 21.3 hr binary system with a companion star mass between 0.4 and 1 M . From the presence of pulsations at different accretion rates we constrain the magnetic field to lie within ∼ 2 108 and ∼ 2.4 1010 G. Together with the value of the neutron star spin period, such an estimate puts this source in the still poorly sampled population of slow, possibly mildly recycled, accreting pulsars thus making it an extremely intriguing scientific case. The presence of a ∼ 815 Hz QPO possibly allows also to test the theories that interpret these features in terms of Keplerian rotation in the inner parts of the accretion disc. IGR J17480-2446 also shows several type I X-ray bursts. For the first time burst oscillations are observed at a relatively low frequency, indicating how fast rotation is not a necessary ingredient for their appearance. As the source shows bursts with a recurrence time down to few minutes it also offers an almost the fundamental opportunity to follow the variations of the bursts energetics with the mass accretion rate, across a wide range of parameters.
50 17 Constraining the mass and moment of iner- tia of neutron stars from quasi-periodic oscil- lations in X-ray binaries
J´erˆomeP´etri Observatoire Astronomique de Strasbourg Neutron stars are the densest objects known in the Universe. Being the final product of stellar evolution, their internal composition and structure is rather poorly constrained by measurements. It is the purpose of this paper to put some constrains on the mass and moment of inertia of neutron stars based on the interpretation of kHz quasi- periodic oscillations observed in low mass X-ray binaries. We use observations of high-frequency quasi-periodic observations (HF- QPOs) in low mass X-ray binaries (LMXBs) to look for the average mass and moment of inertia of neutron stars. This is done by applying our parametric resonance model to discriminate between slow and fast rotators. We fit our model to data from ten LMXBs for which HF-QPOs have been seen and the spin of the enclosed accreting neutron star is known. For a simplified analysis we assume that all neutron stars possess the same properties (same mass M∗ and same moment of inertia I∗). We find an average mass M∗ ≈ 2.0 − 2.2 M . The corresponding average moment of 38 inertia is then I∗ ≈ 1 − 3 × 10 kg m2 ≈ 0.5 − 1.5 (10 km)2 M which equals to dimensionless spin parametera ˜ ≈ 0.05 − 0.15 for slow rotators (neutron stars with a spin frequency roughly about 300 Hz) respectively a˜ ≈ 0.1−0.3 for fast rotators (neutron stars with the spin frequency roughly about 600 Hz).
51 18 The pertinence of Jet Emitting Discs in Micro- quasars
Pierre-Olivier Petrucci Ferreira J., Henri G., Malzac J., Cabanac C.
Laboratoire d’Astrophysique de Grenoble Very broad band observations of microquasars, from radio to X-rays, have revealed the strong connection between accretion and ejection processes in these objects. Not only their radiative outputs are a complex combination of accreting and expelling components but jets appear to play also a crucial role in the dynamics and the energetics of these systems. Based on the complete calculation of the full accretion-ejection set of equations, we present a model that aims at explaining the spectral states and variability behaviors of BH binaries as well as their spectral evolution during outbursts. In our view, the accretion flow is pervaded by a large scale magnetic field of bipolar topology. In the hard state, the field is large enough for a new class of accretion flow to set in where most of the re- leased accretion power feeds self-collimated jets. Such a Jet Emitting Disc (JED) has dynamical properties quite different from both the standard and advection dominated discs. The hot branch of solutions has all the charac- teristics of the so-called ”hot corona” generally invoked in XrB sytems in the Low/Hard states. We will detail the energetics and radiative expectations of our model and show their good agreement with those observed in Cygnus X-1, the proptotype of microquasars. The nature of the X-ray variability is also investigated by assuming an oscillating hot plasma at the jet basis whose temperature varies locally through the propagation of a magnetosonic wave. The variable comptonized spectra are computed through monte-carlo simulation and comparison to observations will be discussed.
52 19 The Advanced X-ray Timing Array (AXTAR)
Paul Ray Deepto Chakrabarty, Colleen Wilson-Hodge for the AXTAR Collaboration
Naval Research Laboratory, Washington AXTAR is a NASA MIDEX mission concept for X-ray timing of compact objects that combines very large collecting area, broadband spectral cov- erage, high time resolution, highly flexible scheduling, and an ability to respond promptly to time-critical targets of opportunity. It is optimized for submillisecond timing of bright Galactic X-ray sources in order to study phenomena at the natural time scales of neutron star surfaces and black hole event horizons, thus probing the physics of ultradense matter, strongly curved spacetimes, and intense magnetic fields. AXTAR’s main instrument is a collimated, thick Si pixel detector with 2-50 keV coverage and over 3 square meters effective area. For timing observations of accreting neutron stars and black holes, AXTAR provides at least a factor of five improve- ment in sensitivity over the RXTE PCA. AXTAR also carries a sensitive sky monitor that acts as a trigger for pointed observations of X-ray tran- sients in addition to providing high duty cycle monitoring of the X-ray sky. We review the science goals and technical concept for AXTAR and present initial results from a preliminary mission design study at the MSFC Ad- vanced Concepts Office.
53 20 A non-magnetar Soft Gamma Repeater
Nanda Rea Intitut de Ciencies de l’Espai (ICE-CSIC, IEEC), Barcelona I will report on the discovery of a Soft Gamma Repeater with a magnetic field in line with that of the normal pulsar population. This discovery opens the possibility of many normal pulsars hiding an internal magnetar-activity.
