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 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 ν L_ν(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.