Uri Keshet, Doron Kushnir, Abraham Loeb, Eli Waxman
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 so far, and the very existence of cluster virial shocks has remained a theory. We report the discovery of a large, ~5 Mpc diameter gamma-ray ring around the Coma cluster, elongated towards the large scale filament connecting Coma and Abell 1367. The gamma-ray ring correlates both with a synchrotron signal and with the SZ cutoff. The gamma-ray, hard-X-ray, and radio signatures agree with analytic and numerical predictions, if the shock deposits a few percent of the thermal energy in relativistic electrons over a Hubble time, and ~1% of the energy in magnetic fields. The implied inverse-Compton and synchrotron cumulative emission from similar shocks dominates the diffuse extragalactic gamma-ray and low frequency radio backgrounds. Our results 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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http://arxiv.org/abs/1210.1574
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