THE COLLIMATION SIGNATURES OF GAMMA-RAY BURSTS: JET PROPERTIES AND ENERGETICS INFERRED FROM X-RAY AFTERGLOW OBSERVATIONS

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1 The Pennsylvania State University The Graduate School Department of Astronomy and Astrophysics THE COLLIMATION SIGNATURES OF GAMMA-RAY BURSTS: JET PROPERTIES AND ENERGETICS INFERRED FROM X-RAY AFTERGLOW OBSERVATIONS A Dissertation in Astronomy and Astrophysics by Judith Lea Racusin c 2009 Judith Lea Racusin Submitted in Partial Fulfillment of the Requirements for the Degree of Doctor of Philosophy August 2009

2 The dissertation of Judith Lea Racusin was reviewed and approved 1 by the following: David N. Burrows Senior Scientist and Professor of Astronomy and Astrophysics Dissertation Adviser Chair of Committee John A. Nousek Professor of Astronomy and Astrophysics Peter Mészáros Eberly Chair Professor of Astronomy and Astrophysics Derek B. Fox Assistant Professor of Astronomy and Astrophysics Paul Sommers Professor of Physics Lawrence Ramsey Professor of Astronomy and Astrophysics Head of the Department of Astronomy and Astrophysics 1 Signatures on file in the Graduate School.

3 iii Abstract Our understanding of gamma-ray bursts (GRBs and their afterglows has progressed dramatically over the last few years thanks to the Swift mission and progress in prompt follow-up observations. These advances have led to frequent near-complete coverage at optical and X-ray wavelengths starting shortly after the bursts and often lasting for weeks. This dissertation explores the properties of X-ray afterglows in the context of the canonical X-ray afterglow model, with particular emphasis on the lack of expected jet collimation signature that was commonly seen in pre-swift optical observations. I find that with slight modifications to the canonical X-ray afterglow model in the context of the Fireball model, by testing many permutations of the closure relations, I can find many more jet breaks than are initially obvious from the data; the remaining residual deficit can be largely resolved by accounting for observing biases. The properties of jet collimation measured from GRB afterglows have direct implications for the inferred energetics of the bursts, which has important theoretical implications. I study the extraordinary naked-eye GRB B as an example of a burst with unusual observational properties of the prompt emission and afterglow suggestive of more complex two-component jet structure than the canonical form. Finally, I explore the expectations for GRB afterglow observations if all GRBs have this two-component structure but are commonly seen off-axis. The implications of this non-canonical two-component jet structure for the total energetics, frequency, and physics of GRBs and their afterglows are far-reaching.

4 iv TABLE OF CONTENTS List of Tables List of Figures Acknowledgments vii viii xv Chapter 1. INTRODUCTION History of Gamma-Ray Bursts Understanding of Gamma-Ray Bursts Prior to the Swift Era The Fireball Model Prompt Emission Afterglows Optical and Near-Infrared Radio X-ray Supernova-GRB Connection The Swift era of Gamma-Ray Burst Science Optical Afterglows X-ray Afterglows The Missing Jet Break Problem and Other Jet Constructions 15 Chapter 2. METHODS XRT Data Analysis Temporal Analysis Light Curve Construction Light Curve Fitting Light Curve Classification Spectral Analysis Chapter 3. THEORETICAL EXPECTATIONS: THE CLOSURE RELATIONS Synchrotron Spectrum and Evolution The Closure Relations Application to the Canonical Afterglow Segment I - Steep Decay Segment II - Plateau Phase Segment III - Normal Decay Segment IV - Post-Jet Break Phase Flares Internal Consistency Checks Classification

5 Chapter 4. THE SEARCH FOR JET BREAKS IN CANONICAL X-RAY AFTER- GLOWS Jet Breaks in the Canonical Scenario Full Canonical Afterglows with Jet Breaks Hidden and Ambiguous Jet Breaks Possible and Unlikely Jet Break Classes Significance of Closure Relation Distinctions GRB Properties Inferred from Closure Relations Exceptions Observing Biases Short Hard Bursts Relevance to Previous Jet Break Studies Chromaticity of Jet Breaks Energetics Estimation of E γ,iso Jet Opening Angles and Implied Energetics Canonical Jet Break Conclusions Chapter 5. THE NON-CANONICAL GRB B Extraordinary Observational Properties Prompt Emission Afterglow X-ray Optical/UV/NIR Radio/mm Interpretation Prompt Emission Afterglow Light Curve Afterglow Spectral Modeling Two-Component Jet Model One-Component Complex Density Medium Model Evidence for the Non-Canonical Two-Component Jet Chapter 6. UNIVERSALITY OF THE TWO-COMPONENT JET Search for Evidence of Other On-Axis Gamma-Ray Bursts Sample Selection Closure Relations, Jet Properties, and Energetics Results Implications for Gamma-Ray Burst Properties and Energetics Testable Predictions for Off-Axis Two-Component Emission Chapter 7. CONCLUSIONS AND FUTURE WORK Appendix A.TEMPORAL AND SPECTRAL FIT PARAMETERS Appendix B.JET BREAK CANDIDATE CATEGORIES v

