Outlook for detection of GW inspirals by GRB-triggered searches in the Advanced detector era

TL;DR

This paper evaluates the prospects of detecting gravitational waves from neutron star mergers associated with short gamma-ray bursts using triggered searches, showing a doubled detection volume and increased association rates in the advanced detector era.

Contribution

It provides analytic estimates, simulations, and formulas to quantify the improved sensitivity and detection prospects of GRB-triggered GW searches compared to blind searches.

Findings

Triggered searches have twice the effective volume of all-sky searches.

Association rates for NS-NS and NS-BH mergers with short GRBs could reach 3-4%.

Statistical excess tests are unlikely to find subthreshold populations before routine GW-GRB associations.

Abstract

Short, hard gamma-ray bursts (GRBs) are believed to originate from the coalescence of two neutron stars (NSs) or a NS and a black hole (BH). If this scenario is correct, then short GRBs will be accompanied by the emission of strong gravitational waves (GWs), detectable by GW observatories such as LIGO, Virgo, KAGRA, and LIGO-India. As compared with blind, all-sky, all-time GW searches, externally triggered searches for GW counterparts to short GRBs have the advantages of both significantly reduced detection threshold due to known time and sky location and enhanced GW amplitude because of face-on orientation. Based on the distribution of signal-to-noise ratios in candidate compact binary coalescence events in the most recent joint LIGO-Virgo data, our analytic estimates, and our Monte Carlo simulations, we find an effective sensitive volume for GRB-triggered searches that is about 2 times greater than for an all-sky, all-time search. For NS-NS systems, a jet angle of 20 degrees, a gamma-ray satellite field of view of 10% of the sky, and priors with generally precessing spin, this doubles the number of NS-NS short-GRB and NS-BH short-GRB associations, to ~3-4% of all detections of NS-NSs and NS-BHs. We also investigate the power of tests for statistical excesses in lists of subthreshold events, and show that these are unlikely to reveal a subthreshold population until finding GW associations to short GRBs is already routine. Finally, we provide useful formulas for calculating the prior distribution of GW amplitudes from a compact binary coalescence, for a given GW detector network and given sky location.