Introduction
We are in a golden era of gravitational wave (GW) physics where the sensitivity of ground-based laser interferometers is rapidly increasing. During observation runs O1,O2, and O3, the GW events can be classified as follows: a) 37 BHBH candidates; b) 7 NSNS candidates. It should be noted that the progenitor of GW190425 is a NSNS system with a total mass of $3.4^{+0.3}_{-0.1}M_\odot$, which is significantly different from the known population of Galactic NSNS systems; c) 4 events in the so-called mass gap (compact objects with masses of $3-5\ M_\odot$); d) 5 BHNS candidates, of which only one event has been confirmed (GW190814) and whose inferred individual masses are $23^{+1}_{-0.9}M_\odot$ and $2.59^{+0.08}_{-0.08}M_\odot$. It is worth noting that, although this event is listed as a BHNS candidate with $>99\%$ probability, due to the lack of any EM counterpart or tidal signature, the nature of the lighter companion is uncertain. If indeed the lighter binary companion is a NS then this would be the heaviest NS yet observed. If, on the other hand, the binary companion is a BH then it would be the lightest BH observed to date.
Unlike the EM counterparts associated with GW170817 (which provides the best direct observational evidence so far that at least some sGRBs are indeed powered by NSNS mergers, or by the merger of a stellar compact binary where at least one of the companions is a NS or a hybrid star), the other candidate EM counterparts were not confirmed by other observatories/satellites operating at the same time. The absence of observable EM counterparts from candidate BHNS mergers may question their role as progenitors of the central engines that power sGRBs. Yet, GRMHD simulations showed that BHNS remnants of $q=3:1$ mergers can potentially launch magnetically-driven jets. Now early population synthesis studies found that the distribution of mass ratios $q$ in BHNSs depends on the metallicity, and peaks at $q= 7 : 1$, but more recent work finds that it is generally less than $10 : 1$, and peaks at $q\approx5 : 1$. The larger the mass ratio, the higher the BH spin required for the NS companion to be tidally disrupted before reaching the innermost stable circular orbit (ISCO). So far in BHBHs reported by the LIGO/Virgo scientific collaboration, BHs have high mass and/or low spins. If this trend continues for LIGO/Virgo BHNSs, then it is expected that LIGO/Virgo BHNS remnants would have negligible accretion disks and ejecta, which might disfavor their role as progenitors of sGRBs and kilonovae. However, the NS spin could have a strong impact on the tidal disruption and dynamical ejection of matter, affecting both sGRB and potential kilonovae signatures. It should be noted that the spins of the binary companions are only weakly constrained by current GW observations.
In this paper, we survey fully relativistic BHNS configurations on a quasicircular orbit undergoing merger in which the BH and/or the NS companions are spinning. We address two questions:
a) Can a moderate high-mass ratio BHNS binary be the progenitor of an engine that powers sGRBs?
b) Can the spin of a NS companion change the fraction of the dynamical ejection of matter that may drive potentially detectable kilonovae signatures?
We consider BHNS configurations with mass ratios $q=3:1$ and $q=5:1$. In the first case the BH spin is $a_{\rm BH}/M_{\rm BH}=0.75$, while in the latter one the BH is nonspinning. The NS spin has a spin $a_{\rm NS}/ M_{\rm NS}=-0.17,\, 0,\, 0.23$ or $0.33$. In all cases, the star is threaded by a dynamically weak poloidal magnetic field that extends from the stellar interior into the exterior (as in a pulsar), and whose dipole magnetic moment is aligned with the orbital angular momentum of the binary. For purposes of comparison with our earlier studies, the NS is modeled by a polytropic equation of state (EOS) with $\Gamma=2$.
We find that the late inspiral and merger phases of the above BHNS binaries are roughly the same as when the magnetic field is confined to the interior of the star. The fraction of the total NS rest mass outside the horizon varies from $\lesssim 1\%$ to $\sim 15\%$ depending strongly on the binary mass ratio. The general trend is that increasing the NS prograde spin increases both the mass of the accretion disk remnant and the unbound material (ejecta). In addition, NS spin leads to GW dephasing, with higher prograde spin increasing the number of GW cycles.
Consistent with our previous results, we find that by $\Delta t\sim 3500-5500M\approx 88-138(M_{\rm NS}/1.4M_\odot)\,\rm ms$ following the GW peak emission a magnetically-driven jet emerges from the BH + disk remnant of BHNSs with mass ratio $q=3:1$ regardless of the initial NS spin. However, the jet launching time depends strongly on the latter. As the initial NS prograde spin increases, the effective ISCO decreases and the separation at which the star is tidally disrupted increases. These two effects induce long tidal tails of matter that result in more baryon-loaded environments. Thus, stronger magnetic fields are required to overcome the baryon ram-pressure, delaying the launch of the jet while the fields amplify. Notice that jet launching may not be possible for all EOSs if the matter fall-back timescale is longer than the disk accretion timescale. The lifetime of the jet [$\Delta t\sim 0.5-0.8 (M_{\rm NS}/1.4M_\odot)\,\rm s$] and outgoing Poynting luminosity [$L_{\rm Poyn}\sim 10^{51.5 \pm 0.5}\,\rm erg/s$] are consistent with typical sGRB, and with the Blandford-Znajek (BZ) luminosities.
