Search for cosmic-ray induced γ-ray emission in Galaxy Clusters The Fermi-LAT Collaboration :

Current theories predict relativistic hadronic particle populations in clusters of galaxies in addition to the already observed relativistic leptons. In these scenarios hadronic interactions give rise to neutral pions which decay into γ rays that are potentially observable with the Large Area Telescope (LAT) on board the Fermi space telescope. We present a joint likelihood analysis searching for spatially extended γ-ray emission at the locations of 50 galaxy clusters in 4 years of Fermi-LAT data under the assumption of the universal cosmic-ray model proposed by Pinzke & Pfrommer (2010). We find an excess at a significance of 2.7σ which upon closer inspection is however correlated to individual excess emission towards three galaxy clusters: Abell 400, Abell 1367 and Abell 3112. We discuss these cases in detail and conservatively attribute the emission to unmodeled background (for example, radio galaxies within the clusters). Through the combined analysis of 50 clusters we exclude hadronic injection efficiencies in simple hadronic models above 25% and establish limits on the cosmic-ray to thermal pressure ratio within the virial radius, R200, to be below 0.012-0.014 depending on the morphological classification. In addition we derive new limits on the γ-ray flux from individual clusters in our sample. Subject headings: galaxy clusters: general – gamma rays, cosmic rays, joint likelihood analysis