GRB980923. A BURST WITH A SHORT DURATION HIGH-ENERGY COMPONENT

The prompt emission of gamma-ray bursts (GRBs) is usually well described by the Band function: two power laws (PLs) joined smoothly at a given break energy. In addition to the Band component, a few bursts (GRB941017, GRB090510, GRB090902B, and GRB090926A) show clear evidence of a distinct high-energy spectral component, which in some cases evolves independently from the prompt keV component and is well described by a PL, sometimes with a cutoff energy; this component is found to have long duration, even longer than the burst itself for all four bursts. Here we report the observation of an anomalous short duration high-energy component in GRB980923. GRB980923 is one of the brightest GRBs observed by BATSE. Its light curve is characterized by a rapid variability phase lasting similar to 40 s, followed by a smooth emission tail lasting similar to 400 s. A detailed joint analysis of BATSE (Large Area Detector and Spectroscopy Detector) and EGRET TASC data of GRB980923 reveals the presence of an anomalous keV to MeV component in the spectrum that evolves independently from the prompt keV one. This component is well described by a PL with a spectral index of -1.44 and lasts only similar to 2 s; it represents one of the three clearly separated spectral components identified in GRB980923, the other two being the keV prompt emission, well described by the Band function and the tail, well fit by a Smoothly Broken Power Law.