The breakout of a fast (>0.1c), yet sub-relativistic shock from a thick
stellar wind is expected to produce a pulse of X-rays with a rise time of
seconds to hours. Here, we construct a semi-analytic model for the breakout of
a sub-relativistic, radiation-mediated shock from a thick stellar wind, and use
it to compute the spectrum of the breakout emission. The model incorporates
photon escape through the finite optical depth wind, assuming a diffusion
approximation and a quasi-steady evolution of the shock structure during the
breakout phase. We find that in sufficiently fast shocks, for which the
breakout velocity exceeds about 0.1c, the time-integrated spectrum of the
breakout pulse is non-thermal, and the time-resolved temperature is expected to
exhibit substantial decrease (roughly by one order of magnitude) during
breakout, when the flux is still rising, because of the photon generation by
the shock compression associated with the photon escape. We also derive a
closure relation between the breakout duration, peak luminosity, and
characteristic temperature that can be used to test whether an observed X-ray
flare is consistent with being associated with a sub-relativistic shock
breakout from a thick stellar wind or not. We also discuss implications of the
spectral softening for a possible breakout event XRT 080109/SN 2008D.Comment: 8 pages, 8 figures, accepted for publication in MNRA