6 Appendix C.CLOSURE RELATION FITS Bibliography vi

7 vii List of Tables 2.1 KS Probability of α Segments KS Probability of β Segments Closure Relations GRB Properties Inferred from Closure Relation Fits GRB Spectral Properties from the Literature Prompt Emission: Band Function Fits Observations of GRB B Two-Component Jet Parameters Two-Component Jet Model Candidates A.1 Temporal Fits A.2 Spectral Fits A.3 Relevant Times and Jet Angles B.1 Prominent Jet Break Candidates B.2 Hidden Jet Break Candidates B.3 Possible Jet Break Candidates B.4 Unlikely Jet Break Candidates B.5 Non-Jet Break Candidates B.6 Jet Break Summary

8 viii List of Figures 1.1 BATSE Durations BATSE Skymap BATSE γ-raylight Curves Spherical Versus Collimated Emission BeppoSAX X-ray Afterglow Light Curve Examples Canonical X-ray Afterglow Light Curve Monte Carlo Error Estimates Distributions of α Distributions of β Synchrotron Spectral Model Synchrotron Temporal Model Prominent Jet Break Light Curves Example Prominent Jet Break Temporal Limits Distributions Hidden Jet Break Example Temporal Behavior Comparisons α α Test Unlikely Jet Break Example Possible Jet break III-IV Example Possible Jet Break II-IV Example Single Power-Laws Limits Single Power-Law Energetics Jet Break Lower Limits Temporal Distributions E γ,iso Distribution Break Times Jet Opening Angles E γ Distributions t b Distributions Broadband Light Curve Prompt Emission Light Curve Prompt Emission Spectra X-ray Afterglow Fit Optical Afterglow Fit Two-Component Jet Schematic Late-Time GRB B Light Curve SED Data

9 5.9 Spectral Components of Two-Component Jet Model and Dependent Temporal Regimes Two-Component Jet Modeling SED Fits for One-Component Complex Density Structure Model ν c as a Function of Time On-Axis Two-Component Jet Light Curve Morphologies Two-Component Jet Candidate GRB Two-Component Jet Candidate GRB Two-Component Jet Candidate GRB Two-Component Jet Candidate GRB Two-Component Jet Candidate GRB Two-Component Jet Candidate GRB Two-Component Jet Candidate GRB Likelihood of Detecting Narrow Jets C.1 GRB Closure Relation Fits C.2 GRB Closure Relation Fits C.3 GRB Closure Relation Fits C.4 GRB B Closure Relation Fits C.5 GRB A Closure Relation Fits C.6 GRB B Closure Relation Fits C.7 GRB Closure Relation Fits C.8 GRB Closure Relation Fits C.9 GRB Closure Relation Fits C. GRB Closure Relation Fits C.11 GRB Closure Relation Fits C.12 GRB Closure Relation Fits C.13 GRB Closure Relation Fits C.14 GRB Closure Relation Fits C.15 GRB 0504 Closure Relation Fits C.16 GRB Closure Relation Fits C.17 GRB A Closure Relation Fits C.18 GRB Closure Relation Fits C.19 GRB Closure Relation Fits C.20 GRB B Closure Relation Fits C.21 GRB Closure Relation Fits C.22 GRB A Closure Relation Fits C.23 GRB Closure Relation Fits C.24 GRB A Closure Relation Fits C.25 GRB Closure Relation Fits C.26 GRB Closure Relation Fits C.27 GRB Closure Relation Fits C.28 GRB Closure Relation Fits C.29 GRB A Closure Relation Fits ix

10 C.30 GRB B Closure Relation Fits C.31 GRB B Closure Relation Fits C.32 GRB Closure Relation Fits C.33 GRB Closure Relation Fits C.34 GRB Closure Relation Fits C.35 GRB Closure Relation Fits C.36 GRB Closure Relation Fits C.37 GRB Closure Relation Fits C.38 GRB Closure Relation Fits C.39 GRB Closure Relation Fits C.40 GRB Closure Relation Fits C.41 GRB Closure Relation Fits C.42 GRB Closure Relation Fits C.43 GRB Closure Relation Fits C.44 GRB A Closure Relation Fits C.45 GRB Closure Relation Fits C.46 GRB Closure Relation Fits C.47 GRB Closure Relation Fits C.48 GRB Closure Relation Fits C.49 GRB Closure Relation Fits C.50 GRB Closure Relation Fits C.51 GRB A Closure Relation Fits C.52 GRB B Closure Relation Fits C.53 GRB Closure Relation Fits C.54 GRB B Closure Relation Fits C.55 GRB C Closure Relation Fits C.56 GRB 0501 Closure Relation Fits C.57 GRB 0506 Closure Relation Fits C.58 GRB 0508 Closure Relation Fits C.59 GRB 0516A Closure Relation Fits C.60 GRB 0516B Closure Relation Fits C.61 GRB 0521A Closure Relation Fits C.62 GRB 0521B Closure Relation Fits C.63 GRB 0522 Closure Relation Fits C.64 GRB 0528 Closure Relation Fits C.65 GRB 0519A Closure Relation Fits C.66 GRB 0519B Closure Relation Fits C.67 GRB Closure Relation Fits C.68 GRB A Closure Relation Fits C.69 GRB B Closure Relation Fits C.70 GRB 0512 Closure Relation Fits C.71 GRB B Closure Relation Fits C.72 GRB A Closure Relation Fits C.73 GRB Closure Relation Fits C.74 GRB 0605 Closure Relation Fits x

