The nearest neighbor statistics for X-ray source counts II. Chandra Deep Field South

TL;DR

This study uses nearest neighbor statistics to estimate the number of faint X-ray sources below detection limits in the Chandra Deep Field-South, revealing a steepening in soft band source counts suggestive of a new population of normal galaxies.

Contribution

It introduces a statistical method to estimate sub-threshold X-ray source counts, extending the understanding of faint source populations in deep X-ray fields.

Findings

Source counts steepen below 10^{-16} cgs in the soft band.

Estimated source counts indicate a slope of about -2.0 for weaker sources.

Steepening suggests a new population of normal galaxies at moderate redshifts.

Abstract

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.