Search for strongly lensed counterpart images of binary black hole mergers in the first two LIGO observing runs

Strong gravitational lensing can produce multiple images of the same gravitational-wave signal, each arriving at different times and with different magnification. Previous work has explored if lensed pairs exist among the known high-significance events from the LIGO and Virgo Collaboration’s GWTC-1 catalog and found no evidence of this. However, the possibility remains that weaker counterparts of these events are present in the data, unrecovered by previous searches. We conduct a targeted search specifically looking for subthreshold lensed images of known binary black hole (BBH) observations from GWTC-1. We recover candidates matching three of the additional events first reported by Venumadhav et al. [Phys. Rev. D 101, 083030 (2020)] but find no evidence for additional BBH events. We also find no evidence that any of the Venumadhav et al. observations are lensed counterparts. We demonstrate how this type of counterpart search can constrain hypotheses about the overall source and lens populations and we rule out at very high confidence the extreme hypothesis that all heavy BBH detections are in fact lensed systems at high redshift with intrinsic masses <15 M_⊙.