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 ∼15 − 50 per cent of transient progenitors reside in z = 1 − 2 galaxies too faint to be detected in surveys that reach AB ≃ 23, such as the Dark Energy Survey. This falls to ≲10 per cent at depths that will be probed by early data releases of LSST (AB ≃ 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 ∼75 per cent of 1015M⊙ strong lensing clusters, rising to ∼100 per cent of 1014M⊙ clusters. Deeper surveys, such as LSST, will miss ∼40 per cent at 1015M⊙, and ∼95 per cent at 1014M⊙. Our results motivate building a cluster watch-list for strongly-lensed transients that includes those found by lens-plane selection.