Galaxy-cluster-stacked Fermi-LAT II: extended central hadronic signal

Abstract

Faint $\gamma$-ray signatures emerge in Fermi-LAT data stacked scaled to the characteristic ${\theta }_{500}$ 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 ($4.7\sigma$ for 75 clusters) and extended central excess, inconsistent with central point sources. The resolved signal is best fit ($3.7\sigma$ TS-test) as hadronic emission from a cosmic-ray ion (CRI) distribution that is flat both spectrally ($p\equiv 1-d\ln u/d\ln E=2.0\pm 0.3$) and spatially (CRI-to-gas index $\sigma =0.1\pm 0.4$), carrying an energy density $du(0.1{\theta }_{500})/d\ln E=10^{-13.6\pm 0.5}$ erg cm${}^{-3}$ at $E=100$ GeV energy; insufficient resolution would raise $p$ and $\sigma$. 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 $\gamma$-ray emission, e.g. from dark-matter annihilation, are imposed.

15 pages, 10 figures

References in corpus (13)

• Diffuse Radio Emission from Galaxy Clusters
• Statistics of X-ray observables for the cooling-core and non-cooling core galaxy clusters
• Do radio core-halos and cold fronts in non major merging clusters originate from the same gas sloshing?
• A statistically-selected Chandra sample of 20 galaxy clusters -- II. Gas properties and cool-core/non-cool core bimodality
• Fermi Large Area Telescope Performance After 10 Years Of Operation
• Fermi Large Area Telescope Fourth Source Catalog Data Release 4 (4FGL-DR4)
• New Limits On Gamma-Ray Emission From Galaxy Clusters
• Detailed study of extended gamma-ray morphology in the vicinity of the Coma cluster with Fermi-LAT
• Common origin for radio relics and halos: galaxy cluster-wide, homogeneous cosmic-ray distribution, and evolving magnetic fields
• Radio halos and relics from extended cosmic-ray ion distributions with strong diffusion in galaxy clusters
• Synchrotron emission from virial shocks around stacked OVRO-LWA galaxy clusters
• Excess cataloged X-ray and radio sources at galaxy-cluster virial shocks
• A Stacked Analysis of GeV Gamma-Ray Emission from SPT-SZ Galaxy Clusters with 16 Years of Fermi-LAT Data

Cited by in corpus (2)

• A MINOT-based Study of Gamma-ray emission from SPT-CL J2012-5649/Abell 3667
• Relativistic ions with power-law spectra explain radio phoenixes