The nearest neighbor statistics for X-ray source counts II. Chandra Deep   Field South
  It is assumed that the unresolved fraction of the X-ray background (XRB) consists of a truly diffuse component and a population of the weak sources below the present detection threshold. Albeit these weak sources are not observed directly, their collective nature could be investigated by statistical means. The goal is to estimate the source counts below the conventional detection limit in the Chandra Deep Field-South 2Ms exposure. The source number counts are assessed using the nearest neighbor statistics applied to the distribution of photon counts. The method is described in the first paper of these series. The source counts down to 3-4 x 10^{-18} cgs in the soft band (0.5-2 keV) and down to 2-3 x 10^{-17} cgs in the hard band (2-8 keV) are evaluated. It appears that in the soft band the source counts steepen substantially below ~10^{-16} cgs. Assuming that the differential slope b = -1.5 to -1.6 in the range 10^{-16} - 10^{-14} cgs, the number of weaker sources indicates the slope of approx. -2.0. The steepening is not observed in the hard band. Steepening of counts in the soft band indicates a new population of sources. A class of normal galaxies at moderate redshifts is a natural candidate. 
A. M. Soltan
astro-ph.CO