Pulsations in Short GRBs from Black Hole-Neutron Star Mergers

TL;DR

This paper proposes that jet precession signals can distinguish black hole-neutron star mergers as the origin of short gamma-ray bursts, providing specific observational signatures and analyzing their expected properties.

Contribution

It introduces the idea that jet precession can serve as an observational marker for black hole-neutron star mergers causing SGRBs, and quantifies the expected precession characteristics.

Findings

Precession periods of ~0.01-0.1 seconds are expected.

Jet precession increases the observable solid angle of SGRBs.

High viscosity may suppress multiple precession cycles.

Abstract

The precise origin of short gamma ray bursts (SGRBs) remains an important open question in relativistic astrophysics. Increasingly, observational evidence suggests the merger of a binary compact object system as the source for most SGRBs, but it is currently unclear how to distinguish observationally between a binary neutron star progenitor and a black hole-neutron star progenitor. We suggest the quasi-periodic signal of jet precession as an observational signature of SGRBs originating in mixed binary systems, and quantify both the fraction of mixed binaries capable of producing SGRBs, and the distributions of precession amplitudes and periods. The difficulty inherent in disrupting a neutron star outside the horizon of a stellar-mass black hole biases the jet precession signal towards low amplitude and high frequency. Precession periods of ~ 0.01-0.1 s and disk-BH spin misalignments ~10 degrees are generally expected, although sufficiently high viscosity may prevent the accumulation of multiple precession periods during the SGRB. The precessing jet will naturally cover a larger solid angle in the sky than would standard SGRB jets, enhancing observability for both prompt emission and optical afterglows.