11 C.75 GRB 0608 Closure Relation Fits C.76 GRB 0609 Closure Relation Fits C.77 GRB 0601 Closure Relation Fits C.78 GRB A Closure Relation Fits C.79 GRB B Closure Relation Fits C.80 GRB Closure Relation Fits C.81 GRB Closure Relation Fits C.82 GRB Closure Relation Fits C.83 GRB Closure Relation Fits C.84 GRB Closure Relation Fits C.85 GRB Closure Relation Fits C.86 GRB Closure Relation Fits C.87 GRB B Closure Relation Fits C.88 GRB Closure Relation Fits C.89 GRB 0602 Closure Relation Fits C.90 GRB A Closure Relation Fits C.91 GRB B Closure Relation Fits C.92 GRB Closure Relation Fits C.93 GRB Closure Relation Fits C.94 GRB A Closure Relation Fits C.95 GRB Closure Relation Fits C.96 GRB Closure Relation Fits C.97 GRB Closure Relation Fits C.98 GRB Closure Relation Fits C.99 GRB Closure Relation Fits C.0GRB Closure Relation Fits C.1GRB Closure Relation Fits C.2GRB Closure Relation Fits C.3GRB Closure Relation Fits C.4GRB Closure Relation Fits C.5GRB A Closure Relation Fits C.6GRB B Closure Relation Fits C.7GRB A Closure Relation Fits C.8GRB Closure Relation Fits C.9GRB Closure Relation Fits C.1GRB 0605A Closure Relation Fits C.111GRB 0605B Closure Relation Fits C.112GRB Closure Relation Fits C.113GRB Closure Relation Fits C.114GRB Closure Relation Fits C.115GRB A Closure Relation Fits C.116GRB B Closure Relation Fits C.117GRB Closure Relation Fits C.118GRB Closure Relation Fits C.119GRB A Closure Relation Fits xi

12 C.120GRB Closure Relation Fits C.121GRB Closure Relation Fits C.122GRB Closure Relation Fits C.123GRB Closure Relation Fits C.124GRB Closure Relation Fits C.125GRB Closure Relation Fits C.126GRB Closure Relation Fits C.127GRB Closure Relation Fits C.128GRB Closure Relation Fits C.129GRB Closure Relation Fits C.130GRB A Closure Relation Fits C.131GRB B Closure Relation Fits C.132GRB Closure Relation Fits C.133GRB Closure Relation Fits C.134GRB Closure Relation Fits C.135GRB Closure Relation Fits C.136GRB Closure Relation Fits C.137GRB A Closure Relation Fits C.138GRB B Closure Relation Fits C.139GRB Closure Relation Fits C.140GRB Closure Relation Fits C.141GRB A Closure Relation Fits C.142GRB Closure Relation Fits C.143GRB A Closure Relation Fits C.144GRB B Closure Relation Fits C.145GRB C Closure Relation Fits C.146GRB Closure Relation Fits C.147GRB Closure Relation Fits C.148GRB Closure Relation Fits C.149GRB 0602 Closure Relation Fits C.150GRB 0604 Closure Relation Fits C.151GRB 0606 Closure Relation Fits C.152GRB 0607 Closure Relation Fits C.153GRB 0619 Closure Relation Fits C.154GRB 0621 Closure Relation Fits C.155GRB 0625 Closure Relation Fits C.156GRB 0612 Closure Relation Fits C.157GRB 0611A Closure Relation Fits C.158GRB 0611B Closure Relation Fits C.159GRB Closure Relation Fits C.160GRB Closure Relation Fits C.161GRB Closure Relation Fits C.162GRB Closure Relation Fits C.163GRB Closure Relation Fits C.164GRB 0612 Closure Relation Fits xii

