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Observational Properties of Gigaelectronvolt-Teraelectronvolt Blazars and the Study of the Teraelectronvolt Blazar RBS 0413 with VERITAS

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Observational Properties of Gigaelectronvolt-Teraelectronvolt Blazars and the Study of the Teraelectronvolt Blazar RBS 0413 with VERITAS Observational Properties of Gigaelectronvolt-Teraelectronvolt Blazars and the Study of the Teraelectronvolt Blazar RBS 0413 with VERITAS G¨une¸sDemet S¸ent¨urk Submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy in the Graduate School of Arts and Sciences COLUMBIA UNIVERSITY 2013 c 2013 G¨une¸sDemet S¸ent¨urk All Rights Reserved ABSTRACT Observational Properties of Gigaelectronvolt-Teraelectronvolt Blazars and the Study of the Teraelectronvolt Blazar RBS 0413 with VERITAS G¨une¸sDemet S¸ent¨urk Blazars are active galactic nuclei with a relativistic jet directed towards the observer’s line of sight. Characterization of the non-thermal continuum emission originating from the blazar jet is currently an essential question in high-energy astrophysics. A blazar spectral energy distribution (SED) has a typical double-peaked shape in the flux vs. energy representation. The low-energy component of the SED is well-studied and thought to be due to synchrotron emission from relativistic electrons. The high-energy component, on the other hand, is still not completely understood and the emission in this part of the blazar spectrum can extend to energies as high as tera electron volts in some objects. This portion of the electromagnetic spectrum is referred to as the very-high-energy (VHE or TeV, E > 0.1 TeV) regime. At the time of this writing, more than half a hundred blazars have been detected to emit TeV gamma rays, representing the high energy extreme of these objects and constituting a population of its own. Most of these TeV blazars have also been detected in the high-energy (HE or GeV, 0.1 GeV <E< 0.1 TeV) gamma-ray range. In this work, we report on our discovery of the TeV emission from the blazar RBS 0413 and perform a detailed data analysis on this source, including contemporaneous multi-wavelength obser- vations to characterize the broad-band SED and test various emission models for the high-energy component. Further, we extend our focus on the high-energy component to all archival TeV- detected blazars and study their spectral properties in the framework of GeV and TeV gamma-ray observations. To do this, we assemble for the first time the GeV and TeV spectra of a complete sample of TeV-detected blazars available in the archive to date. In the Appendix we present an analysis method for improved observations of large zenith angle targets with VERITAS. Table of Contents I TeV Astronomy 3 1 Cherenkov Technique 4 1.1 Extensive Air Showers ................................... 5 1.1.1 Shower Formation ................................. 5 1.1.2 Cherenkov Radiation ................................ 6 1.1.3 Cherenkov Light Pool ............................... 6 1.2 Details of the Cherenkov Technique ............................ 8 1.2.1 Energy of the Primary ............................... 10 1.2.2 Sensitivity ...................................... 11 1.2.3 Gamma/Hadron Separation ............................ 12 1.3 TeV Instruments ...................................... 14 1.3.1 Imaging Atmospheric Cherenkov Telescopes ................... 14 1.3.2 Other Instruments ................................. 15 1.4 GeV Instruments: Fermi Large Area Telescope ..................... 16 2 VERITAS Instrument 18 2.1 Structure and Hardware .................................. 19 2.1.1 Tracking ....................................... 20 2.1.2 Optics and Light Collection ............................ 21 2.1.3 Camera and PMTs ................................. 22 2.2 Trigger and Data Acquisition System ........................... 25 i 2.2.1 Trigger Steps .................................... 25 2.2.2 Data Acquisition .................................. 27 2.3 Observing Modes and Calibration ............................. 28 2.3.1 Data Runs ..................................... 28 2.3.2 Flasher Runs .................................... 28 2.3.3 Pedestals ...................................... 29 2.3.4 Bias Curve ..................................... 30 2.3.5 Pointing Monitors ................................. 30 2.3.6 Mirror Reflectivity and Alignment ........................ 31 2.4 Analysis of VERITAS Data ................................ 33 2.4.1 Calibration Calculation and Image Cleaning ................... 33 2.4.2 Image Parameterization .............................. 35 2.4.3 Shower Reconstruction ............................... 38 2.4.4 Background Rejection and Signal Extraction .................. 40 II Blazars 48 3 Active Galactic Nuclei and Blazars 49 3.1 Active Galaxies ....................................... 50 3.1.1 AGN Continuum and Variability ......................... 