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

TL;DR

This paper presents evidence of virial shocks in galaxy clusters through combined SZ and gamma-ray signals, providing new insights into shock properties and electron acceleration at cluster outskirts.

Contribution

It demonstrates the joint detection of SZ and gamma-ray signals from virial shocks in multiple galaxy clusters, revealing new details about shock properties and particle acceleration.

Findings

Virial shocks detected in Coma, A2319, and A2142 clusters.

Measured electron acceleration efficiency around 0.5%.

Imposed lower limits on shock Mach numbers.

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.