54 21 Theoretical disk spectra and BH spin determi- nation
Agata Rozanska J. Madej, M. Gancarczyk
N. Copernicus Astronomical Centre, Warsaw In my talk I will present angle-dependent, broad-band theoretical spectra from accretion disks around stellar black hole. In our numerical calculations, full radiative transfer of irradiated disk atmospheres was solved including effects of Compton scattering, free-free and all important bound-free transi- tions of 10 main elements. Moreover, we included the fundamental series of iron lines from helium-like and hydrogen-like ions, and fluorescent Kα and Kβ lines from low ionized iron. Our results show, that emission lines of iron can be observed only if heating by external X-rays dominates over thermal processes in hot disk atmospheres. Irradiated disks are usually brighter in X-ray continuum when seen edge on, and fainter when seen face on. The shapes of theoretical multitemperature disk component and iron Kα line are extremely important in black hole spin determination. I will discuss how the disk hardening factor and iron line complex affect the spin of a black hole.
55 22 Kilohertz quasi-periodic oscillations and broad iron emission lines as a probe of strong-field gravity
Andrea Sanna Mariano Mendez, Diego Altamirano, Tomaso Belloni
Kapteyn Astronomical Institute, Groningen Kilohertz quasi-periodic oscillations (kHz QPOs) and broad iron emission lines are thought to be produced at the inner edge of an accretion disk around neutron stars in low mass X-ray binary systems (LMXBs). The frequency of kHz QPOs and the width of iron lines strongly suggest that these phenomena take place just few kilometers above the surface of the neutron star and hence they should probe the strong gravitational field around these stars. Both QPO variability and iron line hold the promise of searching for signatures of strong-field general relativity, e.g. the ISCO or Lense-Thirring precession. Here I present new results of the kHz QPOs in the transient neutron star LMXB XTE J1701- 462, and the kHz QPOs and the broad iron line in the persistent neutron star LMXB 4U 1636-53. In XTE J1701 we find that, contrary to previous claims, properties of the kHz QPO are probably not uniquely determined by the geometry of the space-time. In 4U 1636 we find that there is a complex relation between kHz QPOs and properties of the iron line, which is difficult to explain if both phenomena reflect properties of the same region in the accretion disk.
56 23 HIFI - A High Framerate Imager for GRAVI- TAS
Alexander Stefanescu Max-Planck-Institut Halbleiterlabor, M¨unchen GRAVITAS is a proposed X-ray observatory, designed and optimised to address the Cosmic Vision theme of ”Matter under extreme conditions”. It will have a X-ray mirror system optimized for a very high effective area at the Fe Kα line, and will utilize the resulting statistics to do time-resolved spectroscopy with excellent timing resolution and at high count-rates. HIFI is a high frame-rate imager for GRAVITAS. Using an advanced Active Pixel Sensor array based on DEPFET macropixels, it will be able to offer imaging with excellent energy resolution, while a massively parallel read-out will allow extremely high frame-rates. The APS of HIFI offers very flexible read-out modes, allowing the imager to cope with the flux of bright X-ray bursts. Despite its breakthrough capabilities, all enabling technologies for GRAVITAS and HIFI are already in a high state of readiness.
57 24 All-Sky Monitoring of Variable Sources with Fermi GBM
Colleen Wilson-Hodge for the GBM Pulsar and Earth Occultation teams
NASA/MSFC, Huntsville Using the Gamma ray Burst Monitor (GBM) on Fermi, we monitor the transient hard X-ray/soft gamma ray sky. The twelve GBM NaI detec- tors span 8 keV to 1 MeV, while the two BGO detectors span 150 keV to 40 MeV. We use the Earth occultation technique to monitor a number of sources, including X-ray binaries, AGN, and solar flaring activity. Our mon- itoring reveals predictable and unpredictable phenomena such as transient outbursts and state changes. With GBM we also track the pulsed flux and spin frequency of accretion powered pulsars using epoch-folding techniques. Highlights from the Earth Occultation and Pulsar projects will be presented including our recent surprising discovery of variations in the total flux from the Crab. Searches for quasi-periodic oscillations and X-ray bursts are also possible with GBM all-sky monitoring capabilities. With these results we show how crucial an all-sky monitor is for any future X-ray timing mission.
58 25 The complete sample of type-I X-ray bursts in the low-mass X-ray binary 4U 1636-53 with RXTE
Guo-Bao Zhang Mariano Mendez, Diego Altamirano, Tomaso Belloni and Jeroen Homan
Kapteyn Astronomical Institute, University of Groningen Type-I X-ray bursts are due to unstable burning of H and He on the surface of accreting neutron stars in low-mass X-ray binaries (LMXBs). One of the best studied sources of X-ray bursts is the LMXB 4U 1636-53. Here we present a complete analysis of all X-ray bursts of this source observed to date (300 at the time of writing) with the Rossi X-ray Timing Explorer (RXTE). Using the bolometric light curves of individual bursts we found that about 96% of the bursts show single-peaked profiles, whereas about 4% show a multi-peaked profile. All the multi-peak bursts in this source took place when 4U 1636-53 occupied a relatively narrow region in the colour-colour diagram (CCD), corresponding to a relatively high (inferred) mass-accretion rate. As the source moves from the island state to the banana state on the CCD, both burst duration and recurence time decrease. From fits to time-resolved spectra, we found that about 20% of the single-peaked bursts are photospheric radius-expansion (PRE) bursts, whereas non of the multi- peaked burst is a PRE burst. We also found that on average the X-ray bursts in 4U 1636-53 do not follow the L ∼ T4 relation. We did a statistical study of the fitted blackbody temperature of X-ray bursts at different flux levels during the cooling phase of X-ray bursts. The distribution of blackbody temperature for different type of burst show significant differences at low flux level, where the average temperature in hard non-PRE bursts is higher than in PRE and soft non-PRE bursts. We calculated the colour correction factor (fc) as a function of luminosity for different bursts, and found that fc in hard non-PRE bursts is larger than in PRE and soft non-PRE bursts at the same luminosity level. From the timing analysis we found oscillations appear both in PRE and no-PRE bursts, but mostly when the source is in the soft state.
