Jet Launching from Merging Magnetized Binary Neutron Stars with Realistic Equations of State

TL;DR

This study uses GRMHD simulations to explore how magnetic field configurations in merging neutron stars influence jet formation, revealing conditions that produce jets consistent with short gamma-ray burst observations.

Contribution

It demonstrates that the initial magnetic field configuration and the binary mass relative to the collapse threshold critically affect jet launching in neutron star mergers.

Findings

Jets are only launched from BH+disk remnants after delayed collapse from hypermassive neutron stars.

The delay between gravitational wave peak and jet launch depends on total binary mass and magnetic field setup.

Jet lifetime (~150 ms) and luminosity (~10^{52} erg/s) match short gamma-ray burst central engine characteristics.

Abstract

We perform general relativistic, magnetohydrodynamic (GRMHD) simulations of binary neutron stars in quasi-circular orbit that merge and undergo delayed or prompt collapse to a black hole (BH). The stars are irrotational and modeled using an SLy or an H4 nuclear equation of state. To assess the impact of the initial magnetic field configuration on jet launching, we endow the stars with a purely poloidal magnetic field that is initially unimportant dynamically and is either confined to the stellar interior or extends from the interior into the exterior as in typical pulsars. Consistent with our previous results, we find that only the BH + disk remnants originating from binaries that form hypermassive neutron stars (HMNSs) and undergo delayed collapse can drive magnetically-powered jets. We find that the closer the total mass of the binary is to the threshold value for prompt collapse, the shorter is the time delay between the gravitational wave peak amplitude and jet launching. This time delay also strongly depends on the initial magnetic field configuration. We also find that seed magnetic fields confined to the stellar interior can launch a jet over $\sim 25\,\rm ms$ later than those with pulsar-like magnetic fields. The lifetime of the jet [$\Delta t\lesssim 150\,\rm ms$] and its outgoing Poynting luminosity [$L_{\rm EM}\sim 10^{52\pm 1}\rm erg/s$] are consistent with typical short gamma-ray burst central engine lifetimes, as well as with the Blandford--Znajek mechanism for launching jets and their associated Poynting luminosities. Our numerical results also suggest that the dynamical ejection of matter can be enhanced by the magnetic field. Therefore, GRMHD studies are required to fully understand kilonova signals from GW170818-like events.