Detection of virial shocks in Fermi-LAT galaxy clusters
Galaxy clusters are thought to grow by accreting mass through large scale, strong yet elusive, virial shocks. These collisionless shocks are thought to accelerate relativistic electrons, generating a spectrally-flat leptonic virial ring. However, with the exception of a VERITAS signal from the Coma cluster, attempts to detect virial rings have all failed. By stacking and rescaling Fermi-LAT data at 1-100GeV for the 112 most massive, high latitude, extended clusters we identify (at the 5.9 sigma confidence level) a bright, spectrally flat gamma-ray ring at the expected radius. This indicates that ~0.6% (with an uncertainty factor ~2) of the thermal energy is deposited in relativistic electrons over a Hubble time. This detection is confirmed using two multi messenger analyses of individual clusters: (1) A combined VERITAS (~220GeV), LAT and ROSAT R1 (~0.2keV) elliptical signal in Coma. (2) Planck SZ pressure near the virial radius, coincident with a LAT gamma-ray excess in Coma, A2319, and A2142. This results validate the shock paradigm, calibrate its parameters, and indicate that such shocks significantly contribute to the diffuse extragalactic gamma-ray and radio backgrounds.