59
III Poster contributions
61
1 Preliminary concept of modern X-ray timing mis- sion
Vadim Arefiev M. Pavlinsky, M.Revnivtsev
Space Research Institute, Moscow We present concept of new X-ray timing mission proposed for Russian space program. The preliminary study includes the main scientific tasks for the X- ray timing mission, requirements for scientific payload and spacecraft. The mission will concentrate on X-ray variability study of bright black holes and neutron stars. These objects can give unique information for both strong gravity and nuclear physics. The study of accretion flows variability mecha- nism is essential to successful interpretation of X-ray data and unambiguous determination of fundamental parameters of compact objects. The main in- strument of the observatory is large area X-ray timing spectrometer.
63 2 The 2008 outburst of the new X-ray transient XTE J1719-291
Montserrat Armas Padilla N.Degenaar, R.Wijnands
Astronomical Institute Anton Pannekoek During the last decade a new population of X-ray transients have been discovered. They show anomalously low peak luminosities (2-10 keV) of 1034 to 1036 ergs/sec. A large fraction of them is expected to harbor accreting neutron stars and black holes in binaries systems. These very faint X-ray binaries provide new regimes to study accretion onto compact objects, and therefore they could challenge our understanding of accretion physics and binary evolution models. We present the 2008 outburst of the new X-ray transient XTE J1719- 291, which was observed with the Chandra, Swift, XMM-Newton and RXTE satellites. The spectra are consistent with a power law model affected by interstellar absorption. The photon index has values between 2 and 2.8, and the peak luminosity is 7 x 1035 ergs/sec for an assumed distance of 8 kpc. The X-ray spectrum softens along the outburst (i.e., while the luminosity decreases), which is not typically seen in X-ray transients. We derive a duty cycle of 1 per cent, from which we estimate a long-term time average accretion rate of ∼10−13 M/yr.
64 3 How the fast X-ray timing at extreme count rates will constrain the equation of state of the dense matter from the burst oscillations of the neutron stars
Romain Artigue Didier Barret
CESR/IRAP, Toulouse We show that detailed modeling of burst oscillations can be an extremely powerful probe of neutron star structure (M,R), and thus the equation of state of ultra-dense matter. M and R are encoded in the shape of the os- cillations resulting from spin modulation of the thermonuclear burst flux from the neutron star. We have developed an expanding hot spot model for X-ray burst oscillations, taking into account all relativistic effects affect- ing photon trajectories from the neutron star surface to the observer. We have then simulated several data sets (for an assumed M, R and hot spot geometry and emission pattern) with the statistics currently achieved with the RXTE/PCA and the one expected from a more advanced mission in term of count rate. We first show that thanks to the improved statistics, the harmonic content of the signal (whose amplitude relates directly to the asymmetry of the waveform) will be detected by this new mission. Using a library of theoretical light curves, we have the
65 4 kHz QPO studies with IXO - Testing the Mov- ing Hotspots Model
Matteo Bachetti Centre d’Etude Spatiale des Rayonnements Toulouse According to the moving hot spots hypothesis, kHz quasi-periodic oscilla- tions (kHz QPOs) are originated at the surface of the star from the move- ment of hot spots formed during accretion. In this scenario, the lower peak corresponds to hot spots created by the funnel flow in a hot region around the magnetic pole and moving around it, while the upper peak to hot spots cre- ated through instabilities closer to the equatorial zone, moving at a greater velocity than the polar ones. This model predicts that if the star is not almost perfectly aligned (misalignment angle much smaller than the polar hot region), the same movement originating the lower kHz QPO should also produce a dimmer feature at the frequency of the star. Low and high fre- quency phenomena should also be correlated, the red noise being influenced by the duration and frequency of appearance of the hot spots. The unprecedented collecting area of IXO, increasing the count rate by a factor of ∼ 10 with respect to RXTE, would be an extraordinary tool to study the light curves of LMXBs and look for features at the frequency of the star in observations where the lower QPO is present. Moreover, it would help study LMXB variability on shorter timescales, helping the investigation of the short-term coherence of QPOs and correlations between high and low- frequency phenomena.
66 5 Re Analysis of Timing Parameters of OAO 1657- 415
Altan Baykal Physics Department, Middle East Technical University Using the new orbital parameters of OAO 1657-415, archival data of RXTE observations are analysed. We investigate the spin up/down rate and X-ray flux correlations and re-evaluated the torque noise power density spectra.
67 6 Time lags in 4U 1608-52
Marcio Guilherme Bronzato de Avellar Mendez, Mariano; Sanna, Andrea; Horvath, Jorge
Instituto Astronˆomico,Geof´ısicoe de CiˆenciasAtmosf´ericas, S˜ao Paulo In binary systems with a neutron-star (NS) primary and a low-mass sec- ondary star that fills its Roche lobe, accretion onto the NS proceeds via a disk that extends very close to the surface of the NS. X-ray emission from the disk and the NS surface carries information about the geometry of space- time around the NS. The so-called kilohertz quasi-periodic oscillations (kHz QPOs) are one of the tools that can be used to study this. kHz QPOs are fluctuations in the X-ray emission of these systems, with frequencies between 300 and 1200 Hz, comparable to the orbital frequency 5 km above a 10-km and 1.4 solar mass NS, and are thought to be produced at (or close to) the inner edge of the accretion disk. Since (part of) the high-energy radiation from the surface of the NS is reprocessed an re-emitted at lower energies in the accretion disk, one can constrain the size of emitting region measuring the time lags between low- and high-energy photons on the time scales of the kHz QPOs. We studied the system 4U 1608-52, in which the kHz QPOs change from 540 to 1060 Hz. If these QPOs come from the inner edge of the accretion disk, the time lags should change with frequency, as the inner disc radius changes. We find a significant dependence of the time lags with energy, but a weak (if any) dependence with frequency. We use these results to constrain the location at which these QPOs are produced.
