  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 the preliminary evidence of a probable 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 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.
