Gravitational-wave cutoff frequencies of tidally disruptive neutron star-black hole binary mergers

TL;DR

This paper develops formulas to predict the gravitational-wave cutoff frequency in neutron star-black hole mergers, which varies with system parameters and can reveal neutron star properties, aiding detection and multimessenger astronomy.

Contribution

It introduces a semi-analytical formula for the critical mass ratio for disruption and a fit for the gravitational-wave cutoff frequency based on numerical simulations.

Findings

Derived a formula for the critical mass ratio for disruption.

Provided a fit for the gravitational-wave cutoff frequency.

Enhanced understanding of gravitational wave signatures in disruptive mergers.

Abstract

Tidal disruption has a dramatic impact on the outcome of neutron star-black hole mergers. The phenomenology of these systems can be divided in three classes: nondisruptive, mildly disruptive or disruptive. The cutoff frequency of the gravitational radiation produced during the merger (which is potentially measurable by interferometric detectors) is very different in each regime, and when the merger is disuptive it carries information on the neutron star equation of state. Here we use semianalytical tools to derive a formula for the critical binary mass ratio $Q=M_{\rm BH}/M_{\rm NS}$ below which mergers are disruptive as a function of the stellar compactness $\mathcal{C}=M_{\rm NS}/R_{\rm NS}$ and the dimensionless black hole spin $\chi$. We then employ a new gravitational waveform amplitude model, calibrated to $134$ general relativistic numerical simulations of binaries with black hole spin (anti-)aligned with the orbital angular momentum, to obtain a fit to the gravitational-wave cutoff frequency in the disruptive regime as a function of $\mathcal{C}$, $Q$ and $\chi$. Our findings are important to build gravitational wave template banks, to determine whether neutron star-black hole mergers can emit electromagnetic radiation (thus helping multimessenger searches), and to improve event rate calculations for these systems.