Where is the photospheric emission in GRBs ?

In the standard version of the GRB fireball model, the initial energy content of the flow is essentially thermal. However the absence of bright thermal components in most observed GRB spectra seems in contradiction with this simple picture. A first solution investigated by several authors considers additional dissipative mechanisms (e.g. [1, 2]), which results in a modified, Band-like, spectral shape of the photospheric emission. Here we investigate an alternative scenario where the GRB flow is initially magnetically dominated but contains a subdominant thermal component. The thermal component cools passively up to the photosphere, where it is radiated. We compute the resulting thermal emission and we consistently superimpose a non-thermal contribution produced either by internal shocks or by magnetic dissipation, depending on the remaining magnetization of the jet after acceleration. Within this picture, we explore the parameter space and compare model predictions (light-curves and spectra) to different tentative detections of thermal components in GRB spectra (e.g. the recent Fermi bursts 090902B, [3], and 100724B, [4]).