13 C.165GRB A Closure Relation Fits C.166GRB B Closure Relation Fits C.167GRB 0703 Closure Relation Fits C.168GRB 0707 Closure Relation Fits C.169GRB 0701 Closure Relation Fits C.170GRB Closure Relation Fits C.171GRB Closure Relation Fits C.172GRB Closure Relation Fits C.173GRB Closure Relation Fits C.174GRB Closure Relation Fits C.175GRB Closure Relation Fits C.176GRB Closure Relation Fits C.177GRB Closure Relation Fits C.178GRB Closure Relation Fits C.179GRB Closure Relation Fits C.180GRB Closure Relation Fits C.181GRB Closure Relation Fits C.182GRB Closure Relation Fits C.183GRB Closure Relation Fits C.184GRB Closure Relation Fits C.185GRB A Closure Relation Fits C.186GRB B Closure Relation Fits C.187GRB Closure Relation Fits C.188GRB A Closure Relation Fits C.189GRB Closure Relation Fits C.190GRB Closure Relation Fits C.191GRB Closure Relation Fits C.192GRB Closure Relation Fits C.193GRB Closure Relation Fits C.194GRB A Closure Relation Fits C.195GRB B Closure Relation Fits C.196GRB Closure Relation Fits C.197GRB Closure Relation Fits C.198GRB Closure Relation Fits C.199GRB Closure Relation Fits C.200GRB B Closure Relation Fits C.201GRB Closure Relation Fits C.202GRB Closure Relation Fits C.203GRB Closure Relation Fits C.204GRB Closure Relation Fits C.205GRB A Closure Relation Fits C.206GRB B Closure Relation Fits C.207GRB A Closure Relation Fits C.208GRB B Closure Relation Fits C.209GRB A Closure Relation Fits xiii

14 C.2GRB B Closure Relation Fits C.211GRB Closure Relation Fits C.212GRB Closure Relation Fits C.213GRB Closure Relation Fits C.214GRB 0708A Closure Relation Fits C.215GRB Closure Relation Fits C.216GRB 0701 Closure Relation Fits C.217GRB 0703 Closure Relation Fits C.218GRB 07A Closure Relation Fits C.219GRB 07B Closure Relation Fits C.220GRB 0711 Closure Relation Fits C.221GRB 0720 Closure Relation Fits C.222GRB 0721 Closure Relation Fits C.223GRB 0725 Closure Relation Fits C.224GRB 0728 Closure Relation Fits C.225GRB 0731 Closure Relation Fits C.226GRB C Closure Relation Fits C.227GRB Closure Relation Fits C.228GRB Closure Relation Fits C.229GRB Closure Relation Fits C.230GRB Closure Relation Fits xiv

15 xv Acknowledgments I am profoundly grateful for the support that I have received over the last 6 years while pursuing my Ph.D. from my colleagues, friends, and family. My advisor, Dave Burrows, has always been supportive and encouraging. As a reflection of his teachings and professional style, my dissertation will fill a substantial space on the bookshelf, but only one volume in comparison to his two. My thesis committee consists of some of the world s leading experts in the field of GRBs, and even with their busy schedules they have been nothing but supportive and generous with their time and guidance. I thank Dave Burrows, Derek Fox, Peter Mészáros, John Nousek, and Paul Sommers. I have deep appreciation for the guidance and camaraderie provided by my colleagues at Swift including Claudio Pagani, Jamie Kennea, David Morris, Loredana Vetere, Patricia Schady, Erik Hoversten, Abe Falcone, Sally Hunsberger, Dirk Grupe, Marg Chester, Caryl Gronwall, Pete Roming, Peter Brown, Michael Stroh, and many other members of the XRT, UVOT, BAT, and operations teams at Penn State, Leicester, MSSL, Brera, Goddard, Palermo, and Rome. I have enjoyed working in a team of brilliant dedicated people, and am proud to have been a part of such a successful mission. My work would also have never been possible without the help of my many collaborators. My graduate school experience would have been both boring and miserable without the kindness and generosity of my friends including Laura Pomeroy, Theresa Foley, Petra Klepac, Zeynep Sezen, Marshall Pomeroy, Manodeep Sinha, Cate Tower-Morris, Nino Cucchiara, Nic Ross, and Paola Rodriguez Hidalgo. I am glad that I found many close friends in State College that are a wide mix of astronomers and non-astronomers who have always kept things interesting. Long before I began my Ph.D., my parents encouraged my interest in science. I am forever grateful to them for pushing me to work hard even at things that did always not come easily to me. They have always been supportive of anything about which I feel passionately. I thank my parents, sister, and the rest of my family for their support and understanding.

