Fast radiation mediated shocks and supernova shock breakouts

TL;DR

This paper develops an analytic model for radiation mediated shocks in supernovae, revealing how shock velocity influences the temperature and spectrum of emitted radiation, potentially explaining observed X-ray outbursts.

Contribution

It introduces a simple analytic framework for non-relativistic and relativistic radiation mediated shocks, highlighting the temperature and spectral features at different shock velocities.

Findings

Shock transition region is far from thermal equilibrium at eta_s geq 0.1.

Temperatures T_s can reach tens to hundreds of keV depending on shock velocity.

The model explains the hard X-ray component observed in supernova shock breakouts.

Abstract

We present a simple analytic model for the structure of non-relativistic and relativistic radiation mediated shocks. At shock velocities \beta_s\equiv v_s/c\gtrsim 0.1, the shock transition region is far from thermal equilibrium, since the transition crossing time is too short for the production of a black-body photon density (by Bremsstrahlung emission). In this region, electrons and photons (and positrons) are in Compton (pair) equilibrium at temperatures T_s significantly exceeding the far downstream temperature, T_s\gg T_d\approx 2(\varepsilon n_u \hbar^3c^3)^{1/4}. T_s\gtrsim 10 keV is reached at shock velocities \beta_s\approx 0.2. At higher velocities, \beta_s\gtrsim0.6, the plasma is dominated in the transition region by e^\pm pairs and 60 keV\lesssim T_s \lesssim 200 keV. We argue that the spectrum emitted during the breaking out of supernova shocks from the stellar envelopes (or the surrounding winds) of Blue Super Giants and Wolf-Rayet stars, which reach \beta_s>0.1 for reasonable stellar parameters, may include a hard component with photon energies reaching tens or even hundreds of keV. This may account for the X-ray outburst associated with SN2008D, and possibly for other SN-associated outbursts with spectra not extending beyond few 100 keV (e.g. XRF060218/SN2006aj).