2002ApJ...576..679K

We report evidence for the first time that merging clusters of galaxies are a promising candidate for the origin of high Galactic latitude, steady, unidentified EGRET gamma-ray sources. Instead of using past optical catalogs of eye-selected clusters, we made a matched filter survey of galaxy clusters over 4°x4° areas around seven steady unidentified EGRET sources at|b|>45°, together with a 100 deg² area near the south Galactic pole as a control field. In total, 154 Abell-like cluster candidates and 18 close pairs/groups of these clusters, expected to be possibly merging clusters, were identified within estimated redshift z_est≤0.15. Five among the seven EGRET sources have one or two cluster pairs/groups (CPGs) within 1° from them. We assess the statistical significance of this result by several methods, and the confidence level of the real excess is maximally 99.8% and 97.8% in a conservative method. In contrast, we found no significant correlation with single clusters. In addition to the spatial correlation, we also found that the richness of CPGs associated with EGRET sources is considerably larger than those of CPGs in the control field. These results imply that a part of the steady unidentified EGRET sources at high latitude are physically associated with close CPGs, not with single clusters. We also discuss possible interpretations of these results. We argue that, if these associations are real, they are difficult to explain by hadronic processes but best explained by the inverse-Compton scattering by high-energy electrons accelerated in shocks of cluster formation, as recently proposed.