16 1 Chapter 1 INTRODUCTION Gamma-Ray Bursts (GRBs deliver the most energetic explosions in the Universe, releasing the equivalent of the entire lifetime energy output of the Sun, except only in a matter of seconds. These dramatic demonstrations of power briefly outshine their host galaxies, making them visible from great distances across the Universe. The observational and theoretical understanding of this phenomenon has progressed dramatically over the last fifteen years from the infancy of knowing almost nothing of their origins and mechanisms, to a phase of maturity as we try to understand the details of the physical processes, progenitor systems, and environments of these objects. As advances in observations continue to open new windows into discovery space, they consequently also prompt us to ask many new questions. This dissertation explores the collimation signatures of GRBs, one very specific yet fundamental aspect of this phenomenon, that has important consequences for the greater understanding of how GRBs form and the processes involved in producing their amazing observational behavior. Our current understanding of the collimation signatures is muddied by our expectations from a time when the observational information was scarce, which has required us to reevaluate the logic and origins of such explanations. 1.1 History of Gamma-Ray Bursts The relatively young and often controversial history of the field of GRBs began by one of the most unanticipated means. Their accidental discovery was facilitated by the United States military Vela program that was designed to monitor the nuclear testban treaty (1963 by searching for the telltale gamma-ray signatures of nuclear weapons testing using orbiting spacecraft. After discovering short bursts of gamma-rays, with the

17 2 first firm detection in 1967, and determining their origin as non-terrestrial, they were declassified and announced to the world by Klebesadel et al. (1973. The slight differences in the γ-ray photon arrival times between the different Vela satellites allowed scientists at Los Alamos National Laboratory to determine that the origin of these objects was not from the Earth or the Sun. Preliminary analysis of the sky distribution of these objects (Strong et al suggested they were not clearly aligned with any particular solar system or galactic structures. Even with low counting statistics, the distribution appeared to be isotropic. Early indications of structure and properties of these gamma-ray bursts showed short intense spikes with fine time structure and weak flux continuing for minutes afterwards. The decades following the Vela program, lacking significant improvement to instrumentation, stimulated a wild slurry of theoretical speculation on the origin of these mysterious objects. The Interplanetary Network (IPN, made up of largely solar system exploration and helio-physics missions equipped with γ-ray detectors, continued to discover new GRBs, but their origins remained a mystery (Niel et al Accurate position information and rapid follow-up with ground based telescopes was needed to see if they were aligned with any known categories of astronomical objects. The Burst and Transient Source Experiment (BATSE on-board the Compton Gamma-Ray Observatory (CGRO was the first experiment dedicated to the study of GRBs, and had much greater sensitivity than previous instruments. During 9 years of operation ( , BATSE discovered 2700 new GRBs. Kouveliotou et al. (1993 showed that the BATSE GRBs formed two distinct classes differentiated by their duration (Figure 1.1: short (T 90 < 2 s hard spectrum bursts and long (T 90 > 2 s softer bursts, where T 90 is the time during which 90% of the γ-ray fluence is observed. Different models for the progenitor systems emerged, with the leading theories involving a compact binary merger for the short bursts (Eichler et al. 1989; Narayan et al. 1992, and a massive star collapsing to a compact object (black hole or neutron star origin for the long bursts (Woosley 1993.

18 3 Fig. 1.1 Distribution of BATSE catalog T 90 measurements showing a strong bimodality. Kouveliotou et al. (1993 used this bimodality to distinguish two classes of GRBs (short and long. Plot from BATSE 4G catalog: grb/duration/. The BATSE catalog of GRB positions (Figure 1.2, even with several degree position accuracy, demonstrated that the distribution on the sky was isotropic and inconsistent with any known classes of galactic sources (Meegan et al However, the debate between galactic and extra-galactic origin was not entirely solved with this isotropy measurement. Galactic halo objects such as isolated neutron stars could still possibly account for the distribution. The neutron star hypothesis conveniently seemed to solve the problem of the extreme energetics that would be required of GRBs if they were from an extragalactic origin. However, the debate would not be settled until an accurate distance determination was possible. The debate was finally solved when the Italian-Dutch BeppoSAX satellite discovered X-ray counterparts and afterglows to GRBs in 1997 (Costa et al. 1997, with positions accurate enough for ground-based optical follow-up. Ground based optical follow-up was first attempted on GRB (Gorosabel et al. 1998, and the first optical afterglow was discovered with follow-up observations of GRB (van Paradijs et al. 1997, but it was not until several months later that BeppoSAX pinpointed

19 BATSE Gamma-Ray Bursts Fig. 1.2 BATSE GRB positions from the entire mission showing the isotropic distribution on the sky. Plot from BATSE 4G catalog: grb/skymap/. another burst (GRB , for which follow-up observations led to the redshift determination and the clear extragalactic origin of this phenomenon (Metzger et al Frail et al. (1997 discovered the first radio afterglow for GRB Ironically, all of these afterglow discoveries were shortly preceded by a theoretical prediction of detecting optical, radio, and X-ray counterparts to GRBs as part of the Fireball model (Mészáros & Rees 1997, see section Understanding of Gamma-Ray Bursts Prior to the Swift Era After the discovery of the first afterglow and redshift measurements, the years that followed were filled with new discoveries and new open questions. The High Energy Transient Explorer 2 (HETE-2, , Kawai et al. 1999, Integral (2002-present, Winkler et al. 2003, the InterPlanetary Network (IPN, 1990-present, Cline et al. 1999, and the Rossi Xray Timing Explorer (RXTE/ASM, 1995-present, Bradt & Smith 1999 satellites provided GRB triggers with accurate positions available within minutes to hours of occurring at a rate of approximately once per month. A growing collection

