Faint Gamma-Ray Bursts and Other High-Energy Transients Detected with BATSE by Jefferson Michael Kommers Submitted to the Department of Physics in partial fulfillment of the requirements for the degree of Doctor of Philosophy in Physics at the MASSACHUSETTS INSTITUTE OF TECHNOLOGY February 1999 © Massachusetts Institute of Technology 1999. All rights reserved. C Author........V .V ....... .. .. ...........- ... -- - ..... ...........----.- - Department of Physics December 15, 1998 d4 Certified byV Walter H. G. Lewin Professor of Physics Thesis Supervisor Accepted by ............... /Thomas, .Greytak Professor of Physics Associate Department Head for Education MCH SETTS INSTI TUTE LIBRARIES Faint Gamma-Ray Bursts and Other High-Energy Transients Detected with BATSE by Jefferson Michael Kommers Submitted to the Department of Physics on December 15, 1998, in partial fulfillment of the requirements for the degree of Doctor of Philosophy in Physics ABSTRACT The Burst and Transient Source Experiment (BATSE) onboard the Compton Gamma Ray Observatory detects gamma-ray bursts (GRBs) and other high-energy astronomi- cal transients using a real-time burst detection system running onboard the spacecraft. This thesis describes a search of the archival BATSE data for GRBs, emission from soft gamma-ray repeaters (SGRs), bursts and flares from X-ray binaries, and other transients that were not detected by the onboard system. The search covers six years of the mission, from 1992 December 9.0 to 1997 December 17.0. The search reveals 873 GRB candidates that did not activate the onboard burst detection because they were too faint, because they occurred while the onboard system was disabled for technical reasons, or because their time profile artificially raised the onboard detection threshold. The catalog of these bursts increases the number of GRBs detected with BATSE by 48% during the time period of the search. The intensity distribution of the GRBs detected with the search reaches peak fluxes that are a factor of ~ 2 lower than could be studied previously. The value of the (V/ Vmax) statistic (in Euclidean space) for these bursts, 0.177±0.006, is the lowest so far obtained for a global sample of GRBs. The differential peak flux distribution is consistent with cosmological models in which the co-moving GRB rate approximately traces the star-formation history of the Universe. These results suggest that more sensitive detectors are likely to discover relatively few GRBs (of the kind currently known) that are fainter than the BATSE detection threshold. Thesis Supervisor: Walter H. G. Lewin Title: Professor of Physics ACKNOWLEDGMENTS It has been a pleasure to spend the last 5.5 years working with the members of the MIT astrophysics community and the BATSE team at Marshall Space Flight Center. The time has come to acknowledge the extensive discussions and assistance that they have contributed to this thesis as well as to my other research projects. I am grateful to my thesis advisor, Walter Lewin, for his support, encouragement, and dedication to my success as a scientist. His high standards in research and in pedagogy are a model for my own intellectual development. It has been a privilege and a pleasure to spend these years as a student, colleague, and friend. I would like to thank the other readers on my thesis committee for their discussions at committee meetings and their valuable comments on the manuscript: Jackie Hewitt and Simon Mochrie of MIT and Jerry Fishman of the NASA Marshall Space Flight Center. I am grateful to Jerry, the Principal Investigator on the BATSE project, for acting as the agency sponsor on my NASA Graduate Student Researchers Program Fellowship, for serving as an official reader on this thesis, and for taking the time to travel to MIT for the defense. The research I describe in this thesis was undertaken as part of a collaboration that includes myself, Walter, and Jerry as well as Jan van Paradijs, Chryssa Kouveliotou, Geoff Pendleton, and Chip Meegan. The idea to produce a catalog of nontriggered gamma-ray bursts and other transients from the BATSE archival data is due to Chryssa, who has served as my primary contact with the BATSE group at MSFC throughout this research. Indeed, the entire BATSE team has in one way or another helped make this thesis possible. I am grateful to all these collaborators for their guidance and assistance throughout this project. I would like to thank Jan and Chryssa in particular for their hospitality during my visits to Huntsville. The wide-ranging interests of the MIT astrophysics community ensured that I did not spend all my time on topics directly related to my thesis. I am happy to acknowl- edge discussions on a variety of astrophysical subjects with Deepto Chakrabarty, Al