The Swift short gamma-ray burst rate density: implications for binary neutron star merger rates

TL;DR

This paper estimates the rate density of short gamma-ray bursts using Swift data, accounting for biases, and discusses implications for binary neutron star merger rates relevant to gravitational wave detectors.

Contribution

It provides the first bias-corrected estimates of SGRB rate density and their implications for binary neutron star merger detection rates.

Findings

SGRB rate density lower limit: 8 Gpc^{-3} yr^{-1}

Beaming-corrected upper limit: 1100 Gpc^{-3} yr^{-1}

Detection rates for aLIGO/Virgo: 1-180 per year

Abstract

Short gamma-ray bursts (SGRBs) observed by {\it Swift} are potentially revealing the first insight into cataclysmic compact object mergers. To ultimately acquire a fundamental understanding of these events requires pan-spectral observations and knowledge of their spatial distribution to differentiate between proposed progenitor populations. Up to April 2012 there are only some 30% of SGRBs with reasonably firm redshifts, and this sample is highly biased by the limited sensitivity of {\it Swift} to detect SGRBs. We account for the dominant biases to calculate a realistic SGRB rate density out to $z\approx0.5$ using the {\it Swift} sample of peak fluxes, redshifts, and those SGRBs with a beaming angle constraint from X-ray/optical observations. We find an SGRB lower rate density of $8^{+5}_{-3}$ $\mathrm{Gpc}^{-3}\mathrm{yr}^{-1}$ (assuming isotropic emission), and a beaming corrected upper limit of $1100^{+700}_{-470} $ $\mathrm{Gpc}^{-3}\mathrm{yr}^{-1}$. Assuming a significant fraction of binary neutron star mergers produce SGRBs, we calculate lower and upper detection rate limits of $(1-180)$ yr$^{-1}$ by an aLIGO and Virgo coincidence search. Our detection rate is similar to the lower and realistic rates inferred from extrapolations using Galactic pulsar observations and population synthesis.