Neutrino-induced γ-ray emission from supernovae

During a core-collapse supernova, absorption of ν̄e emitted from the protoneutron star by protons in the hydrogen envelope produces neutrons and positrons. Neutron capture on protons and positron annihilation then produce γ rays of 2.22 and 0.511 MeV, respectively. We calculate the fluxes of these γ rays expected from a supernova with an 11M progenitor. The flux from neutron capture on protons exponentially decays on a time scale of 564 s, which is determined by neutron decay and capture on protons and He3 nuclei. The peak flux is 2.38×10-7cm-2s-1 for a supernova at a distance of 1 kpc. In contrast, the γ-ray flux from positron annihilation follows the time evolution of the ν̄e luminosity and lasts for ∼10s. The peak flux in this case is 6.8×10-5cm-2s-1 for a supernova at a distance of 1 kpc. Detection of the above γ-ray fluxes is beyond the capability of current instruments, and perhaps even those planned for the near future. However, if such fluxes can be detected, they not only constitute a new kind of signal that occurs during the gap of several hours between the neutrino signals and the optical display of a supernova, but may also provide a useful probe of the conditions in the surface layers of the supernova progenitor.