Black hole-neutron star mergers at realistic mass ratios: Equation of state and spin orientation effects

TL;DR

This study investigates how neutron star radius and black hole spin orientation affect black hole-neutron star mergers, revealing their influence on remnant mass, ejecta, and gravitational-wave signals, with implications for multi-messenger astronomy.

Contribution

It provides detailed analysis of the effects of neutron star radius and black hole spin alignment on merger outcomes at realistic mass ratios, highlighting their impact on ejecta and gravitational-wave detectability.

Findings

Remnant mass varies significantly with neutron star radius.

Ejected material can power detectable electromagnetic afterglows.

Few events provide constraints on neutron star equation of state.

Abstract

Black hole-neutron star mergers resulting in the disruption of the neutron star and the formation of an accretion disk and/or the ejection of unbound material are prime candidates for the joint detection of gravitational-wave and electromagnetic signals when the next generation of gravitational-wave detectors comes online. However, the disruption of the neutron star and the properties of the post-merger remnant are very sensitive to the parameters of the binary. In this paper, we study the impact of the radius of the neutron star and the alignment of the black hole spin for systems within the range of mass ratio currently deemed most likely for field binaries (M_BH ~ 7 M_NS) and for black hole spins large enough for the neutron star to disrupt (J/M^2=0.9). We find that: (i) In this regime, the merger is particularly sensitive to the radius of the neutron star, with remnant masses varying from 0.3M_NS to 0.1M_NS for changes of only 2 km in the NS radius; (ii) 0.01-0.05M_sun of unbound material can be ejected with kinetic energy >10^51 ergs, a significant increase compared to low mass ratio, low spin binaries. This ejecta could power detectable optical and radio afterglows. (iii) Only a small fraction (<3%) of the Advanced LIGO events in this parameter range have gravitational-wave signals which could offer constraints on the equation of state of the neutron star. (iv) A misaligned black hole spin works against disk formation, with less neutron star material remaining outside of the black hole after merger, and a larger fraction of that material remaining in the tidal tail instead of the forming accretion disk. (v) Large kicks (v>300 km/s) can be given to the final black hole as a result of a precessing BHNS merger, when the disruption of the neutron star occurs just outside or within the innermost stable spherical orbit.