Preface to the Second Huntsville Gamma-Ray Burst Workshop

It is now three years after the launch of the Compton Gamma-Ray Observatory (CGRO), and the enigma of gamma-ray bursts is as great as ever. The discovery by the Burst and Transient Source Experiment (BATSE) on CGRO that bursts are distributed isotropically and inhomogeneously has altered the thinking of many theorists. Burst models once again cover all distance scales. The once dominant galactic neutron star model of gamma-ray bursts, while still prominent, now occupies a minority position. Numerous new cosmological models have appeared, and old ones have been revived. These models associate gamma-ray bursts with merging compact stars, massive black holes, or other, often more exotic, sources. Even solar system models have their advocates. Despite this diversity, a definite shift to cosmological models has occurred over the past two years, reinforced by the growing BATSE data base.

The Second Huntsville Gamma-Ray Burst Workshop was held in Huntsville, Alabama, USA, from the 20th to the 22nd of October, 1993. It was the largest conference on gamma-ray bursts held to date. While theorists attended in large numbers, the principle focus of the workshop was the presentation and interpretation of recent observational results. Many contentious issues were presented and debated at length.

In these proceedings an attempt has been made to group together similar observations and issues. However, the reader is cautioned that many papers could have fit equally well in more than one section. It is suggested that one should at least superficially peruse all of the titles and abstracts for papers of interests on a particular topic.

Many attempts to solve the mystery of burst origins fall into two categories: statistical tests on ensembles of bursts, and observational attempts to locate burst counterparts. In the first category are attempts to find evidence of burst repetitions, of deviations from isotropy, of time dilation, and of morphological differentiation. In the second category are the searches of burst locations for quiescent sources at other wavelengths, the searches of archives for transient sources, and the simultaneous observation of bursts at other wavelengths. Neither category of test has yet provided unambiguous answers. Although detailed spectral and temporal analyses of large numbers of bursts have been made, the results have been either difficult to characterize in a systematic way or subject to controversy.

In spite of this, there have been two possibly significant breakthroughs: one is that the distribution of burst durations is double peaked; the other is evidence of a correlation of burst duration with burst intensity in the manner expected for a cosmological time dilation. There has also been at least one very significant development in the spectral observations of individual bursts: emission into the GeV energy range is clearly seen by EGRET on CGRO in several bursts. Simultaneous observations of gamma-ray burst spectra by all four experiments on CGRO were presented at the workshop, showing the ability of CGRO to derive an accurate spectrum over five decades of photon energy for the strongest bursts. The issue of whether the continued absence of spectral lines in the bursts observed by BATSE is inconsistent with the observations of KONUS, Ginga, and HEAO was discussed and is represented in this volume by several papers.

The most contentious topic at the workshop was the claim that some burst sources produce more than one gamma-ray burst. The impetus of this claim were analyses by several different groups of the First BATSE Gamma-Ray Burst Catalog, which was released early in 1993 and sparked a rapid flurry of new work. In this volume 11 papers address this topic, and at the workshop two hours of open debate were devoted to this topic. The importance of the outcome is clear: many burst models are unable to produce more than one burst per source.

Observing an optical transient simultaneously with a gamma-ray burst, or finding a counterpart at any wavelength within hours of a burst, is believed by many to be the best hope of solving the burst mystery. For this reason a number of new optical and radio search programs have been implemented or planned. Recent results from some of these programs are presented.

The mystery shrouding soft-gamma repeaters was largely resolved by multiwavelength counterpart observations. Shortly before this workshop, ASCA and BATSE simultaneously observed an outburst from SGR1806--20, placing the source within a galactic radio supernova remnant recently identified by Kulkarni and Frail. We were fortunate to have these results presented at a special session at the workshop and in these proceedings.

Resolution of the mystery surrounding classical gamma-ray bursts may require observations by new experiments or new techniques of analyzing current burst data. Among the ideas discussed at the workshop are improved methods of burst localization, which would enable deeper counterpart searches, and experiments that determine a burster's distance scale by measuring the photoelectric absorption of soft gamma-rays by the galactic interstellar medium.

Many persons helped to make the workshop a success. We are grateful to the following persons for assisting with the organization of the workshop program: C. Kouveliotou, D.Q. Lamb, O. Blaes, and T.L. Cline. Those that served as session chairs did an outstanding job of staying within difficult time limitations for talks, while allowing for stimulating and beneficial discussions. They included E. Fenimore and D. Hartmann, as well as the above named. J.P. Lestrade provided the manuscript TeX macros. M. Rees, on short notice, prepared and gave an enlightening summary of the workshop.

Finally, we thank Dannah McCauley, Paula Cushman, and and other staff members of the Universities Space Research Association for the excellent logistical support they provided for the workshop.

G.J. Fishman, J.J. Brainerd, K. Hurley. February 1994
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