68 7 Flame spreading velocity in Type I X-ray bursts
Yuri Cavecchi Braithwaite, Watts & Levin
Sterrenkundig Instituut Anton Pannekoek, Amsterdam High resolution timing analysis of Type I Burst lightcurves has revealed the presence of oscillations in their intensity. There is evidence of a relation be- tween the frequencies of such oscillations and the spinning frequency of the NS, which in turn can be used to deduce information regarding the EOS. Thus, properly modelling such phenomena could give deep insight into the structure of ultra dense matter, as well as into other processes linked to the rotation of NS, such as i.e. gravitational wave emission. We will present re- sults from simulations of ignition and propagation of thermonuclear burning on the surface of an accreting NS, focusing on the dependence of the velocity of the burning front on physical parameters like conductivity and spinning frequency of the NS, as well as implications for future timing instruments.
69 8 The COSPIX mission : focusing on the ener- getic and obscured Universe
Philippe Ferrando on behalf of the COSPIX consortium
Irfu / Service d’Astrophysique, CEA Saclay Tracing the formation and evolution of all supermassive black holes, includ- ing the obscured ones, and understanding how black holes influence their surroundings, and how matter behaves under extreme conditions, are rec- ognized as key science objectives to be addressed by the next generation of instruments. These are the main goals of the COSPIX proposal, made to ESA in the context of its call for selection of the M3 mission. In addition, COSPIX, will also provide key measurements on the non thermal Universe, particularly in relation to the question of the acceleration of particles, as well as on many other fundamental questions as for example the energetic particle content of clusters of galaxies. COSPIX is proposed as an observatory operating from 0.1 to ∼ 100 keV. The payload features a single long focal length focusing telescope offering an effective area close to ten times larger than any scheduled focusing mission at 30 keV, an angular resolution better than 20 arcseconds in hard X-rays, and polarimetric capabilities. The COSPIX design extensively builds-on results obtained these last years in the studies of missions like Simbol–X and IXO. In this paper, we will describe the science objectives of the mission, its baseline design, and its performances, as proposed to ESA.
70 9 Phase lags and cyclotron lines in 4U 0115+63
Carlo Ferrigno M. Falanga, E. Bozzo
ISDC, University of Geneva Highly magnetized pulsars accreting matter in a binary system are bright sources in the X-ray band (0.1–100 keV). Despite the early comprehension of the basic emission mechanism, their spectral energy distribution is generally described by phenomenological or simplified models. We propose a study of the energy dependent pulse profiles of the high mass X–ray binary pulsars 4U 0115+63 to unveil the role of cyclotron resonant scattering in the pulse profile formation. We analyze archival BeppoSAX and RXTE data of six long observations of the bright phase of the giant outburst occurred during spring 1999 in the energy range 2–50 keV, and study the energy dependency of the pulse profiles by means of a cross correlation technique. We also develop a geometrical model of the column emission to motivate our findings. We find that the pulse profiles are significantly shifted and distorted in correspondence of the cyclotron scattering absorption features and this can be at first approximation explained by a variation of the angular dependent pattern of the radiation emitted by the column. The complex dependency on angle and energy of the beam originating in the accretion column is far from being satisfactorily understood, our analysis reveals that the resonance in the cross section of electrons in the strong magnetic field of this objects plays an important role, never explored in detail from this point of view. Data with high S/N at lower luminosity and a more sophisticated theoretical model will contribute significantly to the comprehension of the the X-ray emission in the strong gravitational and radiative fields of neutron stars.
71 10 Supergiant Fast X-ray Transients and other wind- fed accretors - testing with the Corbet diagram
Sergei Grebenev Space Research Institute, Moscow Supergiant wind-fed X-ray binaries and Be-systems occupy different regions in the Pspin-Porbit diagram (known as the Corbet diagram). The previous attempts to explain this diagram were not fully satisfactory. We propose a new explanation for the observed dependences which takes into account the fact that the accretting matter in such systems has smaller angular momentum than the Keplerian one. We review properties of the Supergiant Fast X-ray Transients recently discovered with INTEGRAL and note that their location at the Corbet diagram can be a key to understanding of the accretion mechanism responsible for their outbursts.
72 11 Aperiodic X-ray variability studies of the black hole candidate MAXI J1659-152
Maithili Kalamkar D. Altamirano, M. van der Klis, P. Casella, J. Homan, M. Linares
Astronomical Institute ‘Anton Pannekoek’, University of Amster- dam MAXI J1659-152 is an X-ray transient discovered in September 2010. The source was identified as a black hole candidate as it exhibited type C QPOs while in the hard intermediate state which is very typical of stellar-mass black hole transients. The source also exhibited the harmonic and sub- harmonic of the type C QPO along-with a strong power law noise. The source is currently in the soft intermediate state and exhibited type B QPO. In this poster, we will present the results of the aperiodic X-ray variability studies with Rossi X-ray Timing Explorer (RXTE). The QPOs are believed to originate in the region of the disk very close to the black hole and therefore can help us to understand the behaviour of matter under extreme conditions present in the vicinity of compact objects. Though the origin of QPOs is very poorly understood, their behaviour has been extensively studied in other transient black hole candidate systems. Similar QPOs have been seen in some neutron star systems. An account will be presented of the properties of the QPOs in MAXI J1659-152 in comparison to these earlier analyses.
