Chandra study of an overdensity of X-ray sources around two distant (z ∼ 0.5) clusters

We present results from a Chandra X-Ray Observatory study of the field X-ray source populations in four different observations: two high-redshift (z similar to 0.5) clusters of galaxies 3C 295 and RX J003033.2+261819; and two noncluster fields with similar exposure time. Surprisingly, the 0.5-2 keV source surface densities (similar to 900-1200 sources deg(-2) at a flux limit of 1.5 x 10(-15) ergs cm(-2) s(-1)) measured in an similar to 8' x 8' area surrounding each cluster exceed by a factor of similar to2 the value expected on the basis of the ROSAT and Chandra log N-log S, with a significance of similar to 2 sigma each, or similar to 3.5 sigma when the two fields are combined (i.e., a probability to be a statistical fluctuation of < 1% and < 0.04%, respectively). The same analysis performed on the noncluster fields and on the outer chips of the cluster fields does not show evidence of such an excess. In both cluster fields, the summed 0.5-10 keV spectrum of the detected objects is well fitted by a power law with Gamma similar to 1.7 similar to active galactic nuclei (AGNs) and shows no sign of intrinsic absorption. The few (similar to 10 of 35) optical identifications available to date confirm that most of them are, as expected, AGNs, but the number of redshifts available is too small to allow conclusions on their nature. We discuss possible interpretations of the overdensity in terms of a statistical variation of cosmic background sources; a concentration of AGNs and/or powerful starburst galaxies associated with the clusters; and gravitational lensing of background QSOs by the galaxy clusters. All explanations, however, are difficult to reconcile with the large number of excess sources detected. Deeper X-ray observations and more redshifts measurements are clearly required to settle the issue.