On building a cluster watch-list for identifying strongly lensed supernovae, gravitational waves and kilonovae

Abstract

Motivated by discovering strongly-lensed supernovae, gravitational waves, and kilonovae in the 2020s, we investigate whether to build a watch-list of clusters based on observed cluster properties (i.e. lens-plane selection) or on the detectability of strongly-lensed background galaxies (i.e. source-plane selection). First, we estimate the fraction of high-redshift transient progenitors that reside in galaxies that are themselves too faint to be detected as being strongly-lensed. We find \(\sim15-50\) per cent of transient progenitors reside in \(z = 1-2\) galaxies too faint to be detected in surveys that reach \({\rm AB}\simeq23\), such as the Dark Energy Survey. This falls to \(\ls10\) per cent at depths that will be probed by early data releases of LSST (\({\rm AB}\simeq25\)). Second, we estimate a conservative lower limit on the fraction of strong lensing clusters that will be missed by magnitude limited searches for multiply-imaged galaxies and giant arcs due to the faintness of such images. We find that DES-like surveys will miss \(\sim75\) per cent of \(10^{15}\)M\(_\odot\) strong lensing clusters, rising to \(\sim100\) per cent of \(10^{14}\)M\(_\odot\) clusters. Deeper surveys, such as LSST, will miss \(\sim40\) per cent at \(10^{15}\)M\(_\odot\), and \(\sim95\) per cent at \(10^{14}\)M\(_\odot\). Our results motivate building a cluster watch-list for strongly-lensed transients that includes those found by lens-plane selection.