Galaxy-cluster-stacked Fermi-LAT. Part II. Extended central hadronic signal

Abstract Faint γ -ray signatures emerge in Fermi -LAT data stacked scaled to the characteristic θ 500 angles of MCXC galaxy clusters. After Paper I of this series thus discovered virial shocks, later supported in other bands, this second paper focuses on cluster cores. Stacking 1–100 GeV source-masked data around clusters shows a significant (4.7 σ for 75 clusters) and extended central excess, inconsistent with central point sources. The resolved signal is best fit (3.7 σ TS-test) as hadronic emission from a cosmic-ray ion (CRI) distribution that is flat both spectrally ( p ≡ 1 - d ln u / d ln E = 2.0 ± 0.3) and spatially (CRI-to-gas index σ = 0.1 ± 0.4), carrying an energy density du (0.1θ 500 )/ d ln E = 10 -13.6±0.5 erg cm -3 at E = 100 GeV energy; insufficient resolution would appear to raise p and σ . Such CRI match the long-predicted distribution needed to power diffuse intracluster radio emission in its various forms (mini-halos, giant halos, standard relics, their transitional forms, and mega-halos), disfavoring models that invoke electron (re)acceleration in weak shocks or turbulence. Stringent upper limits on residual γ -ray emission, e.g. from dark-matter annihilation, are imposed.