The mystery of gamma-ray bursts and its possible solution are textbook examples of the scientific method. These flashes of gamma-rays originating outside the solar system were attributed after their discovery to an impulsive release of energy on nearby neutron stars. This hypothesis had testable consequences, which the Burst and Transient Source Experiment (BATSE) on the Compton Gamma Ray Observatory was built to verify. But BATSE found the predictions were wrong, falsifying the hypothesis. Two new hypotheses were formulated: bursts originate in a large halo surrounding our galaxy, or at cosmological distances. These hypotheses also had testable consequences, and in May, 1997, a clear signature of the cosmological origin of at least one burst was discovered. Other predictions of the simplest cosmological model appear to be invalid, indicating that the phenomenon is more complicated, and therefore more interesting, than previously thought.

Gamma-ray bursts were discovered by the Vela spacecraft. Between 1963 and 1973 the United States launched these satellites whose mission was, among other purposes, to characterize the space environment in which future detectors would attempt to verify that the Soviet Union and other nuclear powers were not detonating nuclear weapons in space to circumvent the newly negotiated Limited Test-Ban Treaty. The Vela program consisted of pairs of satellites of progressively greater complexity. In 1969 R. Klebesadel noticed a burst of radiation had been detected by both Vela 4 satellites on July 2, 1967, verifying that this event was external to the detectors; inspection of the data from subsequent Vela satellites revealed additional bursts. Since these events did not have the signature of nuclear explosions, the existence of the phenomenon was never classified. However the bursts were not reported to the astrophysical community until 1973 after later Vela satellites demonstrated that the sun was not the source. A variety of progressively more sophisticated burst detectors have flown since, and the analysis techniques have advanced accordingly.

This is a time of great change in the study of bursts. I joined the BATSE instrument team just before BATSE was placed in orbit, when we "knew" that bursts occurred on the surface of local neutron stars. Within a few months BATSE showed that this was incorrect, and for six years we debated whether bursts were merely on the outskirts of our galaxy, or at the edge of the universe. Since the beginning of 1997 the Italian-Dutch X-ray satellite Beppo-SAX has localized a number of bursts to small error boxes (less than an arcminute in radius) within hours, resulting in the discovery of transients in other wavelength bands. As I describe in greater depth below, the observations of these transients have shown nearly conclusively that some, and by Occam's Razor probably all, bursts are cosmological. The wealth of new observational data, and the resulting understanding of the burst phenomenon, have led to a serious confrontation of theory and observation. As a member of the spectroscopy group of the BATSE instrument team, for the past eight years I have devoted most of my research efforts to burst phenomenology, while occasionally participating directly in the great debates which have raged in the field; my research interests undoubtedly affect the emphasis of this presentation.

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Last revised January 21, 1999.