Fast radiation mediated shocks and supernova shock breakouts

Abstract

We present a simple analytic model for the structure of non-relativistic and relativistic radiation mediated shocks. At shock velocities βs ≡ vs /c ≳ 0.1, the shock transition region is far from thermal equilibrium since the transition crossing time is too short for the production of a blackbody photon density (by bremsstrahlung emission). In this region, electrons and photons (and positrons) are in Compton (pair) equilibrium at temperatures Ts significantly exceeding the far downstream temperature, Ts ≫ Td≈ 2(εn u3 c 3)1/4. Ts ≳ 10keV is reached at shock velocities βs 0.2. At higher velocities, βs ≳ 0.6, the plasma is dominated in the transition region by e pairs and 60keV ≲ Ts ≲ 200keV. We argue that the spectrum emitted during the breaking out of supernova (SN) shocks from the stellar envelopes (or the surrounding winds) of blue supergiants and Wolf-Rayet stars, which reach βs>0.1 for reasonable stellar parameters, may include a hard component with photon energies reaching tens or even hundreds of keV. Our breakout analysis is restricted to temperatures T s ≲ 50keV corresponding to photon energies hν ≲ 150keV, where pair creation can be neglected. This may account for the X-ray outburst associated with SN2008D, and possibly for other SN-associated outbursts with spectra not extending beyond few 100keV (e.g., XRF060218/SN2006aj). © 2010. The American Astronomical Society. All rights reserved.