Is Thermal Emission in Gamma-Ray Bursts Ubiquitous?

The prompt emission of gamma-ray bursts has yet defied any simple explanation, despite the presence of rich observational material and great theoretical efforts. Here we show that all the types of spectral evolution and spectral shapes that have been observed can indeed be described with one and the same model, namely, a hybrid model with a thermal and a nonthermal component. We further show that the thermal component is the key emission process determining the spectral evolution. Even though bursts appear to have a variety of sometimes complex spectral evolutions, the behaviors of the two separate components are remarkably similar for all bursts, with the temperature describing a broken power law in time. The nonthermal component is consistent with emission from a population of fast-cooling electrons emitting optically thin synchrotron emission or nonthermal Compton radiation. This indicates that these behaviors are the fundamental and characteristic ones for gamma-ray bursts.