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\title{Detection of virial shocks in stacked Fermi-LAT clusters
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\ShortTitle{}

\author{\speaker{Ido Reiss}\\
        Ben Gurion University
 \\
        E-mail: \email{reissi@post.bgu.ac.il
}}

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\abstract{
  In the hierarchical paradigm of structure formation, galaxy clusters are the largest objects ever to virialize.
They are thought to grow by accreting mass through large scale, strong virial shocks.
Such a collisionless shock is expected to accelerate relativistic electrons, thus generating a spectrally flat leptonic virial ring.
However attempts to  detect virial rings have all failed, leaving the shock paradigm unconfirmed.
Here we identify a virial $\gamma$-ray signal by stacking Fermi-LAT data for 112 clusters, enhancing the ring sensitivity by rescaling clusters to their virial radii and utilizing the anticipated spectrum.
In addition to a central unresolved, hard signal (detected at the nominal $5.8\sigma$ confidence level), probably dominated by active galactic nuclei, we identify ($5.9\sigma$) a bright, spectrally flat $\gamma$-ray ring at the expected shock position.
It corresponds to $\sim 0.6\%$ (with an uncertainty factor $\sim2$) thermal energy deposition in relativistic electrons over a Hubble time.
This result validates the shock paradigm, calibrates its parameters, and indicates that the cumulative emission from such shocks significantly contributes to the diffuse extragalactic $\gamma$-ray and radio backgrounds.
}

\FullConference{7th Fermi Symposium 2017\\
		15-20 October 2017\\
		Garmisch-Partenkirchen, Germany}


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