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 black-body 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( nu 3c3)1/4. Ts 10 keV is reached at shock velocities βs≈ 0.2. At higher velocities, βs0.6, the plasma is dominated in the transition region by e pairs and 60 keV Ts 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 βs>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).