The Black Hole Remnant of Black Hole-Neutron Star Coalescing Binaries

TL;DR

This paper introduces an analytical model to accurately predict the spin and mass of black hole remnants after black hole-neutron star mergers, validated against numerical simulations and with implications for gravitational wave analysis.

Contribution

The paper adapts the Buonanno, Kidder, and Lehner method for black hole-neutron star mergers, providing a simple, accurate predictive model for remnant properties.

Findings

Predicted spin parameter with error ≤ 0.02

Mass prediction with relative error ≤ 2%

Potential to distinguish merger types via gravitational wave ringdown

Abstract

We present a model for determining the dimensionless spin parameter and mass of the black hole remnant of black hole-neutron star mergers with parallel orbital angular momentum and initial black hole spin. This approach is based on the Buonanno, Kidder, and Lehner method for binary black holes, and it is successfully tested against the results of numerical-relativity simulations: the dimensionless spin parameter is predicted with absolute error $\lesssim 0.02$, whereas the relative error on the final mass is $\lesssim 2$%, its distribution in the tests being pronouncedly peaked at $1$%. Our approach and the fit to the torus remnant mass reported in Foucart (2012) thus constitute an easy-to-use analytical model that accurately describes the remnant of black hole-neutron star mergers. The space of parameters consisting of the binary mass ratio, the initial black hole spin, and the neutron star mass and equation of state is investigated. We provide indirect support to the cosmic censorship conjecture for black hole remnants of black hole-neutron star mergers. We show that the presence of a neutron star affects the quasinormal mode frequency of the black hole remnant, thus suggesting that the ringdown epoch of the gravitational wave signal may virtually be used to (1) distinguish black hole-black hole from black hole-neutron star mergers and to (2) constrain the neutron star equation of state.