An efficient method for fitting radiation-mediated shocks to gamma-ray burst data: The Kompaneets RMS approximation

TL;DR

This paper introduces the Kompaneets RMS approximation, an efficient method for fitting radiation-mediated shocks in gamma-ray bursts to observational data, enabling extraction of physical shock properties from spectra.

Contribution

The paper develops and validates the KRA, a novel approximate method that accurately models RMS spectra and allows for rapid fitting to GRB data, bridging theory and observations.

Findings

KRA accurately reproduces spectra compared to first-principle simulations.

Application to GRB 150314A reveals shock properties from spectral fitting.

Enables efficient analysis of radiation-mediated shocks in GRBs.

Abstract

Shocks that occur below a gamma-ray burst (GRB) jet photosphere are mediated by radiation. Such radiation-mediated shocks (RMSs) could be responsible for shaping the prompt GRB emission. Although well studied theoretically, RMS models have not yet been fitted to data due to the computational cost of simulating RMSs from first principles. Here, we bridge the gap between theory and observations by developing an approximate method capable of accurately reproducing radiation spectra from mildly relativistic (in the shock frame) or slower RMSs, called the Kompaneets RMS approximation (KRA). The approximation is based on the similarities between thermal Comptonization of radiation and the bulk Comptonization that occurs inside an RMS. We validate the method by comparing simulated KRA radiation spectra to first-principle radiation-hydrodynamics simulations, finding excellent agreement both inside the RMS and in the RMS downstream. The KRA is then applied to a shock scenario inside a GRB jet, allowing for fast and efficient fitting to GRB data. We illustrate the capabilities of the developed method by performing a fit to a non-thermal spectrum in GRB 150314A. The fit allows us to uncover the physical properties of the RMS responsible for the prompt emission, such as the shock speed and the upstream plasma temperature.