Search for γ -Ray Emission from Cluster of Galaxies with Fermi-LAT Data

Abstract In this work, we carry out a systematic search for γ -ray emission from 65 galaxy clusters based on 16 yr of Fermi Large Area Telescope data in the energy range from 500 MeV to 500 GeV. Only two clusters, A2065 and A2244, exhibit significant emission, with local significances of ∼5 σ . The test statistic (TS) values are ∼25.4 for A2065 and ∼25.1 for A2244. We investigate a potential dark matter (DM) origin for these signals. The peak TS value of A2065 is ∼26.2 for the DM annihilation channel χ χ → b b ¯ at a DM mass of m χ ∼ 25.0 GeV. For A2244, the largest TS value is ∼14.5 for χ χ → b b ¯ at m χ ∼ 6.0 GeV. However, the corresponding cross sections conflict with the upper limits derived from observations of dwarf spheroidal galaxies. This indicates that these signals cannot be attributed to DM annihilation. Finally, we consider the γ -ray flux arising from hadronic cosmic-ray (CR) interactions in the intracluster medium, with the corresponding estimated volume-averaged CR-to-thermal pressure ratios being ∼4% and ∼16%, respectively.