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/M²QH = 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 μkick 300 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. [ABSTRACT FROM AUTHOR]