A shock that form below the photosphere of a GRB outflow is mediated by
Compton scattering of radiation advected into the shock by the upstream fluid.
The characteristic scale of such a shock, a few Thomson depths, is larger than
any kinetic scale involved by several orders of magnitudes, hence, unlike
collisionless shocks, radiation mediated shocks cannot accelerate particles to
non-thermal energies. The spectrum emitted by a shock that emerges from the
photosphere of a GRB jet, reflects the temperature profile downstream of the
shock, with a possible contribution at the highest energies from the shock
transition layer itself. We study the properties of radiation mediated shocks
that form during the prompt phase of GRBs, and compute the time integrated
spectrum emitted by the shocked fluid following shock breakout. We show that
the time integrated emission from a single shock exhibits a prominent thermal
peak, with the location of the peak depending on the shock velocity profile. We
also point out that multiple shock emission can produce a spectrum that mimics
a Band spectrum.Comment: 23 pages, 5 figures, revised version with an error corrected and
additional results presente