The Cooling Behavior of Thermal Pulses in Gamma-Ray Bursts

Abstract

We discuss gamma-ray bursts that have very hard spectra, consistent with black-body radiation, throughout their duration. We find that the temperature decay during a pulse can be well described by a broken power-law in time, with an initially constant or weak decay. After the break, most cases are consistent with a decay with index -2/3. Some pulses have a weak non-thermal component overlayed the thermal one, and are better modelled with a combination of a thermal and a non-thermal component. Such a two-component model can explain the whole time-evolution of other bursts, that are found to be only initially thermal and later become non-thermal. The relative strengths between the two components vary with time and this is suggested to, among other things, account for the change in the modelled low-energy power-law slope that is often observed in bursts. We interpret the observations within a model of an optically thick shell that expands adiabatically. The slow, or constant, temperature decrease is from the acceleration phase, during which the bulk Lorentz factor increases, and the faster temperature decay is reached as the flow saturates and starts to coast with a constant speed. We also discuss a Poynting-flux model, in which the saturation radius is reached close to the photosphere.