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

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.

Figures (6)

• Figure 1: Optical afterglows of GRBs in our dataset. The AB mag in the R-band is plotted on the vertical axis, while time since GRB trigger is on the horizontal axis. Each GRB is represented by a distinct color and marker.
• Figure 2: BNS O5 skymaps: simulated skymaps showing the 50% and 90% credible regions obtained using Bilby for the BNS progenitor scenario with GW detectors (LIGO Hanford, LIGO Livingston, Virgo, and KAGRA) operating at O5 sensitivity. The location of the injected source is marked by a blue star. Bottom right: Plot showing enclosed probability percentage for a given area for each of the events.
• Figure 3: NSBH O5 skymaps: simulated skymaps showing the 50% and 90% credible regions obtained using Bilby for the NSBH progenitor scenario with GW detectors (LIGO Hanford, LIGO Livingston, Virgo, and KAGRA) operating at O5 sensitivity. The location of the injected source is marked by a blue star. Bottom right: Plot showing enclosed probability percentage for a given area for each of the events.
• Figure 4: BNS Detectability: The afterglow lightcurves of GRBs with double-arrow (rook-shaped) markers representing the time of source-tile observation and the limiting magnitude of telescopes for shallow (deep) searches in an O5 scenario with BNS progenitor. The markers are color-coded to represent different telescopes, as shown in the plot legend. The shaded blue regions show the uncertainty associated with power-law extrapolation of the afterglow lightcurves.
• Figure 5: NSBH Detectability: The afterglow lightcurves of GRBs with double-arrow (rook-shaped) markers representing the time of source-tile observation and the limiting magnitude of telescopes for shallow (deep) searches in an O5 scenario with NSBH progenitor. The markers are color-coded to represent different telescopes, as shown in the plot legend. The shaded blue regions show the uncertainty associated with power-law extrapolation of the afterglow lightcurves.