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 $\sim 15-50$ per cent of transient progenitors reside in $z=1-2$ galaxies too faint to be detected in surveys that reach $\mathrm{AB}\simeq 23$, such as the Dark Energy Survey. This falls to $\ls10$ per cent at depths that will be probed by early data releases of LSST ($\mathrm{AB}\simeq 25$). 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 $\sim 75$ per cent of $10^{15}$M${}_{\odot }$ strong lensing clusters, rising to $\sim 100$ per cent of $10^{14}$M${}_{\odot }$ clusters. Deeper surveys, such as LSST, will miss $\sim 40$ per cent at $10^{15}$M${}_{\odot }$, and $\sim 95$ 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.

7 pages, 2 figures. MNRAS, accepted