Relativistic shock breakouts - a variety of gamma-ray flares: from low luminosity gamma-ray bursts to type Ia supernovae

A large variety of explosions result in mildly or ultra relativistic shock breakouts. Here we calculate the luminosity and spectrum that these breakouts produce. In order to do so we improve an analytic description of relativistic radiation mediated shocks and follow the system from the breakout itself, through the planar phase and into the spherical phase. We limit our calculation to cases where the post breakout acceleration of the gas ends during the planar phase. (i.e., the final gas Lorentz factor <~30). We find that spherical relativistic breakouts produce a flash of gamma-rays with energy, temperature, and duration that always satisfy a specific relation between them as well as providing the breakout radius and Lorentz factor. The breakout flare is typically followed, on longer time scales, by X-rays that carry a comparable energy. We apply our model to a variety of explosions, including Ia and .Ia supernovae (SNe), AIC, energetic SNe and gamma-ray bursts (GRBs). We find that all these events produce detectable gamma-ray signals, some of which may have already been seen. Some particular examples are: (i) Relativistic shock breakouts provide a natural explanation to the energy, temperature and time scales, as well as many other observed features, of all low luminosity GRBs. (ii) Nearby broad-line Ib/c (like SN 2002ap) may produce a detectable gamma-ray signal. (iii) Galactic Ia SNe produce detectable gamma-ray flares, if their progenitors are single degenerate. We conclude that relativistic shock breakout is a generic process for the production of gamma-ray flares, which opens a new window for the study and detection of a variety of stellar explosions.