On Suggestive Correlations Between GRBs and Clusters of Galaxies

Abstract

Recent claims of angular correlations between gamma-ray bursts (GRBs) and clusters of galaxies are evaluated in light of existing but previously uncorrelated GRB positional data. Additional GRB data sets we use include sub-samples of soft BATSE 3B bursts, bursts located by the Interplanetary Network (IPN), and GRBs localized by COMPTEL. We confirm a previously reported excess by Rood and Struble (1996) of the 185 rich, nearby clusters of galaxies (Abell, Corwin, and Olowin 1989, ACO) in the 1-$σ$ error circles of 74 BATSE 3B positions, but find a typical correlation strength of only $~$2.5-$σ$ for typical sub-samples. However, none of the 185 ACO clusters occur in the 1-$σ$ error boxes of 40 IPN GRBs or 18 COMPTEL GRBs. When all ACO clusters are correlated with BATSE 3B GRBs however, we find an increasingly strong correlation for GRBs with decreasingly small error boxes, reaching above the 3.5-$σ$ level. We also find a slight excess of {\it soft} BATSE GRBs near the positions of 185 rich, nearby ACO clusters, but the significance of the correlation averages only $~$2.5-$σ$ for sub-samples delineated by softness. We caution that the statistical significance of all these correlations is marginal, and so conclude that the excess is at best only suggestive of a physical association. Statistical fluke is still a strong possibility. BATSE could confirm or refute such correlations in a 10-year lifetime.