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R. D. Preece (Dept. of Physics, UAH, Huntsville, AL)
If we accept the hypothesis that some bursts originate as relativistic fireballs, then there should be a period before the start of the prompt gamma-ray emission where the fireball is optically thick. In this early phase, the matter in the fireball is undergoing acceleration, presumably due to radiation pressure. The internal and external shock models for burst emission concentrate on the time after which the blast wave has become optically thin. Although the brightness of the fireball should be diminished, relative to the optically thin portion, it may be detectable, due to relativistic effects such as brightening. There have been reports of spectra observed by detectors on board Ginga that are consistent with thermal emission; however, these results were not conclusive. I will discuss a period of emission at the start of GRB970111, observed by BATSE and the WFC on BeppoSAX, that show the characteristics of blackbody emission with good statistics. There may be several other candidates in the BATSE catalog with similar behavior. This explanation may also account for GRB spectra that cross the synchrotron shock model `death line', without resorting to self-absorption.