Recent observations, especially by the Fermi satellite, reveal the presence of thermal component in GRB spectra and revive strong interest in photospheric emission from relativistic outflows. This topic was pioneered by Goodman and Paczynski in 1986, albeit in different frameworks. The key contribution was given of Abramowicz, Novikov and Paczynski in 1991, who noticed a strong anisotropy of photon scattering, as well as concave shape of the photosphere in highly relativistic wind. These early studies already suggested that the shape of observed spectrum is different from the thermal one.
The photospheric component appears to be subdominant in many GRBs and the origin of the dominant component in the prompt emission is yet unclear. One of popular ideas is that energy dissipation near the photosphere may convert kinetic or magnetic energy of the outflow into radiation with non-thermal spectrum. However, before considering such models one has to determine precise spectral and timing characteristic of the photospheric emission in the simplest possible case. Hence this talk focuses on various physical effects which make the spectrum of photospheric emission different from the black body one. In addition, a new physical effect termed "spotlight" will be discussed. This effect provides a specific mechanism for kinetic energy conversion into radiation above the photosphere, and thus might be part of the radiative mechanism, responsible for the prompt radiation in GRBs.
