Clusters of galaxies in high-energy γ-ray astronomy

Several reasons suggest clusters of galaxies as potential sources of energetic γ radiation. Present models include γ-rays produced hadronically via p-p-interactions of Cosmic rays with the ICM, originating from a secondary population of relativistic electrons or generated as a result of large scale cosmological structure formation. Quantitative estimates, however, remain uncertain by orders of magnitude, but are generally close or below the detection threshold of the EGRET telescope. Only recently, correlation studies have been performed between Abell clusters and unidentified or unresolved EGRET sources. We have reassessed the situation by studying a flux-limited sample of nearby X-ray bright galaxy clusters in the EGRET data between 1991 and 2000. We are only able to report upper limits for 58 individual clusters, also their cumulative overlay yielded only an upper limit of ∼6×10^-9 cm^-2 s^-1 for E>100 MeV. We compare our results with model predictions on the high-energy γ-ray emission from galaxy clusters and contrast it with recently proposed associations between γ-ray sources and galaxy clusters.