Bright transients from strongly-magnetized neutron star-black hole mergers

TL;DR

This paper proposes that highly magnetized neutron star-black hole mergers can produce bright electromagnetic transients detectable in gamma-ray and X-ray wavelengths, serving as valuable counterparts for gravitational-wave observations.

Contribution

It introduces a novel mechanism where neutron star magnetism generates observable electromagnetic signals during mergers, enhancing multi-messenger astronomy.

Findings

Bright electromagnetic transients can occur seconds before merger.

Signals are detectable by Fermi Gamma-Ray Burst Monitor for neutron star fields >10^{14}G.

Event rate could be up to a few per year for highly magnetized neutron stars.

Abstract

Direct detection of black hole-neutron star pairs is anticipated with the advent of aLIGO. Electromagnetic counterparts may be crucial for a confident gravitational-wave detection as well as for extraction of astronomical information. Yet black hole-neutron star pairs are notoriously dark and so inaccessible to telescopes. Contrary to this expectation, a bright electromagnetic transient can occur in the final moments before merger as long as the neutron star is highly magnetized. The orbital motion of the neutron star magnet creates a Faraday flux and corresponding power available for luminosity. A spectrum of curvature radiation ramps up until the rapid injection of energy ignites a fireball, which would appear as an energetic blackbody peaking in the x ray to $\gamma$ rays for neutron star field strengths ranging from $10^{12}$G to $10^{16}$G respectively and a $10M_{\odot} $ black hole. The fireball event may last from a few milliseconds to a few seconds depending on the neutron star magnetic-field strength, and may be observable with the Fermi Gamma-Ray Burst Monitor with a rate up to a few per year for neutron star field strengths $\gtrsim 10^{14}$G. We also discuss a possible decaying post-merger event which could accompany this signal. As an electromagnetic counterpart to these otherwise dark pairs, the black-hole battery should be of great value to the development of multi-messenger astronomy in the era of aLIGO.