Authors: A. L. Piro, P. Chang, and N. N. Weinberg

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 sec, 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 -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 it 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 per year. Archival data sets can also be used to study the early rise dictated by the shock heating (at approximately 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.