Insights from GRBs for optical follow-up of gravitational wave counterparts

TL;DR

This study assesses how well gravitational wave localizations can identify optical counterparts of nearby GRBs during O5, emphasizing the importance of observational depth for detecting faint transients within localized regions.

Contribution

It demonstrates that GW localizations during O5 can be sufficiently precise to find optical counterparts, highlighting the critical role of deep optical observations.

Findings

GW localizations can be a few to tens of square degrees.

Optical counterparts are detectable if telescopes reach beyond 23 mag.

Depth of observations is more critical than localization accuracy.

Abstract

Identifying the electromagnetic counterparts to gravitational wave sources is vital to enabling the myriad of investigations possible with multimessenger astronomy. However, locating faint, fast-varying transients within large localisations remains challenging given the uncertainty in their detailed properties. In this work, we investigate how the nearby merger-induced GRBs would be localised by the LIGO-Virgo-KAGRA detector network during the fifth gravitational wave observing run (O5) and assess whether their optical counterparts could be detected using gravitational wave localisations alone, without additional localisation from gamma-ray instruments. Counterpart detectability is evaluated using the observed optical afterglow lightcurves of these GRBs and the distance-scaled lightcurve of the kilonova AT2017gfo as a fiducial template. We find that such events can be localised to comparatively small regions of the sky, often only a few to tens of square degrees. As a result, counterparts are detectable by at least one of the available optical telescopes during O5. However, detectability depends strongly on observational depth, as the counterparts are fainter than $22$ mag within a day. Facilities capable of reaching depths of $\gtrsim23$ mag therefore play a key role in recovering these faint counterparts. These results indicate that for such events during O5, the primary challenge for multimessenger discovery will be in achieving sufficient observational depth and reliably identifying the true counterpart among unrelated transients rather than gravitational wave localisation itself.