Magnetic interactions in coalescing neutron star binaries

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 electromotive force 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 Rspace. Nevertheless, I show that there are interesting observational and/or dynamical effects irrespective of its exact value. When Rspace is large, electric dissipation as great as ∼1046 erg s-1 (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. When Rspace is small, electric dissipation largely occurs in the surface layers of the magnetic NS. This can reach ∼1049 erg s-1 during the final ∼1 s 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 Rspace, 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. © 2012 The American Astronomical Society. All rights reserved.