Preliminary evidence for a virial shock around the Coma galaxy cluster

Abstract

Galaxy clusters, the largest gravitationally bound objects in the Universe, are thought to grow by accreting mass from their surroundings through large-scale virial shocks. Due to electron acceleration in such a shock, it should appear as a $\gamma$-ray, hard X-ray, and radio ring, elongated towards the large-scale filaments feeding the cluster, coincident with a cutoff in the thermal Sunyaev-Zel'dovich (SZ) signal. However, no such signature was found until now, and the very existence of cluster virial shocks has remained a theory. We find preliminary evidence for a large, $\sim 5$ Mpc minor axis $\gamma$-ray ring around the Coma cluster, elongated towards the large scale filament connecting Coma and Abell 1367, detected at the nominal $2.7\sigma$ confidence level ($5.1\sigma$ using control signal simulations). The $\gamma$-ray ring correlates both with a synchrotron signal and with the SZ cutoff, but not with Galactic tracers. The $\gamma$-ray and radio signatures agree with analytic and numerical predictions, if the shock deposits $\sim 1\%$ of the thermal energy in relativistic electrons over a Hubble time, and $\sim 1\%$ in magnetic fields. The implied inverse-Compton and synchrotron cumulative emission from similar shocks can significantly contribute to the diffuse extragalactic $\gamma$-ray and low frequency radio backgrounds. Our results, if confirmed, reveal the prolate structure of the hot gas in Coma, the feeding pattern of the cluster, and properties of the surrounding large scale voids and filaments. The anticipated detection of such shocks around other clusters would provide a powerful new cosmological probe.

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