Electromagnetic precursor flares from the late inspiral of neutron star binaries

TL;DR

This paper investigates how the interaction of neutron star magnetospheres prior to merger can produce electromagnetic flares, using simulations to analyze magnetic field configurations and their role in precursor emissions.

Contribution

It provides a detailed simulation-based analysis of electromagnetic precursor flares from neutron star binaries, highlighting the importance of magnetic field topology and alignment.

Findings

Flaring occurs for suitable magnetic alignments.

Weaker magnetic fields suppress flaring.

Magnetospheric twisting leads to observable electromagnetic transients.

Abstract

The coalescence of two neutron stars is accompanied by the emission of gravitational waves, and can also feature electromagnetic counterparts powered by mass ejecta and the formation of a relativistic jet after the merger. Since neutron stars can feature strong magnetic fields, the non-trivial interaction of the neutron star magnetospheres might fuel potentially powerful electromagnetic transients prior to merger. A key process powering those precursor transients is relativistic reconnection in strong current sheets formed between the two stars. In this work, we provide a detailed analysis of how the twisting of the common magnetosphere of the binary leads to an emission of electromagnetic flares, akin to those produced in the solar corona. By means of relativistic force-free electrodynamics simulations, we clarify the role of different magnetic field topologies in the process. We conclude that flaring will always occur for suitable magnetic field alignments, unless one of the neutron stars has a magnetic field significantly weaker than the other.