1999ApJ...524L..47G

In this Letter, we present the first evidence in the BATSE data for a prompt high-energy (25-300 keV) afterglow component from a γ-ray burst, GRB 980923. The event consists of rapid variability lasting ∼40 s followed by a smooth power-law emission tail lasting ∼400 s. An abrupt change in spectral shape is found when the tail becomes noticeable. Our analysis reveals that the spectral evolution in the tail of the burst mimics that of a cooling synchrotron spectrum, similar to the spectral evolution of the low-energy afterglows for γ-ray bursts. This evidence for a separate emission component is consistent with the internal-external shock scenario in the relativistic fireball picture. In particular, it illustrates that the external shocks can be generated during the γ-ray emission phase, as in the case of GRB 990123.