Excess cataloged X-ray and radio sources at galaxy-cluster virial shocks

We detect a highly significant excess of X-ray (2RXS) and radio (NVSS, GMRT, VLSSr) catalog sources when stacked around MCXC galaxy clusters and groups, narrowly confined within ≲ 100 kpc of the ∼ 2.4 R500 virial shock radius (inferred from previous continuum stacking), with similar X-ray (∼ 4σ for 443 clusters) and radio (∼ 4σ for 485 clusters) characteristics (> 5σ joint). The excess sources show 10–100 kpc scales, LX(0.1 – 2.4 keV)≃ 1042-43 erg s-1 or νLν(ν = 1.4 GHz) ≃ 1040-41 erg s-1 luminosities, and a preferentially radial radio-polarization. The narrow localization and properties of the excess identify these sources not as AGN, often invoked speculatively for excess X-ray sources at cluster outskirts, but rather as infalling gaseous clumps interacting with the virial shock, probably galactic halos and possibly outflow remnants. The local excess of such discrete, radio-to-γ-ray sources around an object can probe its virial shock also at high redshifts and sub-cluster scales.