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

摘要

The vast majority of the celestial gamma-ray sources detected so far have notyet been identified with secure counterparts at other wavelenghts. Here wereport the preliminary evidence of a probable association between galaxyclusters and unidentified gamma-ray sources of high galactic latitude (|b|>20deg) in the Third EGRET catalog. All of the clusters which are most probablyassociated with EGRET sources show evidence of strong radio emission eitherbecause they host radio galaxies/sources in their environments or because theyhave a radio halo or relic inhabiting their intracluster medium. The clusterradio emission suggests that the relativistic particles (electrons, protons),which are diffusing in the intracluster medium, might be also responsible fortheir gamma-ray emission. Beyond the spatial associations of clusters withunidentified EGRET sources, we found a correlation between the radio flux at1.4 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-clusterassociations we also found a further correlation between the X-ray luminosityof galaxy clusters and the gamma-ray luminosity of the associated gamma-raysource under the hypothesis that the EGRET sources have the same clusterredshifts. Such correlations are consistent with the theoretical expectationsand strengthen the probability of a true, physical association between galaxyclusters and gamma-ray sources.