Black Hole-Neutron Star Mergers with a Hot Nuclear Equation of State: Outflow and Neutrino-Cooled Disk for a Low-Mass, High-Spin Case

TL;DR

This paper presents the first general relativistic simulation of a black hole-neutron star merger with a hot microphysical equation of state and neutrino feedback, revealing how neutrinos influence disk density, composition, and cooling.

Contribution

It introduces a novel simulation incorporating neutrino leakage in a high-spin black hole-neutron star merger with a realistic equation of state.

Findings

Approximately 0.08 solar masses of matter ejected.

A 0.3 solar mass hot, dense accretion disk formed.

Neutrino emission initially very luminous, decreasing over 50 ms.

Abstract

Neutrino emission significantly affects the evolution of the accretion tori formed in black hole-neutron star mergers. It removes energy from the disk, alters its composition, and provides a potential power source for a gamma-ray burst. To study these effects, simulations in general relativity with a hot microphysical equation of state and neutrino feedback are needed. We present the first such simulation, using a neutrino leakage scheme for cooling to capture the most essential effects and considering a moderate mass (1.4 M_{\odot} neutron star, 5.6 M_{\odot} black hole), high spin (black hole J/M^2=0.9) system with the K_0=220 MeV Lattimer-Swesty equation of state. We find that about 0.08 M_{\odot} of nuclear matter is ejected from the system, while another 0.3 M_{\odot} forms a hot, compact accretion disk. The primary effects of the escaping neutrinos are (i) to make the disk much denser and more compact, (ii) to cause the average electron fraction Y_e of the disk to rise to about 0.2 and then gradually decrease again, and (iii) to gradually cool the disk. The disk is initially hot (T~6 MeV) and luminous in neutrinos (L_{\nu}~10^{54} erg s^{-1}), but the neutrino luminosity decreases by an order of magnitude over 50 ms of post-merger evolution.