Detection of virial shocks in stacked Fermi-LAT clusters

Galaxy clusters are thought to grow by accreting mass through large-scale, strong, yet elusive, virial shocks. Such a shock is expected to accelerate relativistic electrons, thus generating a spectrally-flat leptonic virial-ring. However, until now, only the nearby Coma cluster has shown evidence for a $\gamma$-ray virial ring. We stack Fermi-LAT data for the 112 most massive, high latitude, extended clusters, enhancing the ring sensitivity by rescaling clusters to their virial radii and utilizing the expected flat energy spectrum. In addition to a central unresolved, hard signal (detected at the $\sim 5.8\sigma$ confidence level), probably dominated by AGN, we identify (at the $5.8\sigma$ confidence level) a bright, spectrally-flat $\gamma$-ray ring at the expected virial shock position. The ring signal implies that the shock deposits $\sim 0.6\%$ (with an interpretation uncertainty factor $\sim2$) of the thermal energy in relativistic electrons over a Hubble time. This result, consistent with the Coma signal, validates and calibrates the virial shock model, and indicates that the cumulative emission from such shocks significantly contributes to the diffuse extragalactic $\gamma$-ray and low-frequency radio backgrounds.