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

TL;DR

This paper investigates the properties and spectra of radiation mediated shocks below the photosphere in GRBs, revealing thermal peaks and potential spectral mimicking of Band spectra.

Contribution

It provides a detailed analysis of sub-photospheric radiation mediated shocks in GRBs and computes their time integrated spectra, highlighting their spectral features and implications.

Findings

Single shock emission shows a prominent thermal peak.

Multiple shocks can produce spectra resembling Band spectra.

The peak location depends on shock velocity profile.

Abstract

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.