On building a cluster watchlist for identifying strongly lensed supernovae, gravitational waves and kilonovae

Abstract

ABSTRACT Motivated by discovering strongly lensed supernovae, gravitational waves, and kilonovae in the 2020s, we investigate whether to build a watchlist 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). Secondly, 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 1015 M⊙ strong-lensing clusters, rising to ∼100 per cent of 1014 M⊙ clusters. Deeper surveys, such as LSST, will miss ∼40 per cent at 1015 M⊙ and ∼95 per cent at 1014 M⊙. Our results motivate building a cluster watchlist for strongly lensed transients that includes those found by the lens-plane selection.