Preliminary Evidence for a Virial Shock around the Coma Galaxy Cluster

Uri Keshet, Doron Kushnir, Abraham Loeb, Eli Waxman

Research output: Contribution to journalArticlepeer-review

12 Scopus citations


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 γ-ray, hard X-ray, and radio ring, elongated toward 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 γ-ray ring of minor axis around the Coma cluster, elongated toward the large-scale filament connecting Coma and Abell 1367, detected at the nominal confidence level ( using control signal simulations). The γ-ray ring correlates both with a synchrotron signal and with the SZ cutoff, but not with Galactic tracers. The γ-ray and radio signatures agree with analytic and numerical predictions if the shock deposits of the thermal energy in relativistic electrons over a Hubble time and in magnetic fields. The implied inverse Compton and synchrotron cumulative emission from similar shocks can contribute significantly to the diffuse extragalactic γ-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.

Original languageEnglish
Article number24
JournalAstrophysical Journal
Issue number1
StatePublished - 10 Aug 2017


  • galaxies: clusters: individual (Coma cluster)
  • gamma rays: galaxies: clusters
  • large-scale structure of universe
  • shock waves

ASJC Scopus subject areas

  • Astronomy and Astrophysics
  • Space and Planetary Science


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