Missed opportunities: GRB 211211A and the case for continual gravitational-wave coverage with a single observatory

TL;DR

This paper argues that continuous gravitational-wave monitoring with a single observatory can significantly enhance multimessenger detection of neutron star mergers associated with gamma-ray bursts, especially when detectors are not operational.

Contribution

It demonstrates that a single gravitational-wave observatory with sufficient sensitivity can enable confident multimessenger detections, reducing reliance on multiple detectors.

Findings

Approximately 11% of gamma-ray bursts within 600 Mpc will have confident GW associations at O4 sensitivity.

The coincident detection rate is estimated at 0.22 per year with O4, increasing to 0.71 per year with O5 sensitivity.

Single detector operation can be nearly as effective as multiple detectors for confident multimessenger observations.

Abstract

Gamma-ray burst GRB 211211A may have been the result of a neutron star merger at $\approx350$ Mpc. However, none of the LIGO-Virgo detectors were operating at the time. We show that the gravitational-wave signal from a \grb-like binary neutron star inspiral in the next LIGO-Virgo-KAGRA observing run (O4) would be below the conventional detection threshold, however a coincident gamma-ray burst observation would provide necessary information to claim a statistically-significant multimessenger observation. We calculate that with O4 sensitivity, approximately $11\%$ of gamma-ray bursts within 600 Mpc will produce a confident association between the gravitational-wave binary neutron star inspiral signature and the prompt gamma-ray signature. This corresponds to a coincident detection rate of $\unit[0.22^{+8.3}_{-0.22}]{yr^{-1}}$, where the uncertainties are the 90\% confidence intervals arising from uncertainties in the absolute merger rate, beaming and jet-launching fractions. These increase to approximately $34\%$ and $\unit[0.71^{+26.8}_{-0.70}]{yr^{-1}}$ with proposed O5 sensitivity. We show that the above numbers do not depend significantly on the number of gravitational-wave observatories operating with the specific sensitivity. That is, the number of confident joint gamma-ray burst and gravitational-wave detections is only marginally improved with two or three detectors operating compared to a single detector. It is therefore worth considering whether one detector with sufficient sensitivity (post O4) should remain in sky-watch mode at all times to elucidate the true nature of GRB 211211A-like events, a proposal we discuss in detail.