Relativistic shock breakouts - A variety of gamma-ray flares: From low-luminosity gamma-ray bursts to type ia supernovae

The light from a shock breakout of stellar explosions, which carries a wealth of information, strongly depends on the shock velocity at the time of the breakout. The emission from Newtonian breakouts, typical in regular core-collapse supernovae (SNe), has been explored extensively. However, a large variety of explosions result in mildly or ultrarelativistic breakouts, where the observed signature is unknown. Here we calculate the luminosity and spectrum produced by relativistic breakouts. In order to do so, we improve the 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, E _bo, temperature, T _bo, and duration, t ^obs _bo, that provide the breakout radius (≈5 R (t ^obs _bo/10 s)(T _bo/50 keV)²) and the Lorentz factor (≈T _bo/50 keV). They also always satisfy a relativistic breakout relation (t^obs _bo/20 s) ∼ (E_bo/10⁴⁶ erg)^1/2(T _bo/50 keV) ^-2.68. The breakout flare is typically followed, on longer timescales, by X-rays that carry a comparable energy. We apply our model to a variety of explosions, including Type Ia and .Ia SNe, accretion-induced collapse, 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: (1) relativistic shock breakouts provide a natural explanation to the energy, temperature, and timescales of low-luminosity GRBs. Indeed, all observed low-luminosity GRBs satisfy the relativistic breakout relation. (2) Nearby broad-line Type Ib/c (like SN 2002ap) may produce a detectable γ-ray signal. (3) Galactic Type Ia SNe may produce detectable γ-ray flares. We conclude that relativistic shock breakouts provide a generic process for the production of gamma-ray flares.