Signatures of Low Mass Black Hole-Neutron Star Mergers

TL;DR

This paper investigates low-mass black hole-neutron star mergers through simulations, analyzing how parameters like equation of state, spin, and mass ratio influence gravitational waves and ejecta, with implications for multi-messenger astronomy.

Contribution

It provides the first detailed simulation study of low-mass black hole-neutron star mergers, exploring effects of spin, equation of state, and mass ratio on observable signatures.

Findings

Stiffer equations of state produce more fast-moving ejecta.

High-mass ratio systems eject significantly more fast-moving matter.

Black-hole spin reduces fast ejecta but increases total ejected mass.

Abstract

The recent observation of the GW230529 event indicates that black hole-neutron star binaries can contain low-mass black holes. Since lower mass systems are more favourable for tidal disruption, such events are promising candidates for multi-messenger observations. In this study, we employ five finite-temperature, composition-dependent matter equations of state and present results from ten 3D general relativistic hydrodynamic simulations for the mass ratios $q = 2.6$ and $5$. Two of these simulations target the chirp mass and effective spin parameter of the GW230529 event, while the remaining eight contain slightly higher-mass black holes, including both spinning ($a_{BH} = 0.7$) and non-spinning ($a_{BH} = 0$) models. We discuss the impact of the equation of state, spin, and mass ratio on black hole-neutron star mergers by examining both gravitational-wave and ejected matter properties. For the low-mass ratio model we do not see fast-moving ejecta for the softest equation of state model, but the stiffer model produces on the order of $10^{-6}M_\odot$ of fast-moving ejecta, expected to contribute to an electromagnetic counterpart. Notably, the high-mass ratio model produces nearly the same amount of total dynamical ejecta, but yields $52$ times more fast-moving ejecta than the low-mass ratio system. In addition, we observe that the black-hole spin tends to decrease the amount of fast-moving ejecta while increasing significantly the total ejected mass. Finally, we note that the disc mass tends to increase as the neutron star compactness decreases.