The beginning of cosmic ray acceleration at supernova explosions

We investigate the onset of cosmic ray (CR) acceleration at supernovae, from the first day to the first few years following the explosion. We show that, under some circumstances, supernovae occurring in dense winds may accelerate CRs to the knee for a few years. We present a detailed investigation of the maximum CR energy, magnetic field amplification at the shock, and production of secondaries (high-energy neutrinos and gamma-rays). Another important finding is that, CR acceleration must start significantly before shock breakout for some supernovae. Diffusive shock acceleration notably requires the presence of a collisionless shock (CS). During the first few hours following the explosion, this condition is not satisfied. The shock is initially mediated by radiation and cannot accelerate supra-thermal protons to relativistic energies. Shock breakout occurs when the radiation-dominated shock reaches optically thin layers of the progenitor star, or of its surrounding wind if optically thick. The CS is usually assumed to form soon after. We demonstrate here, for the first time, that this does not hold for some progenitors enshrouded in optically thick winds. Secondary neutrinos (at least E > 100 GeV - 1 TeV) can reach the observer several hours before the first photons from breakout, enabling one to study the otherwise inaccessible optically thick layers of the progenitor's wind.