Electromagnetic precursors to black hole - neutron star gravitational wave events: Flares and reconnection-powered fast-radio transients from the late inspiral

TL;DR

This paper uses simulations to show that black hole-neutron star mergers with magnetic fields can produce detectable electromagnetic flares, including fast radio bursts and X-ray emissions, prior to the gravitational wave event.

Contribution

It demonstrates that electromagnetic precursors, such as flares and transients, can occur in black hole-neutron star mergers due to magnetospheric interactions, independent of black hole spin.

Findings

Electromagnetic flares depend on magnetic field orientation.

Flares can produce fast radio burst-like transients.

Luminosity upper bound is approximately 10^{41} erg/s.

Abstract

The presence of magnetic fields in the late inspiral of black hole -- neutron star binaries could lead to potentially detectable electromagnetic precursor transients. Using general-relativistic force-free electrodynamics simulations, we investigate pre-merger interactions of the common magnetosphere of black hole -- neutron star systems. We demonstrate that these systems can feature copious electromagnetic flaring activity, which we find depends on the magnetic field orientation but not on black hole spin. Due to interactions with the surrounding magnetosphere, these flares could lead to Fast Radio Burst-like transients and X-ray emission, with $\mathcal{L}_{\rm EM} \lesssim 10^{41} \left( B_\ast/ 10^{12}\, \rm G \right)^2\, \rm erg/ s$ as an upper bound for the luminosity, where $B_\ast$ is the magnetic field strength on the surface of the neutron star.