Uncovering Faint Lensed Gravitational-Wave Signals and Reprioritizing Their Follow-Up Analysis Using Galaxy Lensing Forecasts with Detected Counterparts

Like light, gravitational waves can be gravitationally lensed by massiveastrophysical objects. For galaxy and galaxy-cluster lenses, one expects to seestrong lensing – forecasted to become observable in the coming years – wherethe original wave is split into multiple copies with the same frequencyevolution but different overall arrival times, phases, amplitudes, and signalstrengths. Some of these images can be below the detection threshold andrequire targeted search methods, based on tailor-made template banks. Thesesearches can be made more sensitive by using our knowledge of the typicaldistribution and morphology of lenses to predict the time delay, magnification,and image-type ordering of the lensed images. Here, we show that when a subsetof the images is super-threshold, they can be used to construct a moreconstrained prediction of the arrival time of the remaining signals, enhancingour ability to identify lensing candidate signals. Our suggested methodeffectively reduces the list of triggers requiring follow-up and generallyre-ranks the genuine counterpart higher in the lensing candidate list.Therefore, in the future, if one observes two or three lensed images, theinformation they provide can be used to identify their sub-thresholdcounterparts, thus allowing identification of additional lensed images. Findingsuch images would also strengthen our evidence for the event being lensed.