An archival search for gamma-ray bursts gravitationally lensed by galaxy clusters

Discoveries of gamma-ray bursts (GRBs) have become commonplace in recent decades, totalling (Formula presented) unique detections across various missions. However, there have been no confirmed discoveries of a gravitationally lensed GRB, despite expected lensing rates of (Formula presented) in (Formula presented). In light of this, we complete an archival search for lensed GRBs by cross-matching well-localized Swift/XRT-detected bursts with a large all-sky sample of galaxy clusters as potential lenses. We find a total of 17 candidate lensed GRBs defined by a 2 arcmin search radius from a cluster in our sample. 14 of our candidates are either confirmed to be at higher redshifts than their cross-matched cluster, or are consistent with a higher redshift origin based on the Amati relation between (Formula presented) and (Formula presented) of GRBs, indicating they are, at some level, lensed by their nearby cluster. Using the Amati relation and the lens-GRB separation, we quantify the magnification experienced by each GRB. We find (Formula presented) for all except for one candidate, GRB 071031, which is consistent with (Formula presented), but is uncertain. Another candidate, GRB 050509B, does not have a directly measured redshift, but was previously assumed to be at the redshift of its nearby cluster, (Formula presented). We produce a lens model of this cluster and show that GRB 050509B is consistent with (Formula presented) and magnified by (Formula presented). We present these findings in anticipation of future lensed GRB discoveries enabled by facilities such as the Vera C. Rubin Observatory in the coming years.