2004A&A...422...11D

We present a statistical analysis of the Chandra observation of the source field around the 3C 295 galaxy cluster (z=0.46) to search for clustering of X-ray sources. We applied three different methods of analysis, all suggesting a strong clustering in the field on scales of a few arcmin. In particular 1) the logN-logS computed separately for the four ACIS-I chips reveals that there is a significant (3.2σ in the 0.5-2keV, 3.3σ in the 2-10keV and 4.0σ in the 0.5-10keV band) excess of sources to the North-North East and a void to the South of the central cluster; 2) the two point, two-dimensional Kolmogorov-Smirnov (KS) test, shows the probability that the sources are uniformly distributed is only a few percent; 3) a strong spatial correlation emerges from the study of the angular correlation function of the field: the angular correlation function (ACF) shows a clear signal on scales of 0.5/5arcmin, correlation angle in the 0.5-7 keV band θ₀=8.5^+6.5_–4.5, 90% confidence limit (assuming a power law ACF with slope γ=1.8). This correlation angle is 2 times higher than that of a sample of 8 ACIS-I field at the 2.5σ confidence level. The above scales translate to 0.2/2Mpc at the cluster redshift, higher than the typical cluster core radius, and more similar to the size of a ``filament'' of the large scale structure.