2024ApJ...970L..12G

The spectral properties of a composite thermal emission arising from a relativistic expanding fireball can be remarkably different from the Planck function. We perform a detailed study of such a system to explore the features of the prompt emission spectra from gamma-ray bursts (GRBs). In particular, we address the effect of optical opacity and its dependence on the density profile between the expanding gas and the observer. This results in a nontrivial shape of the photospheric radius, which in combination with the constraints derived from the equal arrival time can result in a mild broader spectrum compared to the Planck function. Further, we show the time-integrated spectrum from the expanding fireball deviates significantly from the instantaneous emission and is capable of explaining the observed broad spectral width of GRBs. We also show that the demand of the spectral width of the order of unity, obtained through statistical analysis, is consistent with the scenario where the dynamics of the expanding fireball are governed predominantly by the energy content of the matter.