73 12 Pulse profile variations in GX 1+4
Peter Kretschmar R. Ruiz Carmona, E. Kuulkers, A. Gonzalez Galan, C. Ferrigno
ESA/ESAC The symbiotic accreting X-ray pulsar GX 1+4 shows significant changes in its pulse profile together with strong, irregular luminosity variations typical for this source class. We present results of a comparative study of profiles obtained over several years with INTEGRAL.
74 13 Dipping and Absorption in the stellar wind in GX 301-2
Ingo Kreykenbohm Fuerst, F.; Gogus, E.; Belloni, T.; Schmid, C.; Wilms, J.; Pottschmidt, K.; Suchy, S.; Rothschild, R.E.; Kretschmar, P.
Dr. Karl Remeis Observatory & ECAP, Bamberg We present a detailed study of the absorption and dipping behavior of the accreting High Mass X-ray Binary GX 301-2 using high quality XMM timing mode data taken during the pre-periastron flare. We accumulate spectra with an exposure time of the order of 30 seconds and perform pulse phase resolved spectroscopy on single pulses. Keeping the continuum parameters fixed, we obtain a light curve of the column density NH and the iron line parameters, thus allowing us to track the evolution of the NH and the iron line with time. By tracking the NH and iron line with very high time resoultion, we can investigate the absorbing material in the stellar and its rapid changes This NH lightcurve shows that the density in the stellar wind fluctuates strongly on timescales of 100 seconds or less. Analysis of RXTE data shows that the source is also strongly variable outside the pre-periastron flare: the source exhibits intense flaring as well as dipping activity. The source countrate as recorded by the RXTE/PCA drops to almost zero during some of the dips in the lightcurve and no pulsations are observed, whereas the pulsations are clearly visible outside the dip. Similar phenomena have been observed in Vela X-1 (Kreykenbohm et al. 2008, 1999). These remarkable phenomena show that these objects need to be ob- served with IXO with high time resolution to be able to study the absorbing material and the pulsations in detail and thus track structures in the stellar wind. To assess the feasibility of such observations, we perform detailed simulations of observations of bright sources with the HTRS.
75 14 Pulse-phase spectroscopy as a tool for the study of X-ray pulsars
Alexander Lutovinov S.Tsygankov
Space Research Institute, Moscow We proposed to use the pulse-phase resolved spectroscopy as a tool for the study of the accretion flows structure in X-ray pulsars at different luminosi- ties. Such a comprehensive analysis require a new generation of instruments with high timing and spectroscopic capabilities like HTRS. Based on the current results of the analysis of the emission from transient X-ray pulsars obtained with the RXTE observatory we debated the hypothesis about a ge- ometrical origin of the spectral parameters variations with the pulse phase. We also shown that properties of the fluorescent iron line emission (partic- ularly a variability of its equivalent width on the time scale of the pulse period) can be used to investigate the spatial distribution and physical con- ditions of the matter around the compact object as well as in the binary system as a whole.
76 15 On the nature of low frequency quasi periodic oscillations: a variability study of the BHC GX 339-4
Sara Motta Osservatorio Astronomico di Brera A few distinct types of low-frequency oscillations showing rather different properties have been discovered in the X-ray emission of black-hole binaries. An exhaustive classification has not yet been obtained, however, three main oscillation types stand out in the present scenario. In the past years, it has become clear that the best way to characterize the spectral evolution of an X-ray transient is to produce a Hardness-Intensity Diagram, analogous to a stellar CMD but for each source separately. The three types of oscillations are observed only when the source is in very delimited areas of this dia- gram, with a very clear and well-defined phenomenology. Here I present the analysis of all the RXTE data of the bright black hole transient GX 339-4, which has shown five complete outbursts. I examine the parameters of the oscillations and compare them with the result of a full spectral analysis of the data. The three types display different dependences on the spectral and timing parameters, further strengthening their different nature.
77 16 X-ray bursts and burst oscillations from the slowly spinning X-ray pulsar IGR J17480-2446
Sara Motta Osservatorio Astronomico di Brera The newly discovered 11 Hz accreting pulsar, IGR J17480-2446, located in the globular cluster Terzan 5, has shown several bursts with a recurrence time as short as few minutes. The source shows the shortest recurrence time ever observed from a neutron star. We present a study of the morphologi- cal, spectral and temporal properties of 107 of the bursts observed by the Rossi X-ray Timing Explorer. The recurrence time and the fluence of the bursts clearly anticorrelate with the increase of the persistent X-ray flux. The ratio between the energy generated by the accretion of mass and that liberated during bursts indicate that Helium is ignited in a Hydrogen rich layer. Therefore we conclude that all the bursts shown by IGR J17480-2446 are Type-I X-Ray bursts.
78 17 Spectroscopy of the Pulsar-like White Dwarf AE Aquarii from Chandra and Swift-XRT Data
Bosco Oruru P. J. Meintjes
Department of Physics and UFS-Boyden Observatory, University of the Free State The novalike variable AE Aqr is perhaps the best source to study among accretion driven systems. It consists of a fast rotating highly magnetic WD orbiting, and accreting material from, a low-mass main sequence compan- ion. It has been observed in almost all wavelengths, with unique charac- teristics which include flares and coherent oscillations. The system is in a propeller state, and most of its emission properties are associated with the propeller process. We have analysed its X-ray spectra from data observed with Chandra and the Swift X-ray telescope, and the results show that the X-rays emission in the system has both thermal and non-thermal character- istics. In this presentation, results of the X-ray spectroscopy, based on the constraints on the emission mechanisms, thermal and non-thermal, will be discussed.
