2015MNRAS.449.3693G

During a supernova explosion, a radiation-dominated shock (RDS) travels through its progenitor. A collisionless shock (CS) is usually assumed to replace it during shock breakout (SB). We demonstrate here that for some realistic progenitors enshrouded in optically thick winds, such as possibly SN 2008D, a CS forms deep inside the wind, soon after the RDS leaves the core, and therefore significantly before SB. The RDS does not survive the transition from the core to the thick wind when the wind close to the core is not sufficiently dense to compensate for the r^–2 dilution of photons due to shock curvature. This typically happens when the shock velocity is i≲ 0.1c (u_w/10 km/s)({dot}M/(5x10^–4M_☉/yr))^–1(|r_*|/10¹³cm) , where u_w, {dot}M and r^* are, respectively, the wind velocity, mass-loss rate and radius of the progenitor star. The radiative CS results in a hard spectrum of the photon flash at breakout, which would produce an X-ray flash. Cosmic ray acceleration would start before SB, for such progenitors. A fraction of secondary TeV neutrinos can reach the observer up to more than 10 h before the first photons from breakout, providing information on the invisible layers of the progenitor.