Electromagnetic Precursors to Gravitational-wave Events: Numerical Simulations of Flaring in Pre-merger Binary Neutron Star Magnetospheres

The detection of gravitational waves from neutron star merger events has opened up a new field of multimessenger astronomy linking gravitational-wave events to short gamma-ray bursts and kilonova afterglows. A further—yet to be discovered—electromagnetic counterpart is a precursor emission produced by the nontrivial interaction of the magnetospheres of the two neutron stars prior to merger. By performing special-relativistic force-free simulations of orbiting neutron stars we discuss the effect of different magnetic field orientations and show how the emission can be significantly enhanced by differential motion present in the binary, either due to stellar spins or misaligned stellar magnetospheres. We find that the buildup of twist in the magnetic flux tube connecting the two stars can lead to the repeated emission of powerful flares for a variety of orbital configurations. We also discuss potential coherent radio emission mechanisms in the flaring process.