2011A&A...532A..19S

Most of the X-ray background (XRB) is generated by discrete X-ray sources. It is likely that the population of weak sources below the present detection threshold contributes to the remaining fraction of the XRB. The nature of these weak sources remains a matter of discussion. It is assumed that the unresolved XRB consists of a truly diffuse component and a population of such weak sources. Albeit these sources are not observed directly, their collective nature an be investigated by statistical means. The goals are: a) to explore the effectiveness of the nearest neighbor statistics (NNST) of the photon distribution in investigating of the number counts of the very weak sources, b) to estimate the source counts below the conventional detection limit in the Chandra Deep Field-South 2Ms exposure. All the sources generating at least two counts each induce a nonrandom distribution of counts. This distribution is analyzed by means of the NNST. Using the basic probability equations, the relationships between the source number counts N(S) and the NNST are derived. The method is tested on the medium deep Chandra pointing to assess the source counts N(S) at flux levels attainable only with the very deep exposures. It is shown that the method yields constraints on the N(S) relationship below the regular discrete source detection threshold. The Chandra 2Ms exposure was used to assess the source counts down to 3-4x10^–18cgs in the soft band (0.5-2keV) and down to 2-3x10^–17cgs in the hard band (2-8keV). In the soft band, the source counts appear to steepen substantially below ∼10^–16cgs. Assuming that the differential slope b≃1.5-1.6 in the range 10^–16-10^–14cgs, the number of weaker sources indicates that the slope ≃-2.0. The steepening is not observed in the hard band. The steepening of counts in the soft band indicates that we detect a new population of sources. A class of normal galaxies at moderate redshifts is a natural candidate.