Coincident SZ and $\gamma$-ray signals from cluster virial shocks
2018
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 signals were reported only in Coma and in stacked Fermi-LAT clusters, and an SZ shock signal was reported only in A2319. We point out that a few clusters --- presently Coma, A2319, and A2142 --- already show sharp drops in Planck SZ pressure near the virial radius, coincident with a 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. Lower limits of order a few are imposed on the shock Mach numbers.
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