Electromagnetic precursor flares from the late inspiral of black hole - neutron star binaries

Most neutron star – black hole gravitational wave events are thought to feature negligible remnant mass after the merger, suppressing potential electromagnetic counterpart production. There- fore, it is an essential question for current multi-messenger astronomy to understand what type of other electromagnetic transients can potentially be sourced by these merger events. In this work, we numerically investigate pre-merger interactions of the common magnetosphere of neutron star–black hole systems. Using general-relativistic force-free electrodynamics simulations, we demonstrate that this system can feature strong electromagnetic flaring activity. We quantify the energy released by these flaring events with a particular emphasis on GW200115-like gravitational wave events, including their ability to source fast-radio and X-ray transients.