Observational characteristics of radiation-mediated shocks in photospheric gamma-ray burst emission

Abstract

Emission from the photosphere in gamma-ray burst (GRB) jets can be substantially affected by subphotospheric energy dissipation, which is typically caused by radiation-mediated shocks (RMSs). We study the observational characteristics of such emission, in particular the spectral signatures. Relevant shock initial conditions are estimated using a simple internal collision framework, which then serve as inputs for an RMS model that generates synthetic photospheric spectra. Within this framework, we find that if the free fireball acceleration starts at $r_0 \sim 10^{10}~$cm, in agreement with hydrodynamical simulations, then the typical spectrum consists of a broad, soft power-law segment with a cutoff at high energies and a hardening in X-rays. The synthetic spectra are generally well fitted with a standard cutoff power-law (CPL) function, as the hardening in X-rays is commonly outside the observable energy range of current detectors. The CPL-fits yield values for the low-energy index, $\alpha$, and the peak energy, $E_{\rm peak}$, that are centered around $\sim -0.8$ and $\sim 220~$keV, respectively, similar to typical observed values. We also identify a non-negligible parameter region for what we call ``optically shallow shocks'': shocks that do not accumulate enough scatterings to reach a steady-state spectrum before decoupling and thereby produce more complex spectra. These occur for optical depths $\tau \lesssim 55 \, u_u^{-2}$, where $u_u = \gamma_u\beta_u$ is the dimensionless specific momentum of the upstream as measured in the shock rest frame.