Jet launching from binary black hole-neutron star mergers: Dependence on black hole spin, binary mass ratio and magnetic field orientation

TL;DR

This study uses general relativistic magnetohydrodynamic simulations to explore how black hole spin, mass ratio, and magnetic field orientation affect jet launching in black hole-neutron star mergers, crucial for understanding short gamma-ray bursts.

Contribution

It systematically investigates the impact of various binary parameters on jet formation, extending previous work by including different spins, mass ratios, and magnetic field orientations.

Findings

Jets are launched in cases with high black hole spin ($a/M_{BH}=0.5$ or $0.75$).

Jet lifetimes are approximately 0.5 to 0.7 seconds, consistent with short gamma-ray bursts.

Jets exhibit Poynting luminosities around 10^{51} erg/s, supporting their role as central engines for sGRBs.

Abstract

Black hole-neutron star (BHNS) mergers are one of the most promising targets for multimessenger astronomy. Using general relativistic magnetohydrodynamic simulations of BHNS undergoing merger we showed that a magnetically--driven jet can be launched by the remnant if the NS is endowed with a dipole B field extending from the interior into the exterior as in a radio pulsar. These self-consistent studies considered a BHNS system with mass ratio $q=3:1$, BH spin $a/M_{BH}=0.75$ aligned with the total orbital angular momentum (OAM), and a NS that is irrotational, threaded by an aligned B field, and modeled by an $\Gamma$--law equation of state with $\Gamma=2$. Here, as a crucial step in establishing BHNS systems as viable progenitors of central engines that power short gamma--ray bursts (sGRBs) and thereby solidify their role as multimessenger sources, we survey different BHNS configurations that differ in BH spin ($a/M_{BH} =-0.5,\,0,\,0.5,\,0.75$), in the mass ratio ($q=3:1$ and $q=5:1$), and in the orientation of the B field (aligned and tilted by $90^\circ$ with respect to the OAM). We find that by $\Delta t\sim 3500M-4000M \sim 88(M_{NS}/1.4M_\odot){\rm ms}-100(M_{NS}/1.4M_\odot)\rm ms$ after the peak gravitational wave signal a jet is launched in the cases where the initial BH spin is $a/M_{BH}= 0.5$ or $0.75$. The lifetime of the jets[$\Delta t\sim 0.5(M_{NS}/1.4M_\odot){\rm s-0.7}(M_{NS}/1.4M_\odot)\rm s$] and their Poynting luminosities [$L_{jet}\sim 10^{51\pm 1}\rm erg/s$] are consistent with sGRBs, as well as with the Blandford--Znajek mechanism. By the time we terminate our simulations, we do not observe either an outflow or a large-scale B field collimation in the other configurations we simulate. These results suggest that future multimessenger detections from BHNSs are more likely produced by binaries with highly spinning BH companions and small tilt-angle B fields.