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

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 (MBH~7MNS) and for black-hole spins large enough for the neutron star to disrupt (JBH/MBH²=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 0.3MNS to 0.1MNS for changes of only 2 km in the NS radius; (ii) 0.01M_--0.05M_ of unbound material can be ejected with kinetic energy 10⁵¹ 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 ~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 vkick300 km/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.