Neutrino-Induced Gamma-Ray Emission from Supernovae

During a core-collapse supernova, absorption of anti-nu_e emitted from the proto-neutron star by protons in the hydrogen envelope produces neutrons and positrons. Neutron capture on protons and positron annihilation then produce gamma rays of 2.22 and 0.511 MeV, respectively. We calculate the fluxes of these gamma rays expected from a supernova with an 11 M_sun progenitor. The flux from neutron capture on protons exponentially decays on a timescale of 564 s, which is determined by neutron decay and capture on protons and 3He nuclei. The peak flux is 2.38x10^{-7}/cm^2/s for a supernova at a distance of 1 kpc. In contrast, the gamma-ray flux from positron annihilation follows the time evolution of the anti-nu_e luminosity and lasts for ~10 s. The peak flux in this case is 6.8x10^{-5}/cm^2/s for a supernova at a distance of 1 kpc. Detection of the above gamma-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 signals that occur 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.