79 18 On the time evolution of the lower kHz QPO in 4U 1608-52
St´ephanePaltani D. Barret, M. S¨uveges
ISDC, University of Geneva We follow the evolution of the lower kHz QPO in the neutron-star X-ray binary 4U 1608-52 over 55 RossiXTE observations, for a total of more than 120 ks over 10 years. The evolution is found to differ significantly from a power-law in the sense that it evolves more slowly. The QPO is found to move symmetrically in frequency space, which implies non-symmetric steps in radius, if the QPO frequency is determined by the Keplerian frequency at a given distance to the central source. We propose a model of random-walk with a deterministic frequency drift that is able to reproduce the time series properties of the QPO frequency. If applied to the frequency evolution, the parallel-track model fails to reproduce the power-spectrum properties of the source flux.
80 19 A filter wheel for the HTRS
St´ephanePaltani Franck Cadoux, Ludovic Genolet
ISDC, University of Geneva The HTRS requires a filter wheel to perform several important tasks: to pro- tect the detector, to reduce the optical load on the detector when observing optically bright sources and to place a radioactive source in the optical beam for calibration purposes. The very compact footprint of the HTRS FPA and its very tight mass budget impose strong constraints on the filter wheel me- chanical envelope and mass. We present here the current design of the filter wheel, together with a full-scale bread-board model of the mechanism.
81 20 Central Engines of GRBs
Tsvi Piran Racah Institute for Physics Most GRB models are based on rapid accretion onto a newborn compact object. An alternative model is based on tapping the rotational energy of the compact object. This model has been suggested either for powering the prompt emission or as the source of the ”energy injection” observed in the plateau phase of the afterglow of many GRBs. I examine pros and cons of this model as well as the prospects of searching for an observed signature that would enable us to distinguish between the two.
82 21 INTEGRAL and the unified model of AGN
Claudio Ricci Roland Walter, Thierry Courvoisier, St´ephanePaltani
ISDC, University of Geneva We present the study of the average hard X-ray spectra of a sample of about 200 hard X-ray selected radio-quiet Active Galactic Nuclei (AGN) detected by INTEGRAL. Using all the available IBIS/ISGRI public data (as of May 2010), we produced the average stacked spectra of different classes of AGN in the 17-250 keV energy range. The sample consists of 68 Seyfert 1, 38 Seyfert 1.5, 77 Compton thin Seyfert 2, 12 Narrow Line Seyfert 1 (NLS1), and 13 Compton thick (CT) Seyfert 2. These subsamples have exposures up to 45 Ms. We found that, in agreement with the unified model of AGN, the av- erage spectra of Seyfert 1 and Seyfert 1.5 are consistent, and can be well represented by a cutoff power law with a photon index Γ = 1.9, a cutoff at Ec>300 keV, and a reflection parameter of R<0.4. The spectrum of Compton thin Seyfert 2 shows instead a much greater curvature, and, when normalized and compared to those of Seyfert 1 and Seyfert 1.5, it presents a strong excess in the 20-60 keV band. We found that this excess is due to the contribution of heavily obscured (Nh in the 1-7 1023 cm−2 range) Seyfert 2, which show a reflection dominated spectrum. The unabsorbed reflector of these objects has an average value of R>1, which might imply the existence of partially covering CT clumps in the line of sight. Taking into account reflection, the spectrum of heavily obscured Sy2 has the same continuum of Sy1 and Sy1.5. We will discuss the impact of these results on the Cosmic X-ray Background, and on the unified model of AGN.
83 22 Type I and type II X-ray bursts of the Rapid Burster as seen by Swift
Gloria Sala F. Haberl, J. Jos´e,A. Parikh, W. Pietsch
Univeristat Polit`ecnicade Catalunya, Barcelona The Rapid Burster (MXB1730-335) is a unique object showing both type I (thermonuclear) and type II (accretion driven) X-ray bursts. Swift ob- servations obtained during a long outburst in February-March 2009 show intense bursting activity, with bursts intervals between 25 and 200 seconds. During the first observation, most Type II bursts peak at fluxes higher than 10−8 erg/s cm2 (L=(1-1.6)x1038 erg/s (1-10 keV)), with the peak intensity decreasing in the last Swift observations. A Type I burst is observed on March 5, showing indications of photospheric radius expansion.
84 23 A Catalogue of X-ray bursters detected by JEM- X on board INTEGRAL
Celia Sanchez-Fernandez E. Kuulkers
INTEGRAL Science Operations Center, ESAC The wide field of view of JEM-X on board INTEGRAL allows concurrent monitoring of several sources in a single pointing, thus allowing serendipitous detection of interesting events from any of these targets. Taking advantage of this capability, we have performed a systematic search of Type-I X-ray bursts in the light curves of known X-ray bursters. For this task, all the the public data in the INTEGRAL archive have been analyzed. We present here the preliminary results of this work. In total, ∼ 70 000 science windows of observations have been analyzed, allow- ing the detection of 2243 Type-I X-ray burst candidates, with short and intermediate durations (≤ 100 sec).
85 24 Simulations of the performance of the HTRS on IXO
Christian Schmid Wilms J., Martin M., Kendziorra E., M¨uck B., Barret D., Rambaud D.
Dr. Karl Remeis-Observatory & ECAP, Bamberg We present the results of our analysis of the HTRS performance studied with a Monte-Carlo simulation of the photon imaging and detection pro- cesses. Based on the effective area of the individual mirror shells in the IXO silicon pore optics design we have analysed the photon distribution on the HTRS detector at its out-of-focus position taking into account misalignment effects according to the instrument alignment requirements. We have stud- ied different pixel geometries and analysed the bright source performance with respect to pile-up for analog and digital readout electronics.
