Monte-Carlo simulations of relativistic radiation mediated shocks: II. photon starved regime

TL;DR

This paper uses Monte-Carlo simulations to study relativistic radiation mediated shocks in photon-starved regimes, revealing how shock velocity influences temperature, opacity, and emitted spectra, with implications for astrophysical transients.

Contribution

It provides the first self-consistent Monte-Carlo simulation results for photon-starved relativistic RMS across a wide velocity range, detailing shock microphysics and emission characteristics.

Findings

Downstream temperature regulated by pair creation, from 50 keV to 200 keV.

Opacity dominated by pair creation at relativistic velocities.

Spectra are softer than Planck distribution below peak, affecting optical emission predictions.

Abstract

Radiation mediated shocks (RMS) play a key role in shaping the early emission observed in many transients. In most cases, e.g., shock breakout in supernovae, llGRBs and neutron star mergers, the upstream plasma is devoid of radiation, and the photons that ultimately reach the observer are generated predominantly inside and downstream of the shock. Predicting the observed spectrum requires detailed calculations of the shock structure and thermodynamic state that account properly for the shock microphysics. We present results of self-consistent Monte-Carlo simulations of photon-starved RMS, that yield the shock structure and emission for a broad range of shock velocities, from sub-relativistic ($\beta_{sh} = 0.1$) to highly relativistic ($\Gamma_{sh} = 20$). Our simulations confirm that in relativistic RMS the immediate downstream temperature is regulated by exponential pair creation, ranging from $50$ keV at $\beta_{sh}=0.5$ to $200$ keV at $\Gamma_{sh}=20$. At lower velocities the temperature becomes sensitive to the shock velocity, with $kT\sim 0.5$ keV at $\beta_{sh}=0.1$. We also confirm that in relativistic shocks the opacity is completely dominated by newly created pairs, which has important implications for the breakout physics. We find the transition to pair dominance to occur at $\beta_{sh}=0.5$ roughly. In all cases examined, the spectrum below the $\nu F_\nu$ peak has been found to be substantially softer than the Planck distribution. This has important implications for the optical emission in fast and relativistic breakouts, and their detection. The applications to GRB 060218 and GRB 170817A are discussed.