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

TL;DR

This study conducts an archival search for gravitationally lensed gamma-ray bursts by cross-matching Swift/XRT detections with galaxy clusters, identifying 17 candidates and estimating their magnifications, with implications for future observations.

Contribution

It provides the first systematic search for lensed GRBs using archival data and develops methods to estimate lensing magnifications and redshifts.

Findings

Identified 17 candidate lensed GRBs near galaxy clusters.

Most candidates show magnifications less than 10, with one possibly exceeding 10.

Produced a lens model suggesting GRB 050509B is magnified by a factor of 2-6.

Abstract

Discoveries of gamma-ray bursts (GRBs) have become commonplace in recent decades, totalling $\mathcal{O}(10^4)$ unique detections across various missions. However, there have been no confirmed discoveries of a gravitationally-lensed GRB, despite expected lensing rates of $\sim1$ in $10^{3}$. In light of this, we complete an archival search for lensed GRBs by cross-matching well-localised \emph{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 arcminute 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 $E_{p,i}$ and $E_{\rm iso}$ 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 $\mu < 10$ for all except for one candidate, GRB~071031, which is consistent with $\mu > 10$, 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, $z=0.225$. We produce a lens model of this cluster and show that GRB~050509B is consistent with $z>1$ and magnified by $\mu\simeq2-6$. 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.