86 25 Measuring strong magnetic fields of neutron stars with the next-generation of X-ray instru- ments
Gabriele Sch¨onherr P. Kretschmar, J. Wilms, I. Kreykenbohm, F. Schwarm, K. Pottschmidt, R. Rothschild
Astrophysikalisches Institut Potsdam To date, the only direct method to assess the strong magnetic fields of High- Mass X-ray Binaries containing accreting neutron stars is the observation and analysis of electron cyclotron resonance features (”cyclotron lines”) in their high-energy X-ray spectra. We discusss how the upcoming area of new observatories and instrumentation like IXO will change our ”magnetic” view of those objects.
87 26 Investigating the disc-jet coupling in accreting compact objects by studying the “radio quiet” sources
Paolo Soleri Rob Fender
Kapteyn Astronomical Institute, University of Groningen The coupling between accretion processes and ejection mechanisms in ac- creting black hole binaries can be investigated by empirical relations between the X-ray/radio and X-ray/optical-infrared luminosities. These correlations are valid over several orders of magnitude and can connect stellar-mass and supermassive black holes. Despite these relations were supposed to be uni- versal, many black hole binaries have been found to produce jets that, given certain accretion-powered luminosities, are fainter than expected from the correlations. This shows that black holes with similar accretion flows can produce a broad range of outflows in power, suggesting that some other parameters or factors might be tuning the accretion/ejection coupling. We discuss whether typical parameters of the binary system, as well as the prop- erties of the outburst, produce any effect on the energy output in the jet. No obvious dependence is found. We also define a jet-toy model in which the bulk Lorentz factor becomes larger than ∼1 above ∼0.1% of the Eddington luminosity. Despite several limits, this model can qualitatively describe the scatter of the X-ray/radio correlation and the ”radio quiet” population. We also compare the ”radio quiet” black holes with the neutron stars. We show that if a mass correction is applied, the possibility that they are statistically indistinguishable in the X-ray:radio plane can not be completely ruled out. Our result suggests that some of the outliers could actually be neutron stars. Another possibility is that the disc-jet coupling in the ”radio quiet” black holes is more similar to the one in neutron stars.
88 27 XTE J1752-223: A broad band X-ray spectral investigation combined with timing studies
Holger Stiele T. Munoz-Darias, S. Motta, T. Belloni
INAF Osservatorio astronomico di Brera We report on X-ray monitoring observations of the transient black hole candidate (BHC) XTE J1752-223 with the Rossi X-ray Timing Explorer (RXTE). The source was discovered on 2009 October 23 and during its low/hard state (LHS), which lasted for at least 25 days, all timing and spectral properties were similar to those of Cyg X-1 during its canonical hard state (Munoz-Darias et al. 2010, MNRAS 404, L94). The combined PCA/HEXTE spectra were well fitted by an absorbed broken power law with high energy cut off. When RXTE observations were resumed, after an observational gap due to solar constrains, the source was in the hard inter- mediate state (HIMS), showing type C QPOs. The source evolved through the soft intermediate state, characterized by a 5 - 10 % rms variability and type B QPOs, into the high soft state. In the following it showed several transitions between these states. The combined PCA/HEXTE spectra were well fitted by an absorbed disk black body plus broken power law. After a further 59 days XTE J1752-223 passed through another HIMS at lower lu- minosity into the LHS. We discuss the different states, including a discussion of the variability diagram, and present the results of our combine spectral (PCA/HEXTE) and timing investigations. In addition, we will discuss the results of our spectral studies in which we combined RXTE and Swift data.
89 28 X-ray bursting neutron star atmosphere mod- els
Valery Suleimanov J. Poutanen, K. Werner
IAAT, University of Tuebingen X-ray bursting neutron stars in low mass X-ray binaries constitute an ap- propriate source class to constrain masses and radii of neutron stars. We computed a necessary for these investigations set of model atmospheres and emergent spectra in a plane-parallel, hydrostatic, and LTE approximation with Compton scattering taken into account. The models were calculated for six different chemical compositions: pure hydrogen and pure helium at- mospheres, and atmospheres with solar mix of hydrogen and helium, and various heavy element abundances Z = 1, 0.3, 0.1, and 0.01 Z . For each chemical composition the models are computed for three values of surface gravity, log g =14.0, 14.3, and 14.6, and for 20 values of the luminosity in units of the Eddington luminosity, L/LEdd, in the range 0.001–0.98. The emergent spectra of all models are redshifted and fitted by a diluted black- body in the RXTE/PCA 3–20 keV energy band, and corresponding values of the color correction.
90 29 Magnetized neutron star atmospheres: beyond the cold plasma approximation
Valery Suleimanov G.G. Pavlov, K. Werner
IAAT, University of Tuebingen Harmonically spaced absorption features are observed in thermal soft X-ray spectra of some isolated neutron stars. The interpretation of the features as quantum resonances of magnetic free-free opacity has been recently sug- gested, and neutron star atmosphere models calculated in the cold plasma approximation. This approximation neglects the cyclotron emission of elec- trons at the harmonics of the cyclotron frequency, as well as the Doppler broadening of the cyclotron resonances, which may be important even at the relatively low temperatures of neutron star atmospheres. To demonstrate this effect, we calculate models of hydrogen neutron star atmospheres with 10 11 B ∼ 10 –10 G (i.e., the electron cyclotron energy Ec,e ∼ 0.1–1 keV) and Teff = 1–3 MK with the thermal cyclotron opacities taken into account. Such conditions are thought to be typical for the so-called central compact objects in supernova remnants, such as 1E 1207.4–5209 in PKS 1209–51/52. We show that the ”warm plasma effects” are very significant in first three cyclotron harmonics, and they should be taken into account for the proper interpretation of the observed spectra in addition to the quantum oscillations in the magnetic free-free opacity.
