Properties of Neutrino Transfer in a Deformed Remnant of a Neutron Star Merger

Abstract

We study properties of neutrino transfer in a remnant of a neutron star merger, consisting of a massive neutron star and a surrounding torus. We perform numerical simulations of the neutrino transfer by solving the Boltzmann equation with momentum-space angles and energies of neutrinos for snapshots of the merger remnant having elongated shapes. The evaluation of the neutrino distributions in multiple dimensions enables us to provide detailed information on the angle and energy spectra and neutrino reaction rates. We demonstrate features of asymmetric neutrino fluxes from the deformed remnant and investigate the neutrino emission region by determining the neutrinosphere for each energy. We examine the emission and absorption of neutrinos to identify important ingredients of heating rates through neutrino irradiation. We show that the contributions of mu- and tau-type neutrinos are important for the heating in the region above the massive neutron star. We also examine the angle moments and the Eddington tensor calculated directly from the neutrino distribution functions and compare them with those obtained by a moment closure approach, which is often used in the study of neutrino-radiation hydrodynamics. We show that the components of the Eddington tensor have non-monotonic behaviors, and the approximation of the closure relation may become inaccurate for high-energy neutrinos, whose fluxes are highly aspherical due to the extended merger remnant.