Properties of neutrino transfer in a deformed remnant of neutron star merger

TL;DR

This paper investigates neutrino transfer in a neutron star merger remnant using detailed numerical simulations, revealing asymmetric neutrino fluxes, energy-dependent emission regions, and limitations of common approximation methods.

Contribution

It provides a comprehensive multi-dimensional analysis of neutrino distributions and fluxes in a deformed merger remnant, highlighting the importance of muon and tau neutrinos for heating and evaluating the accuracy of moment closure approximations.

Findings

Asymmetric neutrino fluxes from the deformed remnant.

Energy-dependent neutrinosphere locations.

Limitations of moment closure methods for high-energy neutrinos.

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.