Evidence for early high-energy afterglow: BATSE observations of GRB980923

The relation between the prompt gamma -ray emission of a Gamma-Ray Burst (GRB) and its afterglow emission is unclear. We present evidence in the BATSE data for a prompt high-energy (25-300 keV) afterglow component associated with GRB980923 that is initiated during the burst. The event exhibits rapid variability lasting similar to 40 s, followed by a smooth power-law emission tail lasting similar to 400 s. The energy spectrum changes abruptly to a cooling synchrotron spectrum when the tail becomes noticeable, consistent with the internal-external shock scenario in the fireball model. This overlap of prompt and afterglow emission illustrates that the external shocks can be generated during the gamma -ray emission phase, as also suggested by the simultaneous optical emission of GRB991023.