Levine, Ed Morgan, Vicky Kaspi, Ron Remillard, George Ricker, and Paul Schechter. The graduate student community has been the source of countless hours of helpful discussions and lasting friendships. Bob Rutledge, Derek Fox, Patrick Wojdowski, and Bob Guerriero have been colleagues, running partners, and friends. There have been hours of serious and whimsical discussions with the other residents of our sixth- floor computer room-Dave Buote, Froney Crawford, Taotao Fang, Eric Gaidos, Jon Miller, Dave Pooley, and Julia Steinberger-as well as with those working downstairs with the CCD lab and the XTE group: Chris Becker, Glen Monnelly, Mike Pivovaroff, Bob Shirey, and Don Smith. I am happy to acknowledge financial support from a National Science Foundation Graduate Fellowship during my first three years at MIT and from a NASA Graduate Student Researchers Program Fellowship under grant NGT 8-52816 for the remainder of my graduate studies. My parents and grandparents, Jim, Nancy, Clarence, Vernetta, and Betty, deserve special thanks. Their support, encouragement, and strong family commitment to education motivated me to pursue the doctoral degree. Finally, I am personally indebted to Arthur Pugsley IV. His support, patience, and companionship over the last 3.5 years have been indispensable and deeply appreciated. Contents 1 Introduction 17 1.1 Organization ... ... .. ... .. 18 1.2 Terms, Naming Conventions, and Units 19 1.3 Publications ... ... ...... 20 1.3.1 Refereed Journals . 21 1.3.2 In Preparation . ... 22 1.3.3 Conference Proceedings 22 1.3.4 IAU Circulars .. .... 23 2 Gamma-Ray Bursts and Other High-Energy Astronomical Tran- sients 25 2.1 Gamma-Ray Bursts . ... ... 26 2.1.1 Historical Overview . .. 26 2.1.2 Gamma-Ray Properties 29 2.2 Soft Gamma-Ray Repeaters ... ... 35 2.3 Bursts and Flares from X-ray Binaries 36 2.4 Solar and Terrestrial Phenomena . .. 37 2.4.1 Solar Flares ... ... ... .. 38 2.4.2 Radiation Belts .. .. .. ... 38 2.4.3 Orbiting Nuclear Reactors . 40 2.4.4 Terrestrial Gamma-Ray Flashes 41 2.5 Unknown Phenomena .. ... ... .. 42 7 2.6 Effects of High-Energy Transients on Earth .... ..... ..... .4 42 3 The Burst and Transient Source Experiment 45 3.1 The Compton Gamma Ray Observatory .. .... .... ... ... 4 5 3.2 BATSE Instrument Description ... .... ... .. .... ... 46 3.3 Onboard Data Processing and Burst Detection . .. ... .... .. 49 3.4 Detector Characteristics .. ...... ..... .. ... ... .... 5 2 3.5 Detector Background ..... ...... ..... .... ... ... 5 5 4 The Search for Nontriggered Events 61 4.1 Motivations for the Off-line Search ..... .... .. .. .. .. .. 62 4.2 Data Selection and Scope ...... ..... ... ... ... .... 6 4 4.3 Off-line Search Algorithm .. .... .... .... ... ...... 6 5 4.4 Detection Efficiency and Sky Exposure ... .... .. .... ... 7 2 4.5 Classification of Off-line Triggers ... ... .... .... ... ... 7 5 4.5.1 Occultation Steps and Aperiodic Variability .. .... ... 7 8 4.5.2 Phosphorescence Spikes ... .... ... ... ... .... 8 0 4.5.3 Magnetospheric Particle Precipitations .. ....... ... 8 0 4.5.4 Solar Flares ... ... ... ... ... ... .. ... .. .. 8 1 4.5.5 Terrestrial Gamma-Ray Flashes ... ... .... ... ... 8 3 4.5.6 Gamma-Ray Bursts ... .... .... ... .. ... .. .. 83 4.5.7 Bursts and Flares Associated with Known Sources in Outburst 87 4.5.8 Soft Gamma-Ray Repeaters ... .... .... ..... ... 87 4.5.9 "Unknown" Low-Energy Events . .... .... ..... ... 90 5 The Nontriggered Event Catalogs 93 5.1 Off-line Triggers ..... ...... ..... ...... ..... ... 93 5.2 Nontriggered GRB Candidates .......... ........... 94 5.3 Estimation of Physical Parameters ......... .......... 96 5.3.1 Source Directions ....... ....... ....... .... 97 8 5.3.2 Durations . .... .... .... .... .... ... .... 98 5.3.3 Peak Photon Fluxes .. .... ... ... .... ... ... 99 5.3.4 Fluences .. .... .... .... .... .... ... .... 100 5.3.5 Cmax/Cmin . .. .. .. .. .. .. .. .. .. .. .. 100 5.3.6 Accuracy of Off-line Burst Parameters . ... .. ... .. .. 100 5.4 Classification Accuracy .. .. .. .. .. .. .. .. .. .. 104 5.4.1 Contamination by Non-GRB Events . .. .. .. .. .. 104 5.4.2 Incompleteness by Misclassification of GRBs .. ... .. .. 109 5.5 GRB Direction Distribution . .. ... ... ... .. ... ... ... 111 5.6 GRB Duration Distribution . .. .. .. .. .. .. .. .. .. .. 114 5.7 GRB Rate . .. .. .. .. .. .. .. .. .. .. .. .. .. .. 114 5.8 SGR Events . .. .. .. .. .. .. .. .. .. .. .. .. 116 5.9 "Unknown" Low-Energy Events ... ... ... .. ... ... ... 118 5.9.1 Direction Distribution . .. .. .. .. .. .. .. .. .. 118 5.9.2 Spectral Hardness . .. ... .. .. ... .. .. ... .. .. 119 5.9.3 Event Rate . .. .. .. .. ... .. .. .. .. .. .. .. .. 123 6 Gamma-Ray Burst Intensity Distribution 127 6.1 Abstract ........ .................................