Author: Indrek Vurm
Abstract: Photospheric emission of a heated GRB jet has a nonthermal Band-type spectrum. We investigate the expected spectrum using global radiative transfer simulations that start deep in the opaque zone and follow the evolution of radiation in the expanding jet until it is released to distant observers. The main new feature of our simulations is the self-consistent calculation of photon production and thermalization at large optical depths, which has a strong impact on the emerging radiation. The radius where radiation first deviates from blackbody is found at huge optical depths, as large as 105. We explore the effects of subphotopsheric heating and bulk acceleration on the number of produced photons and the shape of the GRB spectrum. We identify the heating histories that give spectra consistent with observations and discuss implications for the heating mechanism.