20 of detailed multi-band optical and near-infrared light curves documented the detailed structure of afterglow emission The Fireball Model The fireball model (Rees & Meszaros 1992; Mészáros & Rees 1997; Wijers et al. 1997; Zhang & Mészáros 2004; Sari et al describes the physical mechanisms that produce GRBs after whichever progenitor collapses to form the central engine. This central engine must be compact (a black hole or neutron star and powerful enough to accelerate the ejecta to relativistic velocities. The fireball model explains the different observational components of GRBs as the results of interactions of relativistic shocks between blobs of material ejected from the central engine and each other, and between this ejected material and the surrounding medium. These internal shocks between the blobs themselves traveling at slightly different relativistic velocities produce synchrotron radiation responsible for the prompt γ-ray emission. The external shocks between the outgoing ejecta and the surrounding medium cause the optical, X-ray, and radio afterglows. Optical emission is also produced in a reverse shock that propagates backwards from the external shock front that is suddenly slowed by the dense circumstellar environment Prompt Emission The short energetic bursts of γ-rays are the telltale signature of the GRB phenomenon, yet the diversity of γ-ray behavior suggests complex and heterogeneous conditions. As Chryssa Kouveliotou once said, if you ve seen one gamma-ray burst, then you ve seen one gamma-ray burst. Figure 1.3 shows several examples demonstrating the diverse properties of GRB prompt emission observed by BATSE, which range in duration from to 3 seconds. The physical mechanisms used to explain this diversity of observations must account for both the temporal and spectral variations creating durations that range from tens of milliseconds to hundreds of seconds, peaking at energies ranging from a few kev

21 BATSE Trigger BATSE Trigger Ch: (1: 4 Time Res: s Ch: (1: 4 Time Res: s Rate (counts s Rate (counts s Seconds Since Trigger (9201 : BATSE Trigger 8073 Ch: (1: 4 Time Res: s Seconds Since Trigger ( : BATSE Trigger 85 Ch: (1: 4 Time Res: s Rate (counts s Rate (counts s Seconds Since Trigger ( : Seconds Since Trigger ( : Fig. 1.3 Demonstration of the diversity of γ-ray light curves. Top left: the multi-peaked light curve of GRB 9201 with dramatic fine time structure. Top right: the Fast Rise Exponential Decay (FRED light curve of GRB with earlier weak precursors. Bottom left: a short hard burst GRB Bottom right: the light curve of GRB with multiple peaks but limited small-timescale structure. Plots taken from the BATSE light curve catalog

22 7 to more than an MeV, with temporal behavior varying on millisecond timescales in a variety of large scale shapes, and diverse spectra that may contain multiple components. Prompt γ-ray spectra are non-thermal and empirically best fit by a smoothlyjoined broken power-law such as the Band function (Band et al. 1993, described in Chapter 2. Higher energy emission was also detected by the Energetic Gamma Ray Experiment Telescope (EGRET also aboard CGRO with a response between 20 MeV and 30 GeV. Most GRBs detected by EGRET were consistent with the extrapolation of the softer γ-ray Band function, however GRB 9417 (González et al and GRB (Hurley et al showed distinct longer-lived components. There has been much speculation as to the origin of this emission from mechanisms such as Synchrotron Self-Compton (SSC emission (Pe er & Waxman 2004, SSC from the reverse shock (Granot & Guetta 2003; Wang et al. 2005, and inverse Compton scattering off thermal X-rays (Wang & Mészáros The new addition of the Astro Rivelatore Gamma a Immagini Leggero (AGILE, Pittori et al. 2006, 2007-present and Fermi Gamma-ray Space Telescope (2008-present with their high energy γ-ray capabilities, will undoubtedly shed new light onto the properties and origin of this component. The discovery of the first prompt optical flash associated with a GRB by the Robotic Optical Transient Search Experiment (ROTSE telescope from GRB (Akerlof et al introduced a new component to the GRB mechanism. In a few cases of very fast (within s of seconds robotic wide-field optical follow-up, a bright optical flash has been observed simultaneously with the γ-ray flash. Prompt optical emission was also observed from GRB 0204 (Fox et al and GRB (Li et al These sparsely sampled prompt optical light curves did not appear to correlate with the γ-ray emission, and were therefore assumed to be due to a different emission mechanism, namely a synchrotron reverse shock. It was not until later that new observations of GRB B (Vestrand et al and GRB B (Racusin et al. 2008, see also Chapter 5 would challenge this hypothesis with strong evidence for an additional prompt optical component.

