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Astrophysics, High Energy Astrophysical Phenomena, astro-ph.HE
Abstract:
We study properties of neutrino transfer in a remnant of 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 the multi-dimensions enable us to provide the detailed
information of 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 $\tau$-types 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 by 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.
