EGRET Upper Limits on the High-Energy Gamma-Ray Emission of Galaxy Clusters

We report EGRET upper limits on the high-energy gamma-ray emission from clusters of galaxies. EGRET observations between 1991 and 2000 were analyzed at positions of 58 individual clusters from a flux-limited sample of nearby X-ray-bright galaxy clusters. Subsequently, a co-added image from individual galaxy clusters has been analyzed using an adequately adapted diffuse gamma-ray foreground model. The resulting upper 2 σ limit for the average cluster is ~6 × 10-9 cm-2 s-1 for E > 100 MeV. Implications of the nondetection of prominent individual clusters and of the general inability to detect the X-ray-brightest galaxy clusters as a class of gamma-ray emitters are discussed. We compare our results with model predictions on the high-energy gamma-ray emission from galaxy clusters as well as with recent claims of an association between unidentified or unresolved gamma-ray sources and Abell clusters of galaxies and find these contradictory.