Massive disk formation in the tidal disruption of a neutron star by a nearly extremal black hole

TL;DR

This paper presents the highest-spin black hole-neutron star merger simulation to date, revealing massive, stable accretion disks and significant tidal tails influenced by near-extremal black hole spins.

Contribution

It introduces a fully relativistic simulation of a BHNS merger with a black hole spin of 0.97, surpassing previous spin limits and demonstrating the impact on disk mass and stability.

Findings

Largest accretion disk observed in BHNS mergers to date

High black-hole spin results in more massive and stable disks

Black hole spin decreases after merger due to accretion and merger dynamics

Abstract

Black hole-neutron star (BHNS) binaries are important sources of gravitational waves for second-generation interferometers, and BHNS mergers are also a proposed engine for short, hard gamma-ray bursts. The behavior of both the spacetime (and thus the emitted gravitational waves) and the neutron star matter in a BHNS merger depend strongly and nonlinearly on the black hole's spin. While there is a significant possibility that astrophysical black holes could have spins that are nearly extremal (i.e. near the theoretical maximum), to date fully relativistic simulations of BHNS binaries have included black-hole spins only up to $S/M^2$=0.9, which corresponds to the black hole having approximately half as much rotational energy as possible, given the black hole's mass. In this paper, we present a new simulation of a BHNS binary with a mass ratio $q=3$ and black-hole spin $S/M^2$=0.97, the highest simulated to date. We find that the black hole's large spin leads to the most massive accretion disk and the largest tidal tail outflow of any fully relativistic BHNS simulations to date, even exceeding the results implied by extrapolating results from simulations with lower black-hole spin. The disk appears to be remarkably stable. We also find that the high black-hole spin persists until shortly before the time of merger; afterwards, both merger and accretion spin down the black hole.