  A unique feature of deflagration-to-detonation (DDT) white dwarf explosion
models of SNe of type Ia is the presence of a strong shock wave propagating
through the outer envelope. We consider the early emission expected in such
models, which is produced by the expanding shock-heated outer part of the
ejecta and precedes the emission driven by radioactive decay. We expand on
earlier analyses by considering the modification of the pre-detonation density
profile by the weak-shocks generated during the deflagration phase, the time
evolution of the opacity, and the deviation of the post-shock equation of state
from that obtained for radiation pressure domination. A simple analytic model
is presented and shown to provide an acceptable approximation to the results of
1D numerical DDT simulations. Our analysis predicts a thousand second long
UV/optical flash with a luminosity of ~1 to 3*1e39 erg/s. Lower luminosity
corresponds to faster (turbulent) deflagration velocity. The predicted
luminosity of the UV flash is an order of magnitude lower than that of earlier
estimates, and is expected to be strongly suppressed at times longer than an
hour due to the deviation from pure radiation domination.
