Radially polarized synchrotron from galaxy-cluster virial shocks

Radio-To-γ-ray signals, recently found narrowly confined near the characteristic 2.4R500 scaled radii of galaxy clusters and groups, have been associated with their virial (structure-formation accretion) shocks based on spectro-spatial characteristics. By stacking high-latitude Global Magneto-Ionic Medium Survey radio data around galaxy clusters from the Metacatalog of x-ray Detected Clusters of galaxies, we identify (3σ-4σ) excess radially polarized emission at the exact same scaled radius, providing directional support, and indicating tangential magnetic fields induced by the shocked inflow. The results suggest a strong mass dependence, a flat energy spectrum, and a high polarization fraction, consistent with synchrotron emission from electrons accelerated by strong virial shocks. The narrow radial range of such stacked virial-shock signals suggests that although the shocks are theorized to have diverse, irregular morphologies, they share similar ∼2.4R500 minimal radii.