Connecting GRBs and galaxies: the probability of chance coincidence

TL;DR

This study assesses the likelihood of chance alignments between GRB afterglows and galaxies, finding that such coincidences are rare but significant, especially for X-ray localized GRBs, impacting host galaxy identification accuracy.

Contribution

It provides a statistical analysis of chance superpositions in GRB host galaxy identification, highlighting the importance of considering these probabilities in astrophysical studies.

Findings

Approximately 1% chance of optical afterglow and galaxy coincidence.

Half of the X-ray superpositions are likely chance alignments.

Statistical studies of X-ray localized GRBs may be unreliable.

Abstract

Studies of GRB host galaxies are crucial to understanding GRBs. However, since they are identified by the superposition in the plane of the sky of a GRB afterglow and a galaxy there is always a possibility that an association represents a chance alignment, rather than a physical connection. We examine a uniform sample of 72 GRB fields to explore the probability of chance superpositions. There is typically a ~1% chance that an optical afterglow will coincide with a galaxy by chance. While spurious host galaxy detections will, therefore, be rare, the possibility must be considered when examining individual GRB/host galaxy examples. It is also tempting to use the large and uniform collection of X-ray afterglow positions to search for GRB-associated galaxies. However, we find that approximately half of the 14 superpositions in our sample are likely to occur by chance, so in the case of GRBs localized only by an X-ray afterglow, even statistical studies are suspect.