Coincident SZ and $\gamma$-ray signals from cluster virial shocks

Abstract

Virial shocks around galaxy clusters are expected to show a cutoff in the thermal Sunyaev-Zel'dovich (SZ) signal, coincident with a leptonic ring. However, until now, leptonic virial signals were reported only in Coma and in stacked Fermi-LAT clusters, and an SZ virial shock signal was reported only in A2319. We point out that a few clusters --- presently Coma, A2319, and A2142 --- already show a sharp drop in Planck SZ pressure near the virial radius, coincident with a weak LAT $\gamma$-ray excess. These signatures are shown to trace the virial shocks of the clusters, at joint medium to high confidence levels. The electron acceleration rates inferred from $\gamma$-rays are consistent with previous measurements. The combined signal allows a separate measurement of the $\sim0.5\%$ acceleration efficiency and of the accretion rate. Lower limits of order a few are imposed on the shock Mach numbers.