Gravitational waves from nonspinning black hole-neutron star binaries: dependence on equations of state

TL;DR

This study uses numerical simulations to explore how the gravitational wave signals from nonspinning black hole-neutron star mergers depend on the neutron star's equation of state, revealing key relationships between waveform features and stellar properties.

Contribution

It provides new insights into how the neutron star's equation of state influences gravitational wave spectra and remnant disk characteristics in black hole-neutron star mergers.

Findings

Tidal disruption occurs before black hole swallowing for small mass ratios.

Cutoff frequency of gravitational waves correlates with neutron star compactness.

Remnant disk mass depends on the neutron star's EOS.

Abstract

We report results of a numerical-relativity simulation for the merger of a black hole-neutron star binary with a variety of equations of state (EOSs) modeled by piecewise polytropes. We focus in particular on the dependence of the gravitational waveform at the merger stage on the EOSs. The initial conditions are computed in the moving-puncture framework, assuming that the black hole is nonspinning and the neutron star has an irrotational velocity field. For a small mass ratio of the binaries (e.g., MBH/MNS = 2 where MBH and MNS are the masses of the black hole and neutron star, respectively), the neutron star is tidally disrupted before it is swallowed by the black hole irrespective of the EOS. Especially for less-compact neutron stars, the tidal disruption occurs at a more distant orbit. The tidal disruption is reflected in a cutoff frequency of the gravitational-wave spectrum, above which the spectrum amplitude exponentially decreases. A clear relation is found between the cutoff frequency of the gravitational-wave spectrum and the compactness of the neutron star. This relation also depends weakly on the stiffness of the EOS in the core region of the neutron star, suggesting that not only the compactness but also the EOS at high density is reflected in gravitational waveforms. The mass of the disk formed after the merger shows a similar correlation with the EOS, whereas the spin of the remnant black hole depends primarily on the mass ratio of the binary, and only weakly on the EOS. Properties of the remnant disks are also analyzed.