Early afterglows of short gamma-ray bursts

In the relativistic fireball model, the afterglow of a gamma-ray burst (GRB) is produced by synchrotron radiation of the electrons accelerated in the external shock that emerges as the relativistic flow moves. According to this model, the afterglow peaks on a time scale of ～10 s when observed in the soft gamma-ray band. The peak flux can be high enough to be detected by modern all-sky monitors. We investigate the emission from short (ΔT<1 s) GRBs on a time scale t≈10 s using BATSE/CGRO data. A significant flux is recorded for ～20% of the events. In most cases, the observed persistent emission can be explained in terms of the model as an early burst afterglow. No early afterglows of most short GRBs are observed. The model parameters for these bursts are constrained.