23 Afterglows The discovery of GRB afterglows in 1997 changed the basic phenomenology implied by the name of the objects. As the GRB field became more automated, broadband prompt observations became more common allowing for follow-up while the GRB afterglows were still bright enough for detection and could be observed while they faded away. The afterglow light curves showed approximately power-law decay behavior with some possible initial early rising in the optical. It also became obvious that a substantial fraction ( 40% of afterglows are detected only in the X-ray band and not the optical, with radio afterglows in only about % of all cases. Those afterglows without detected optical emission were deemed dark and are probably either too far away, faded too fast, or are highly extinguished by intervening material either in our Galaxy, the host galaxy, or in between Optical and Near-Infrared In the pre-swift era, multi-band optical and near-infrared (NIR afterglow data sets provided the bulk of afterglow information. They generally showed a decaying power-law with temporal index α 1 and spectral index β 0.7, with a large amount of scatter, following the form: F opt (t, ν t α opt ν β opt. (1.1 There was evidence for early sharp rises (GRB , Galama et al. 1998, GRB , Akerlof et al. 1999, and other deviations from simple power-law behavior in several other bursts (Zhang & Mészáros The most significant light curve feature in the pre-swift era is the characteristic sharp steepening a few days after the GRB triggers to a slope of α 2. The pre-swift optical data sets showed tens of cases these breaks (Frail et al. 2001; Bloom et al. 2001; Zeh et al This steepening was interpreted as evidence for the collimation of the burst ejecta with physical half-angle θ j. The ejecta moves at relativistic velocities with a bulk Lorentz factor, Γ, and the radiation is relativistically beamed into an angle

24 9 θ = 1/Γ. As the ejecta sweeps up the surrounding material, the fireball decelerates, with the beaming angle eventually exceeding the physical collimation angle, causing a sudden increase in the rate of decay of the flux (i.e. the jet break, Figure 1.4. At the same time, sideways expansion of the ejecta with relativistic speeds also causes a sudden flux decrease (Sari et al. 1999; Rhoads 1999; Zhang & Mészáros Most likely both of these effects contribute to the jet breaks, and therefore, both models must be considered. Jet breaks are expected to be achromatic based on the assumption that the afterglow emission regions and mechanisms are the same for various spectral regimes, and should therefore only reflect ejecta geometry. Achromaticity has indeed been confirmed between the optical and near-infrared bands in pre-swift GRBs (Kulkarni et al. 1999b; Harrison et al. 2001; Klose et al Radio Radio afterglow observations in the pre-swift era provided an interesting insight into the spectral evolution. Most radio observations were taken in the 5 and 8.5 GHz bands, the spectral indices were usually negative (F R ν β, and the light curves were not simple power-law declines. Radio afterglows are very long lived, in some cases persisting for years (Frail et al In addition, early short lived radio flares have also been observed (Kulkarni et al. 1999a; Frail et al X-ray The status of X-ray afterglow observation in the pre-swift era observed with BeppoSAX involved 40 light curves that began approximately eight hours after the GRB trigger and displayed mostly sparsely sampled single power-laws (de Pasquale et al In a handful of cases, follow-up observations with the Chandra X-ray Observatory or the XMM-Newton observatory revealed later fainter behavior. An afterglow was identified in 90% of BeppoSAX GRBs, with only 42% also having optical afterglows. This limited early indication of dark bursts foreshadowed important observational behavior in the Swift era.

25 Fig. 1.4 Illustration from Ghisellini (2001 demonstrating how the observer only sees a fraction of the emitting area inside a cone with angle 1/Γ prior to the jet break, and it cannot be distinguished between spherical emission and collimated emission. In the collimated case (lower figures, the jet becomes apparent only when θ > 1/Γ.

26 11 BeppoSAX X-ray afterglows generally showed temporally decaying power-law behavior with an index of α 0.9, and a spectral decay of β 1.4 (Zhang & Mészáros 2004, both with a wide range of scatter, but generally following the form: F X (t, ν t α X ν β X. (1.2 There were also slight indications of more complicated behavior (flattening, bumps, flares, etc. seen in a few bursts (de Pasquale et al Despite the limited observations of X-ray afterglows, it was assumed that they would follow the same decay rates and break times as the optical afterglows. The light curve breaks discussed in were thought to be a purely geometrical effect suggesting that with longer deeper observations with a more sensitive instrument, the X-ray light curve structure would show the same features Supernova-GRB Connection A new clue into the progenitors of at least one category of GRBs came from the temporal and positional coincidence of GRB with SN 1998bw (Galama et al The GRB position error was large (> 8 arcmin and the association somewhat ambiguous, but included the optical, X-ray, and radio SN. Five years later, the nearby luminous GRB was spectroscopically confirmed to be associated with Type Ic SN 2003dh (Stanek et al These strong cases and several weaker observations of rising SN components as part of GRB light curves confirmed that some population of long GRBs were due to core collapse supernova type events. The fact that only the most nearby GRBs have shown this SN signature suggests that higher redshift GRBs probably also have hidden SNe, but they are too faint. If a SN dominates the light curve around the jet break time, it can be difficult to distinguish the individual contributions from the afterglow, host, and the SN. However, if multi-band observations are available, there are indications from color evolution, and an exponential rather than power-law decaying light curve (e.g. Tanvir et al

