Deep and rapid observations of strong-lensing galaxy clusters within the sky localisation of GW170814

TL;DR

This study conducted rapid, deep optical observations of galaxy clusters within the GW170814 localization area to search for electromagnetic counterparts, demonstrating the feasibility of such targeted searches and setting new sensitivity limits.

Contribution

First to perform a rapid, sensitive search for EM counterparts in galaxy clusters within GW localization, establishing detection limits and constraining lensing scenarios for GW170814.

Findings

No EM counterparts detected in the observed clusters.

Set the most sensitive limits to date for transient detection in GW counterpart searches.

Ruled out lensing of GW170814 by these clusters with kilonova-like EM signals.

Abstract

We present observations of two strong-lensing galaxy clusters located within the $90$ per cent credible sky localization maps released following LIGO-Virgo's discovery of the binary black hole (BH-BH) gravitational wave (GW) source GW170814. Our objectives were (1) to search for candidate electromagnetic (EM) counterparts to GW170814 under the hypothesis that it was strongly-lensed, and thus more distant and less massive than inferred by LIGO-Virgo, and (2) to demonstrate the feasibility of rapid target of opportunity observations to search for faint lensed transient point sources in crowded cluster cores located within GW sky localizations. Commencing $20$ hours after discovery, and continuing over $12$ nights, we observed Abell 3084 ($z=0.22$) and SMACSJ0304.3$-$4401 ($z=0.46$) with GMOS on the Gemini-South telescope, and Abell 3084 with MUSE on ESO's Very Large Telescope. We detect no candidate EM counterparts in these data. Calibration of our photometric analysis methods using simulations yield $5\sigma$ detection limits for transients in difference images of the cores of these clusters of $i=25$. This is the most sensitive photometric search to date for counterparts to GW sources, and rules out the possibility that GW170814 was lensed by these clusters with a kilonova-like EM counterpart. Based on the detector frame masses of the compact objects, and assuming that at least one Neutron Star (NS) is required in the merging system to produce a kilonova-like counterpart, implies that GW170814 was neither a NS-NS nor NS-BH merger at $z>8$ lensed by either of these clusters. Also, in the first ever emission line search for counterparts to GW sources, we detected no lines down to a $5\sigma$ detection limit of $5\times10^{-17}{\rm erg\,s^{-1}\,cm^{-2}}$.