Clusters and Structure Shocks in the Background: An Upper Bound from Non-Detection

Galaxy clusters should glow in gamma rays if cosmic rays fill their intracluster media, and the Fermi Large Area Telescope's non-detection of nearby clusters limits how bright that glow can be. Scaling that limit to the full cluster population, this paper places an upper bound on the cluster and structure-shock fraction of the isotropic background from 0.3 to 30 GeV. It finds max fraction at 1GeV = 0.03; max fraction at 10GeV = 0.0198. Clusters are, at most, a few-percent component of the background, so the background's bulk must be sought among compact source populations rather than diffuse structure. The study is simulated and computed, not measured.