Discovery and characterization of the binary system LSI +61◦303 in very high energy gamma-rays with MAGIC Ph.D. Dissertation by N´uria Sidro Mart´ın supervised by Juan Cortina Blanco & F. Javier Rico Castro Institut de F´ısica d’Altes Energies Universitat Aut`onoma de Barcelona Barcelona, June 5th 2008 N´uria Sidro Mart´ın Institut de F´ısica d’Altes Energies Universitat Aut`onoma de Barcelona E-08291 Barcelona [email protected] i Contents 1 Introduction 1 2 Very High Energy Gamma Ray Astronomy 3 2.1 Cosmic rays ..................................... 3 2.2 Mechanisms of γ-ray production ......................... 5 2.3 Sources of VHE γ-rays ............................... 6 2.4 Detection techniques ................................ 10 2.4.1 Direct detections .............................. 10 2.4.2 Indirect detections: Extended air showers ................ 11 2.4.3 Cerenkovˇ light from EAS ......................... 14 2.4.4 Cerenkovˇ telescopes ............................ 16 3 The MAGIC Telescope 21 3.1 The telescope frame, mirror dish and drive system ............... 22 3.2 The camera ..................................... 24 3.3 Trigger and data acquisition systems ....................... 27 3.4 The calibration system ............................... 28 3.5 The slow control .................................. 30 3.5.1 Camera subsystems and drivers ...................... 30 3.5.2 State-machine decision manager ..................... 32 4 Data analysis technique 39 4.1 Signal extraction: charge and arrival time .................... 40 4.1.1 Pulse shape reconstruction ........................ 40 4.1.2 Criteria for optimal signal extraction ................... 41 4.1.3 Extraction algorithms ........................... 42 4.1.4 Monte-Carlo studies of signal extraction ................. 42 4.1.5 Pedestal reconstruction .......................... 52 4.1.6 Reconstruction of calibration pulses ................... 53 4.1.7 Signal extraction conclusions ....................... 54 4.2 Event reconstruction ................................ 55 4.2.1 Calibration ................................. 55 4.2.2 Identification of bad pixels ........................ 56 4.2.3 Image cleaning ............................... 56 4.2.4 Image parameters ............................. 57 4.2.5 MAGIC Monte-Carlo simulation ..................... 59 4.2.6 Data quality checks ............................ 60 4.2.7 Gamma/Hadron separation ........................ 61 4.2.8 Energy reconstruction ........................... 63 4.2.9 Source position reconstruction ...................... 65 4.3 Signal evaluation and spectral reconstruction .................. 66 4.3.1 Evaluation of the significance of a positive detection .......... 66 4.3.2 Upper Limit calculation .......................... 67 4.3.3 Reconstruction of the energy spectrum .................. 68 ii N´uria Sidro Mart´ın, 2008 4.4 Observation modes ................................. 71 4.4.1 On/Off and Wobble ............................ 71 4.4.2 Observations in presence of moonlight .................. 72 4.5 Systematic errors .................................. 74 5 LSI +61◦303: description 77 5.1 X-ray binaries .................................... 77 5.2 Optical observations and the nature of the objects in the system ....... 78 5.3 Radio and X-ray observations ........................... 82 5.3.1 Time and spectral features ........................ 82 5.3.2 Morphological features ........................... 86 5.4 Previous γ-ray observations ............................ 89 5.5 Accretion vs colliding wind models ........................ 90 5.5.1 The microquasar scenario ......................... 90 5.5.2 The colliding winds scenario ........................ 93 5.6 Other γ-ray binaries ................................ 95 6 VHE spectral and temporal properties of LSI +61◦303 97 6.1 Introduction ..................................... 97 6.2 Observations and data selection .......................... 98 6.2.1 Campaign I ................................. 98 6.2.2 Campaign II ................................ 100 6.3 Source Detection .................................. 103 6.4 Light Curve ..................................... 104 6.4.1 Intranight variability ............................ 107 6.5 Spectral Variability ................................. 111 6.6 Periodicity Analysis ................................ 115 6.6.1 Statistical tests ............................... 115 6.6.2 Periodic analysis of the LSI +61◦303 data ................ 129 6.6.3 Effect of the time binning ......................... 134 6.6.4 Peak frequency and its error ....................... 137 6.6.5 Periodicity conclusions ........................... 138 6.7 Simultaneous and follow up observations ..................... 139 6.7.1 Campaign I ................................. 