Is Thermal Emission in Gamma-Ray Bursts Ubiquitous?

The prompt emission of gamma-ray bursts has yet defied any simpleexplanation, despite the presence of a rich observational material and greattheoretical efforts. Here we show that all the types of spectral evolution andspectral shapes that have been observed can indeed be described with one andthe same model, namely a hybrid model of a thermal and a non-thermal component.We further show that the thermal component is the key emission processdetermining the spectral evolution. Even though bursts appear to have a varietyof, sometimes complex, spectral evolutions, the behaviors of the two separatecomponents are remarkably similar for all bursts, with the temperaturedescribing a broken power-law in time. The non-thermal component is consistentwith emission from a population of fast cooling electrons emittingoptically-thin synchrotron emission or non-thermal Compton radiation. Thisindicates that these behaviors are the fundamental and characteristic ones forgamma-ray bursts.Comment: ApJ Letters Accepte