Discovering gravitationally lensed gravitational waves: predicted rates,
  candidate selection, and localization with the Vera Rubin Observatory
  The first secure detection of a gravitationally lensed gravitational (GW)
will be a watershed moment, as it will bring together these two pillars of
General Relativity for the first time. Accurate selection and interpretation of
candidate lensed GWs is challenging for numerous reasons, including large sky
localization uncertainties for most GW detections, the broad range of
gravitational lenses spanning galaxy/group/cluster-scales in the dark matter
halo mass function, and uncertainty in the intrinsic mass function of compact
object remnants of stellar evolution. We introduce a new magnification-based
approach to predicting the rates of lensed GWs that is agnostic to the mass and
structure of the lenses and combine it with expressions for arrival time
difference for representative lenses and their catastrophes to delineate the
range of expected arrival time differences. We also predict the lightcurves of
lensed kilonova counterparts to lensed binary neutron star (NS-NS) mergers and
assess the feasibility of detection with the Vera Rubin Observatory. Our main
conclusions are: (1) selection of candidate lensed NS-NS mergers from the mass
gap between NS and black holes in low latency is an efficient approach with a
rate approaching one per year in the mid-2020s, (2) the arrival time
differences of lensed NS-NS/kilonovae are typically $\lesssim1\,$year, and thus
well-matched to the operations of GW detectors and optical telescopes, and (3)
detection of lensed kilonovae is feasible with the Vera Rubin Observatory.
Whilst our predictions are motivated by lensing, they provide a physically
well-understood approach to exploring the mass gap electromagnetically as the
number of detections in this exciting region of parameter space grows.
