First gamma-rays from galaxy clusters. Preliminary evidence of the association of galaxy clusters with EGRET unidentified gamma-ray sources

The vast majority of the celestial gamma-ray sources detected so far have not yet been identified with secure counterparts at other wavelenghts. Here we report preliminary evidence for an association between galaxy clusters and unidentified gamma-ray sources of high galactic latitude ($|b|>20$ $\deg$) in the Third EGRET catalog. All of the clusters which are most probably associated with EGRET sources show evidence of strong radio emission either because they host radio galaxies/sources in their environments or because they have a radio halo or relic inhabiting their intracluster medium. The cluster radio emission suggests that the relativistic particles (electrons, protons), which are diffusing in the intracluster medium, might be also responsible for their gamma-ray emission. Beyond the spatial associations of clusters with unidentified EGRET sources, we found a correlation between the radio flux at 1.4 GHz of the cluster's brightest source and the gamma-ray flux, $F({>}100 ~{\rm MeV})$, of the associated EGRET source. For the most probable EGRET-cluster associations we also found a further correlation between the X-ray luminosity of galaxy clusters and the gamma-ray luminosity of the associated gamma-ray source under the hypothesis that the EGRET sources have the same cluster redshifts. Such correlations are consistent with the theoretical expectations and strengthen the probability of a true, physical association between galaxy clusters and gamma-ray sources.