91 30 Extreme-value analysis of the X-ray emission of Cygnus X-1
Maria S¨uveges S. Paltani
ISDC Data Centre for Astrophysics, University of Geneva The multiplicative model of Uttley et al. (2005) for the X-ray emission from X-ray binaries accounts well for the observed global characteristics of the time series such as the linear flux-rms relationship and the lognormal distribution of the fluxes. However, models that are fitted to the bulk of the data may not provide satisfactory models for the extreme events in the series: these might be produced by other unrecognized processes. As the model of Uttley et al. (2005) implies a well-determined structure of events above high thresholds, we applied methods from extreme-value statistics to check whether the same process can be responsible for the extremes in the data. We first considered the limiting distribution of large events as ex- pected from the model, and compared to that of a model fitted to data from Cygnus X-1. Next, the clustering characteristics of the observed large events was compared to what is expected. Both comparisons show that the mul- tiplicative model can indeed generate the observed extreme events, though there are indications of non-stationarity in the baseline level of the process and also of short periods where the observed extreme-value characteristics are incompatible with the model implications.
92 31 Synchrotron self-Compton model for optical/X- ray correlation in black hole X-ray binaries
Alexandra Veledina J. Poutanen, I. Vurm
Department of Physics, University of Oulu Simultaneous optical and X-ray observations of compact objects in low-mass X-ray binaries allow us to constrain radiative processes operating in these systems. Such observations for the first time were carried out by Motch et al. (1983) for X-ray binary GX 339-4. Although the time resolution of data was not very good, the authors were first to report on complicated cross- correlation function (CCF) between the two bands, with an anti-correlation preceding positive correlation. The same structure of CCF was later found in XTE J1118+480 (Kanbach et al. 2001) and in SWIFT J1753.5-0127 (Durant et al. 2008). All these objects are black-hole candidates, however, recently similar CCFs were found for two binaries, Sco X-1 and Cyg X-2, hosting neutron stars (Durant et al. 2010). The physical nature of the CCF structure is unknown. Number of toy models were developed to explain such a behavior, most of them include jets as a source of optical emission. Detailed calculations of resulting CCFs were made so far only by Malzac et al. (2004), where the optics and the X-rays are fed by magnetic energy reservoir. However, the model has a number of problems, for example, it is not clear how to maintain such an energetic magnetized zone. Also in the case of SWIFT J1753.5-0127, the jet seems to be too faint to contribute to the optical luminosity (Soleri et al. 2010). We propose a novel model capable of explaining timing properties of these objects. We argue that the X-ray emission is produced by synchrotron self-Compton mechanism, which also contributes to the optical luminosity. Small variations in the accretion rate cause changes of parameters of the flow (optical depth, total luminosity), leading to variations in X-rays and optical luminosities. By simulating spectra for specific parameters, we show that fluxes in the two bands are anti-correlated. Additional contribution to the optics comes from reprocessing of the X-rays by the outer regions of the accretion disc. We find that our model is consistent with the CCFs recently observed from Galactic black holes. From the presence of the dips in the CCFs, we can estimate the size of the synchrotron-emitting regions, and from the absolute value of the dips we constrain the contribution of synchrotron radiation in the optical energy band.
93 32 Fast variability of gamma-ray emission from supermassive black hole binary OJ 287
Ievgen Vovk A. Neronov
ISDC Data Centre for Astrophysics, University of Geneva We report the discovery of fast variability of gamma-ray flares from blazar OJ 287. This blazar is known to be powered by the binary system of super- massive black holes. The observed variability time scale Tvar < 3 hr is much shorter than the light crossing time of the more massive (1.8 x 1010 solar masses) black hole and is comparable to the light crossing time of the less massive (2 x 108 solar masses) black hole. This indicates that the observed gamma-ray emission is produced by relativistic jet ejected by the black hole of smaller mass. Detection of gamma-rays with energies in excess of 10 GeV during the fast variable flares constrains the bulk Lorentz factor of the jet to be larger than 4. Possibility of the study of the orbital modulation of emis- sion from relativistic jet makes OJ 287 a unique laboratory for the study of the mechanism(s) of formation of jets by black holes, in particular, of the response of the jet parameters to the changes of the parameters of the medium
94 33 The Digital Data Processing Unit for the HTRS on board IXO
Henning Wende G. Distratis, C. Tenzer, E. Kendziorra, A. Santangelo
IAAT, University of Tuebingen The Institute for Astronomy and Astrophysics in T¨ubingenparticipates in the development of two of the IXO instruments - one of them is the High Timing Resolution Spectrometer (HTRS). The soft- and hardware for its Data Processing Unit (DPU) is being developed and will be tested in T¨ubingen. We will give a brief overview of the challenges that the DPU faces, in particular the instrument data rate. The two main solutions to this problem and thus the primary tasks of the DPU are then presented in more detail. Reducing the incoming data rate to a value limited by the available telemetry rate is achieved in two steps: First the data handling FPGA will implement several highly configurable detector operation modi to reduce the amount of data while enabling the observer to optimize sci- entific output of the observation. The second step is lossless on-board data compression and intelligent wrapping of the data done by a Leon3 CPU ad- ditionally implemented in the FPGA. We present first simulation results of the DPU operations that show the achieved data rate reduction and give an update on the development and performance testing of an operational DPU prototype done in T¨ubingen.
95 34 LATE POSTER: Are the two peaks of the Cathedral QPO real harmonics?
J´erˆomeRodriguez P. Vari`ere
CEA Saclay We present a complete study of the two main peak of the so-called cathedral QPO in XTE J1859+226. While looking at the temporal evolution of the two features we show that they do not manifest the same amplitude of variations of their power, and do not seem to follow the flux variation in the same way. We then present their RMS-spectra and show that they do not have the same shape, slope and cut-off energy. We discuss these different facts and try to answer the question regarding the genuineness of their harmonic relationship.
96