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

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 postmerger remnant are very sensitive to the parameters of the binary (mass ratio, black-hole spin, neutron star radius). In this paper, we study the impact of the radius of the neutron star and the alignment of the black-hole spin on black-hole--neutron-star mergers within the range of mass ratio currently deemed most likely for field binaries (${M}_{\mathrm{BH}}\ensuremath{\sim}7{M}_{\mathrm{NS}}$) and for black-hole spins large enough for the neutron star to disrupt (${J}_{\mathrm{BH}}/{M}_{\mathrm{BH}}^{2}=0.9$). We find that (i) In this regime, the merger is particularly sensitive to the radius of the neutron star, with remnant masses varying from $0.3{M}_{\mathrm{NS}}$ to $0.1{M}_{\mathrm{NS}}$ for changes of only 2 km in the NS radius; (ii) $0.01{M}_{\ensuremath{\bigodot}}--0.05{M}_{\ensuremath{\bigodot}}$ of unbound material can be ejected with kinetic energy $\ensuremath{\gtrsim}{10}^{51}\text{ }\text{ }\mathrm{ergs}$, a significant increase compared to low mass ratio, low spin binaries. This ejecta could power detectable postmerger optical and radio afterglows. (iii) Only a small fraction 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 (at best $\ensuremath{\sim}3%$ of the events for a single detector at design sensitivity). (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 ${v}_{\mathrm{kick}}\ensuremath{\gtrsim}300\text{ }\text{ }\mathrm{km}/\mathrm{s}$ can be given to the final black hole as a result of a precessing black-hole--neutron-star merger, when the disruption of the neutron star occurs just outside or within the innermost stable spherical orbit.