Cosmic-ray acceleration and gamma-ray signals from radio supernovæ

Abstract

Core collapse supernovae (SNe) are among the most extreme events in the universe. The are known to harbor among the fastest (but non- or midly-relativistic) shock waves. Once it has crossed the stellar atmosphere, the SN blast wave expands in the wind of the massive star progenitor. In type IIb SNe, the progenitor is likely a Red SuperGiant (RSG) star which has a large mass loss rate and a slow stellar wind producing a very dense circumstellar medium. A high velocity shock and a high density medium are both key ingredients to initiate fast particle acceleration, and fast growing instabilities driven by the acceleration process itself. We have reanalyzed the efficiency of particle acceleration at the forward shock right after the SN outburst for the particular case of the well-known SN 1993J. We find that plasma instabilities driven by the energetic particles accelerated at the shock front grow over intraday timescales. This growth, and the interplay of non-linear process, permit a fast amplification of the magnetic field at the shock, that can explain the magnetic field strengths deduced from the radio monitoring of the source. The maximum particle energy is found to reach 1–10 PeV depending on the instability dominating the amplification process. We derive the time dependent particle spectra and the associated hadronic signatures of secondary particles (gamma-ray, leptons and neutrinos) arising from proton proton interactions.

We find that the Cherenkov Telescope Array (CTA) should easily detect objects like SN 1993J in particular above 1 TeV, while current generation of Cherenkov telescopes such as H.E.S.S. could only marginaly detect such events. The gamma-ray signal is found to be heavily absorbed by pair production process during the first week after the outburst. We predict a low neutrino flux above 10 TeV, implying a detectability horizon with a KM3NeT-type telescope of 1 Mpc only. We finally discuss the essential parameters that control the particle acceleration and gamma-ray emission in other type of SNe.