Neutrino acceleration by bulk matter motion and explosion mechanism of gamma-ray bursts

The neutrino annihilation is one of the most promising candidates for the jet-production process of gamma-ray bursts. Although neutrino interaction rates depend strongly on the neutrino spectrum, the estimations of annihilation rate have been done with an assumption of the neutrino thermal spectrum based on the presence of the neutrinospheres, in which neutrinos and matter couple strongly. We consider the spectral change of neutrinos caused by the scattering by infalling materials and amplification of the annihilation rate. We solve the kinetic equation of neutrinos in spherically symmetric background flow and find that neutrinos are successfully accelerated and partly form non-thermal spectrum. We find that the accelerated neutrinos can significantly enhance the annihilation rate by a factor of similar to 10, depending on the injection optical depth.