50 3.1.2 Emission lines .................................... 53 3.1.3 Active Galaxy Types ................................ 53 3.1.4 AGN Structure and Unification .......................... 55 3.2 Blazars ........................................... 56 3.2.1 Superluminal Motion ................................ 58 3.2.2 Relativistic Beaming ................................ 59 3.2.3 Spectral Properties and Radiation Mechanisms ................. 60 3.2.4 Blazar Sequence .................................. 63 ii 3.3 TeV Blazars ......................................... 64 3.3.1 Correlations Between TeV Emission and Other Bands ............. 64 3.3.2 Indirect Studies Involving TeV Blazar Emission ................. 65 III Results 69 4 The TeV Discovery of the BL Lac Object RBS 0413 70 4.1 Historical Background ................................... 70 4.2 VERITAS Observations and Results ........................... 71 4.3 Contemporaneous Multiwavelength Data ......................... 75 4.4 Modeling of Spectral Energy Distribution ........................ 76 4.5 Discussion .......................................... 78 5 GeV-TeV Properties of TeV Blazars 80 5.1 Data Sample ........................................ 80 5.2 Fermi Analysis ....................................... 82 5.3 Results and Discussion ................................... 87 5.3.1 IC Peak Frequency ................................. 87 5.3.2 Hard TeV BL Lac Objects ............................. 91 5.3.3 Spectral Variability ................................. 93 5.3.4 Spectral Features .................................. 95 5.3.5 Quasi-simultaneous GeV-TeV Spectra ...................... 100 IV Discussion and Conclusion 102 V Bibliography 105 Bibliography 106 iii VI Appendices 120 A The Disp Method 121 A.1 Description of the algorithm ................................ 122 A.2 Application and Results .................................. 124 A.2.1 LZA Crab Nebula observations .......................... 124 A.2.2 Galactic Center Analysis .............................. 124 A.3 Further Tests and Improvements ............................. 124 A.3.1 Performance of Disp as a function of energy ................... 125 A.3.2 Off-axis Reconstruction .............................. 125 A.3.3 Effect of the loss cut ................................ 126 A.3.4 Effect of the distance dimension ......................... 126 A.4 Combining Geo and Disp ................................. 127 A.5 Conclusion and Outlook .................................. 127 iv List of Figures 1.1 Development of extensive air showers ........................... 7 1.2 Cherenkov light pool .................................... 9 1.3 Simulated air showers for a 1 TeV gamma ray and a 1 TeV nucleon .......... 13 1.4 Lateral distribution of Cherenkov photons on the ground ................ 14 1.5 Fermi instrument ...................................... 17 2.1 The VERITAS array .................................... 19 2.2 A VERITAS telescope ................................... 20 2.3 Hexagonal mirrors mounted on a VERITAS telescope ................. 21 2.4 A VERITAS camera .................................... 22 2.5 Inner structure of a photomultiplier tube (PMT) .................... 23 2.6 A Cherenkov pulse generated in a PMT pixel ...................... 24 2.7 Photon detection efficiency ................................ 24 2.8 VERITAS trigger system ................................. 27 2.9 Bias curve .......................................... 30 2.10 Point spread function vs. elevation ............................ 31 2.11 Telescope reflectivity .................................... 32 2.12 Relative gain distributions ................................. 34 2.13 A cosmic ray shower image ................................ 36 2.14 Hillas parameterization of a shower image on the camera plane ............ 37 2.15 Reconstructed direction of a shower image ........................ 39 v 2.16 Core location reconstruction ................................ 40 2.17 θ2 distribution for the galactic center data ........................ 42 2.18 Normalized MSW distribution of gamma and CR events ................ 43 2.19 Camera Acceptance .................................... 44 2.20 ON and OFF regions used in background rejection ................... 46 2.21 Excess and significance maps for the galactic center data ................ 47 3.1 SED of the radio-loud quasar 3C 273 ........................... 52 3.2 AGN structure ....................................... 57 3.3 Superluminal motion observed in the radio images of the quasar 3C 279 ....... 67 3.4 Schematic diagram for superluminal motion ....................... 68 3.5 Blazar sequence ....................................... 68 4.1 RBS 0413 excess counts and significance maps ...................... 72 4.2 RBS 0413 cumulative significance and θ2 distribution .................. 73 4.3 VERITAS photon spectrum for RBS 0413 .......................
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