27 The Swift era of Gamma-Ray Burst Science The largest distinctions between the pre-swift and Swift eras are the prevalence of prompt GRB follow-up by the narrow field Swift instruments (the X-Ray Telescope (XRT and the UltraViolet Optical Telescope (UVOT within minutes, and the increased sensitivity of the Swift-Burst Alert Telescope (BAT compared to previous instruments. This has led to many new discoveries about early optical and X-ray afterglow behavior as well as challenging some of the previously established trends from prompt emission and afterglow studies. BAT detects GRBs at a rate of 0 per year, yet observes only 1/6 of the sky at any given time. BATSE observed GRBs at a higher total rate due to larger sky coverage, but did not provide accurate enough prompt positions for much afterglow follow-up. The majority of detected pre-swift afterglows were from GRBs discovered by HETE-2, BeppoSAX, and RXTE-ASM. These afterglows were only detected for the brightest bursts due to the sensitivity of those instruments. In addition to the fluence sensitivity differences, BAT s energy range ( kev is different than that of pre-swift instruments (BATSE:25 kev - 2 MeV, HETE-2-FREGATE: kev, BeppoSAX-WFC: 2-30 kev, RXTE-ASM: 2- kev. Therefore, BAT detects a somewhat different population of GRBs than previous instruments. On average they are at a higher redshift (z Swift 2.5 versus z pre Swift 1.5, and therefore the afterglows are more likely to be visible in the NIR rather then the ultraviolet or optical bands. Swift GRBs are also on average less energetic in the isotropic equivalent γ-ray output Optical Afterglows The Swift-era canonical ultraviolet/optical afterglow form (Oates et al often catches the rising emission at the beginning of observations. This is possibly due to either the onset of the forward shock, or as a geometrical effect due to a slightly off-axis observer watching more of the jet come into their line of sight. With or without the rising behavior, the afterglows demonstrate a power-law decay occasionally superimposed with short re-brightening events. A reverse shock signature is occasionally also observed. In

28 13 a few cases, jet breaks are also observed but more rarely than was expected from the pre-swift era. The GRB community is much more prepared in the Swift-era for rapid multiwavelength follow-up of GRBs with robotic and rapidly repointed telescopes. In some cases, after receiving the automatic GRB alerts, these robotic telescopes repoint and begin observations before the Swift spacecraft has even completed its slew ( 1 minute. This has led to new insight into how the prompt emission transitions into the later better known afterglow properties X-ray Afterglows In the Swift era, it is X-ray afterglow light curves that provide the most homogeneous data set to study GRB afterglows. With the rapid GRB triggers provided by the Swift-BAT, and the autonomous prompt Swift-XRT observations that frequently begin within 1-2 minutes of the trigger, X-ray afterglows have gone from sparsely sampled single power-laws (BeppoSAX-era, de Pasquale et al. 2006, Figure 1.5 to a rich database of light curves with widely varying properties and durations. If observations begin promptly, X-ray afterglows are also detected in nearly all ( 98% of all cases, compared to only 30 60% in UV/optical/NIR bands. A common canonical shape of the Swift-XRT X-ray light curves emerged (Nousek et al. 2006; Zhang et al with five components (Figure 1.6. These 4 segments and additional component are I: the initial steep decay often attributed to high-latitude emission also known as the curvature effect (Kumar & Panaitescu 2000; Qin et al. 2004; Liang et al. 2006; Zhang et al. 2007b; II: the plateau, which is frequently attributed to continuous energy injection from the central engine (Rees & Mészáros 1998; Dai & Lu 1998; Sari & Mészáros 2000; Zhang & Mészáros 2001; Granot & Kumar 2006; Zhang et al. 2006; Liang et al. 2007; III: the normal decay due to the deceleration of an adiabatic fireball (Mészáros 2002; Zhang et al. 2006; IV: the post-jet break phase (Rhoads 1999; Sari et al. 1999; Mészáros 2002; Piran 2005; V: flares, which are seen in 1/3 of all Swift GRB X-ray afterglows during any phase (I-IV and are believed to be caused by sporadic emission from the central engine (Burrows et al. 2005a; Zhang et al. 2006; Chincarini