Evidence for an early high-energy afterglow observed with BATSE from GRB 980923

In this Letter, we present the first evidence in the BATSE data for a promp t high-energy (25-300 keV) afterglow component from a gamma-ray burst, GRB 980923. The event consists of rapid variability lasting similar to 40 a fol lowed by a smooth power-law emission tail lasting similar to 400 s. An abru pt change in spectral shape is found when the tail becomes noticeable. Our analysis reveals that the spectral evolution in the tail of the burst mimic s that of a cooling synchrotron spectrum, similar to the spectral evolution of the low-energy afterglows for gamma-ray bursts. This evidence for a sep arate emission component is consistent with the internal-external shock sce nario in the relativistic fireball picture. In particular, it illustrates t hat the external shocks can be generated during the gamma-ray emission phas e, as in the case of GRB 990123.