Magnetic Interactions in Coalescing Neutron Star Binaries

TL;DR

This paper explores magnetic interactions in coalescing neutron star binaries using a unipolar inductor model, predicting observable electromagnetic signals and effects on inspiral dynamics relevant for gravitational wave detection.

Contribution

It introduces a detailed analysis of magnetic interactions in neutron star binaries, highlighting potential electromagnetic precursors and their impact on inspiral evolution.

Findings

Electric dissipation can reach ~10^{46} erg/s with large R_space, producing X-ray precursors.

Dissipation can reach ~10^{49} erg/s in the final second, similar to short gamma-ray bursts.

Magnetic torques can spin up the magnetized NS, affecting inspiral timescales.

Abstract

It is expected on both evolutionary and empirical grounds that many merging neutron star (NS) binaries are composed of a highly magnetized NS in orbit with a relatively low magnetic field NS. I study the magnetic interactions of these binaries using the framework of a unipolar inductor model. The e.m.f. generated across the non-magnetic NS as it moves through the magnetosphere sets up a circuit connecting the two stars. The exact features of this circuit depend on the uncertain resistance in the space between the stars R_space. Nevertheless, I show that there are interesting observational and/or dynamical effects irrespective of its exact value. When R_space is large, electric dissipation as great as ~10^{46} erg/s (for magnetar-strength fields) occurs in the magnetosphere, which would exhibit itself as a hard X-ray precursor in the seconds leading up to merger. With less certainty, there may also be an associated radio transient, but this would be observed well past merger (~hrs) because of interstellar dispersion. When R_space is small, electric dissipation largely occurs in the surface layers of the magnetic NS. This can reach ~10^{49} erg/s during the final ~1 sec before merger, similar to the energetics and timescales of short gamma-ray bursts. In addition, for dipole fields greater than ~10^{12} G and a small R_space, magnetic torques spin up the magnetized NS. This drains angular momentum from the binary and accelerates the inspiral. A faster coalescence results in less orbits occurring before merger, which would impact matched-filtering gravitational-wave searches by ground-based laser interferometers and could create difficulties for studying alternative theories of gravity with compact inspirals.