Gravitational waves from spinning black hole-neutron star binaries: dependence on black hole spins and on neutron star equations of state

TL;DR

This study investigates how black hole spins and neutron star equations of state influence the merger dynamics, tidal disruption, and gravitational wave signals in black hole-neutron star binaries using full general relativity simulations.

Contribution

It provides new insights into the dependence of tidal disruption and disk formation on black hole spin and neutron star properties in relativistic merger simulations.

Findings

Prograde black hole spins promote tidal disruption across various mass ratios.

Large remnant disks (>0.1 solar masses) form with prograde spins, supporting short gamma-ray burst models.

A clear relation exists between neutron star compactness and the gravitational-wave cutoff frequency.

Abstract

We study the merger of black hole (BH)-neutron star (NS) binaries with a variety of BH spins aligned or anti-aligned with the orbital angular momentum, and with the mass ratio in the range MBH/MNS = 2--5, where MBH and MNS are the mass of the BH and NS, respectively. We model NS matter by systematically parametrized piecewise polytropic equations of state. The initial condition is computed in the puncture framework adopting an isolated horizon framework to estimate the BH spin and assuming an irrotational velocity field for the fluid inside the NS. Dynamical simulations are performed in full general relativity by an adaptive mesh refinement code, SACRA. The treatment of hydrodynamic equations and estimation of the disk mass are improved. We find that the NS is tidally disrupted irrespective of the mass ratio when the BH has a moderately large prograde spin, whereas only binaries with low mass ratios, MBH/MNS <~ 3 or small compactnesses of the NSs, bring the tidal disruption when the BH spin is zero or retrograde. The mass of the remnant disk is accordingly large as >~ 0.1 Msun, which is required by central engines of short gamma-ray bursts, if the BH spin is prograde. Information of the tidal disruption is reflected in a clear relation between the compactness of the NS and an appropriately defined "cutoff frequency" in the gravitational-wave spectrum, above which the spectrum damps exponentially. We find that the tidal disruption of the NS and excitation of the quasinormal mode of the remnant BH occur in a compatible manner in high mass-ratio binaries with the prograde BH spin. The correlation between the compactness and the cutoff frequency still holds for such cases. It is also suggested by extrapolation that the merger of an extremely spinning BH and an irrotational NS binary does not lead to formation of an overspinning BH.