We present a simple analytic model for the structure of non-relativistic and relativistic radiation mediated shocks .
At shock velocities \beta _ { s } \equiv \textrm { 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 ^ { 3 } c ^ { 3 } ) ^ { 1 / 4 } . T _ { s } \gtrsim 10 \mbox { keV } is reached at shock velocities \beta _ { s } \approx 0.2 .
At higher velocities , \beta _ { s } \gtrsim 0.6 , the plasma is dominated in the transition region by e ^ { \pm } pairs and 60 \mbox { keV } \lesssim T _ { s } \lesssim 200 \mbox { 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 .
Our breakout analysis is restricted to temperatures T _ { s } \lesssim 50 \mbox { keV } corresponding to photon energies h \nu \lesssim 150 \mbox { 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 \mbox { keV } ( e.g .
XRF060218/SN2006aj ) .