Coincident Sunyaev–Zel’dovich and Gamma-Ray Signals from Cluster Virial Shocks

Abstract

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 have only been reported in Coma and in stacked Fermi Large Area Telescope (LAT) clusters, and an SZ virial shock signal reported only in A2319. We point out that a few clusters—presently Coma, A2319, and A2142—already show a significant ( ) sharp drop in the Planck y parameter near the virial radius, coincident with a (2.2σ–3.9σ) LAT γ-ray excess. These signatures are naturally interpreted as tracers of the virial shocks of these clusters, at joint medium to high confidence levels. The electron acceleration rates inferred from the γ-rays are consistent with previous measurements. The combined signal, along with galaxy count data, allows a separate measurement of the ∼0.5% (with a factor of ∼2 uncertainty) acceleration efficiency and of the accretion rate. Lower limits on order of a few are imposed on the shock Mach numbers.