FAST RADIATION MEDIATED SHOCKS AND SUPERNOVA SHOCK BREAKOUTS

We present a simple analytic model for the structure of non-relativistic and relativistic radiation mediated shocks. At shock velocities beta(s) = v(s)/c greater than or similar to 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 T(s) significantly exceeding the far downstream temperature, T(s) >> T(d) approximate to 2(epsilon n(u)h(3)c(3))(1/4). T(s) >= 10 keV is reached at shock velocities beta(s) approximate to 0.2. At higher velocities, beta(s) greater than or similar to 0.6, the plasma is dominated in the transition region by e(+/-) pairs and 60 keV less than or similar to T(s) less than or similar to 200 keV. 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 beta(s) > 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) less than or similar to 50 keV corresponding to photon energies h nu less than or similar to 150 keV, 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 100 keV (e. g., XRF060218/SN2006aj).