2008A&A...484..303D

We present a detailed analysis of the overdensity of X-ray sources close to the 3C295 galaxy cluster (z=0.46) to assess whether it is associated with a filament of the large-scale structure of the Universe. We obtained optical spectra of the optical counterparts of eleven sources associated with the filament, finding that one is at z=0.474. This is a type 1 AGN at 1.5arcmin from the cluster center. We found three more sources with a redshift in the range 0.37/0.53. We extracted the stacked X-ray spectrum of 47 X-ray sources belonging to the putative filament. We found a significant narrow (at the resolution of the Chandra ACIS-I) line at E ∼4.4keV, the energy of the iron Kα line at the redshift of the cluster. The detection of this line is confirmed at a confidence level of better than 3σ and its energy is constrained to be in the range 6.2-6.47 (at a 90% confidence level), excluding an identification with the 6.7 helium-like iron line from the hot cluster ICM at better than 4σ. We conclude that the detection of the redshifted line is a strong indication that at least several of the excess sources lie at z∼0.46 and that AGNs are efficient tracers of the ``filament'' connected with the central cluster of galaxies.