Signatures of a Maxwellian component in shock-accelerated electrons in GRBs

Recent particle-in-cell simulations suggest that a large f raction of the energy dissipated in a relativistic shock is deposited into a Maxwellian distribu tion of electrons that is connected to the high-energy power-law tail. Here, we explore the observational implications of such a mixed thermal-nonthermal particle distribution for the a fterglow and prompt emission of gamma-ray bursts. When the Maxwellian component dominates the energy budget, the afterglow lightcurves show a very steep decline phase followed by a more shallow decay when the characteristic synchrotron frequency crosses the observe d band. The steep decay appears in the X-rays at∼100 sec after the burst and is accompanied by a characteristi c hard-soft-hard spectral evolution that has been observed in a large number of early afterglows. If internal shocks produce a similar mixed electron distribution, a bump is expected at the synchrotron peak of theν fν spectrum.