The Observable Effects of a Photospheric Component on GRB and XRF Prompt Emission Spectrum

Abstract

A thermal radiative component is likely to accompany the first stages of the prompt emission of Gamma-ray bursts (GRB's) and X-ray flashes (XRF's). We analyze the effect of such a component on the observable spectrum, assuming that the observable effects are due to a dissipation process occurring below or near the thermal photosphere. We consider both the internal shock model and a 'slow heating' model as possible dissipation mechanisms. For comparable energy densities in the thermal and the leptonic component, the dominant emission mechanism is Compton scattering. This leads to a nearly flat energy spectrum (u F_u \propto u^0) above the thermal peak at ~10-100 keV and below 10-100 MeV, for a wide range of optical depths 0.03