A follow-up strategy enabling discovery of electromagnetic counterparts to highly magnified gravitationally lensed gravitational waves

Making an unambiguous detection of lensed gravitational waves (GWs) is challenging with current-generation detectors due to large uncertainties in sky localizations and other inferred parameter distributions. However, in the case of binary neutron star (BNS) mergers, this challenge can be overcome by detecting multiple images of its lensed kilonova (KN) counterpart, simultaneously confirming the lensing nature of the event and locating it precisely—further enabling a wealth of lensed multi-messenger science. Such a strategy demands answers to two key problems: (i) How can candidate lensed BNS events be identified fast enough to ensure the lensed KN is still detectable? (ii) What is the most economical observing strategy on telescope time for following up candidate lensed events to discover lensed KNe? In this article we discuss solutions to both points, specifically how GW detections of progenitors in the approximately 2.5–5 M⊙ black hole ‘mass gap’ can be interpreted as candidate lensed BNS events, giving evidence for lensing from just a single detection, and we present a strategy that can actively be employed for follow-up of such events in the O4 run of LIGO-Virgo-KAGRA (LVK) and beyond.