Fully General Relativistic Simulations of Black Hole-Neutron Star Mergers

TL;DR

This paper presents advanced simulations of black hole-neutron star mergers using full general relativity, revealing details about mass ejection, disk formation, and gravitational wave signatures relevant for gravitational wave detection and neutron star physics.

Contribution

It introduces high-accuracy, fully relativistic simulations of BHNS mergers with detailed analysis of matter dynamics and gravitational waveforms, improving understanding of these events.

Findings

Most neutron star material is promptly accreted by the black hole.

Less than 3% of the neutron star's mass is ejected into a bound disk.

Gravitational waveforms show measurable differences from binary black hole mergers.

Abstract

Black hole-neutron star (BHNS) binaries are expected to be among the leading sources of gravitational waves observable by ground-based detectors, and may be the progenitors of short-hard gamma ray bursts (SGRBs) as well. Here, we discuss our new fully general relativistic calculations of merging BHNS binaries, which use high-accuracy, low-eccentricity, conformal thin-sandwich configurations as initial data. Our evolutions are performed using the moving puncture method and include a fully relativistic, high-resolution shock-capturing hydrodynamics treatment. Focusing on systems in which the neutron star is irrotational and the black hole is nonspinning with a 3:1 mass ratio, we investigate the inspiral, merger, and disk formation in the system. We find that the vast majority of material is promptly accreted and no more than 3% of the neutron star's rest mass is ejected into a tenuous, gravitationally bound disk. We find similar results for mass ratios of 2:1 and 1:1, even when we reduce the NS compaction in the 2:1 mass ratio case. These ambient disks reach temperatures suitable for triggering SGRBs, but their masses may be too small to produce the required total energy output. We measure gravitational waveforms and compute the effective strain in frequency space, finding measurable differences between our waveforms and those produced by binary black hole mergers within the advanced LIGO band. These differences appear at frequencies corresponding to the emission that occurs when the NS is tidally disrupted and accreted by the black hole. The resulting information about the radius of the neutron star may be used to constrain the neutron star equation of state.