1205.3227 (Amir Levinson)

Observational signatures of sub-photospheric radiation mediated shocks in the prompt phase of GRBs    [PDF]

Amir Levinson

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 from a shock that breaks out of 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 for shocks that form just below the photosphere, at optical depths $\tau\simlt10$, the emitted spectrum has a Wien shape. The time integrated spectrum emitted from shocks that form at moderate optical depths is modified by adiabatic cooling. Typically, it exhibits a thermal peak with a power law extension that depends on the geometry of the unshocked jet, with $\nu F_\nu\propto\nu^{-1/2}$ for a conical jet, and a steeper slope for a collimating jet. At large optical depths, $\tau\simgt10^3$, thermalization processes affect the shape of the emitted spectrum.

View original: http://arxiv.org/abs/1205.3227