Black hole-neutron star coalescence: effects of the neutron star spin on jet launching and dynamical ejecta mass

TL;DR

This study uses magnetohydrodynamic simulations to explore how neutron star spin affects jet launching and ejecta mass in black hole-neutron star mergers, with implications for gravitational wave and electromagnetic observations.

Contribution

It provides the first detailed analysis of neutron star spin effects on jet formation and ejecta in BHNS mergers using dynamical spacetime simulations.

Findings

Prograde neutron star spin increases accretion disk and ejecta mass.

Magnetically-driven jets form only for specific mass ratios and timescales.

Ejected matter can power observable kilonovae.

Abstract

Black hole-neutron star (BHNS) mergers are thought to be sources of gravitational waves (GWs) with coincident electromagnetic (EM) counterparts. To further probe whether these systems are viable progenitors of short gamma-ray bursts (sGRBs) and kilonovae, and how one may use (the lack of) EM counterparts associated with LIGO/Virgo candidate BHNS GW events to sharpen parameter estimation, we study the impact of neutron star spin in BHNS mergers. Using dynamical spacetime magnetohydrodynamic simulations of BHNSs initially on a quasicircular orbit, we survey configurations that differ in the BH spin ($a_{\rm BH}/M_{\rm BH}=0$ and $0.75$), the NS spin ($a_{\rm NS}/M_{\rm NS}=-0.17,\,0,\,0.23$ and $0.33$), and the binary mass ratio ($q\equiv M_{\rm BH}:M_{\rm NS}=3:1$ and $5:1$). The general trend we find is that increasing the NS prograde spin increases both the rest mass of the accretion disk onto the remnant black hole, and the rest mass of dynamically ejected matter. By a time~$\Delta t\sim 3500-5500M\sim 88-138(M_{\rm NS}/1.4M_\odot)\,\rm ms$ after the peak gravitational wave amplitude, a magnetically--driven jet is launched only for $q=3:1$ regardless of the initial NS spin. The lifetime of the jets [$\Delta t\sim 0.5-0.8(M_{\rm NS}/1.4 M_\odot)\,\rm s$] and their outgoing Poynting luminosity [$L_{\rm Poyn}\sim 10^{51.5\pm 0.5}\,\rm erg/s$] are consistent with typical sGRBs luminosities and expectations from the Blandford-Znajek mechanism. By the time we terminate our simulations, we do not observe either an outflow or a large-scale magnetic field collimation for the other systems we considered. The mass range of dynamically ejected matter is $10^{-4.5}-10^{-2}~(M_{\rm NS}/1.4M_\odot)M_\odot$, which can power kilonovae with peak bolometric luminosities $L_{\rm knova}\sim 10^{40}-10^{41.4}$ erg/s with rise times $\lesssim 6.5\,\rm h$ and potentially detectable by the LSST.