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

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 $5\times10^{-17}{\rm erg\,s^{-1}\,cm^{-2}}$.