139 6.7.2 Campaign II ................................ 140 7 Conclusions and perspectives 145 A Future LSI +61◦303 scheduling 149 B Camera and calibration control 153 1 Chapter 1 Introduction The window of very high energy (VHE) gamma-ray astrophysics was opened less than two decades ago, when the Crab Nebula was detected for the first time, with photon energies from 100 GeV to 100 TeV. After several years of development, the technique used by imaging atmospheric Cerenkovˇ telescopes is now allowing to conduct sensitive observations in the VHE γ-ray regime. The instruments detect cosmic γ-rays by the detection of secondary showers produced by interactions of primary γ-rays with Earth’s atmosphere, seen either directly (shower arrays) or through their Cerenkovˇ radiation (Imaging Atmospheric Cerenkovˇ telescopes – IACTs). In this short time, the field evolved fast, and several extragalactic and galactic source classes have been established as TeV emitters, showing a variety of interesting phenomena that are boosting theory in VHE γ-ray astrophysics. The Major Atmospheric Gamma Imaging Cerenkovˇ (MAGIC) telescope is currently the largest single dish IACT, characterized by a 17 m diameter reflector. Located on La Palma (Spain), it features the lowest energy threshold IACT nowadays, allowing the reconstruction of γ-rays with energies from 60 GeV to 10 TeV. It combines a huge, ultralight reflector with a large number of technical innovations. The camera has a total field of view of about 3.5◦. When this work was started in 2004, no stellar X-ray binary object was known to emit VHE γ-rays, although theoretical predictions for theses systems were promising. Among them, LSI +61◦303 is a peculiar binary, characterized by a variable radio emission, modulated with a 26.5 days orbital period. The source was also known to be periodic in infrared, optical∼ and X-ray bands. LSI +61◦303 was initially proposed as a possible counterpart of the high-energy COS B γ-ray source, which was also detected by EGRET above 100 GeV (3EG J0241+6103). However, the association was unclear due to the low angular resolution of EGRET. This possible association together with the fact that jet flows were claimed from radio interferometric observations, made LSI +61◦303 a promising microquasar candidate. The detection and detailed study of LSI +61◦303 at VHE γ-rays with MAGIC was proposed as the main objective of this thesis, as an attempt to better understand the nature and physical properties of this peculiar system. This thesis is structured as follows. In chapter 2, I give an overview of the production mechanisms and sources of VHE • γ-ray. I describe the physics of the extended air showers produced by the cosmic rays entering in the atmosphere and the techniques developed to detect them. In chapter 3, the MAGIC telescope is presented. I will introduce in detail the re- • mote camera control software, since is the part of the detector in which I contributed technically. The analysis chain of the MAGIC data is explained in chapter 4: from the reconstruction • of the image recorded by the telescope of the air shower produced by the cosmic ray in the atmosphere to the selection of the γ-ray candidates among these recorded images. And the final reconstruction of the physical physical quantities, which finally yields 2 N´uria Sidro Mart´ın, 2008 the γ-ray estimated energy and incident direction, the significance of the γ-ray source detection, the lightcurve and the energy spectrum. The LSI +61◦303 X-ray binary, its physical properties and phenomenology are presented • in chapter 5. There I also review the theoretical models of VHE γ-ray production. In chapter 6, I present the results of the analysis of LSI +61◦303 observations with • MAGIC during years 2005 and 2006. I report on the discovery of VHE γ-rays from the direction of LSI +61◦303. The position, extension, energy spectra and differential fluxes of this source are presented. The flux variations with time are studied in detail, which yields a periodic γ-ray signal, that points to the mechanism for VHE emission being related to the orbital period of the system. The conclusions of the results of my analysis are the subject of chapter 7. • At the time of writing this dissertation, there are already 4 galactic γ-ray binary objects: LS 5039, PSR B1259-63, Cyg X1 and LSI +61◦303 that are detected at VHEs and established as a new category named γ-ray binaries. 3 Chapter 2 Very High Energy Gamma Ray Astronomy In this chapter we review the physical mechanisms by which very high energy γ-rays are pro- duced, and the astrophysical objects where these mechanisms may take place. We move on to describe the Cerenkovˇ technique used by Imaging Air Cerenkovˇ Telescopes to indirectly detect γ-rays