  The mode of explosive burning in Type Ia SNe remains an outstanding problem.
It is generally thought to begin as a subsonic deflagration, but this may
transition into a supersonic detonation (the DDT). We argue that this
transition leads to a breakout shock, which would provide the first unambiguous
evidence that DDTs occur. Its main features are a hard X-ray flash (~20 keV)
lasting ~0.01 s with a total radiated energy of ~10^{40} ergs, followed by a
cooling tail. This creates a distinct feature in the visual light curve, which
is separate from the nickel decay. This cooling tail has a maximum absolute
visual magnitude of M_V = -9 to -10 at approximately 1 day, which depends most
sensitively on the white dwarf radius at the time of the DDT. As the thermal
diffusion wave moves in, the composition of these surface layers may be
imprinted as spectral features, which would help to discern between SN Ia
progenitor models. Since this feature should accompany every SNe Ia, future
deep surveys (e.g., m=24) will see it out to a distance of approximately 80
Mpc, giving a maximum rate of ~60/yr. Archival data sets can also be used to
study the early rise dictated by the shock heating (at about 20 days before
maximum B-band light). A similar and slightly brighter event may also accompany
core bounce during the accretion induced collapse to a neutron star, but with a
lower occurrence rate.
