What X-ray source counts can tell about the large scale matter distribution. (arXiv:1212.5891v1 [astro-ph.CO])

What X-ray source counts can tell about the large scale matter distribution. (arXiv:1212.5891v1 [astro-ph.CO]):
Sources generating most of the X-ray background (XRB) are dispersed over a
wide range of redshifts. Thus, statistical characteristics of the source
distribution carry the information on the matter distribution on very large
scales. We test the possibility to detect the variation of the X-ray source
number counts over the celestial sphere. A large number of Chandra pointings
spread over both galactic hemispheres is investigated. A search for all the
point-like sources in the soft band of 0.5 - 2 keV is performed, and
statistical assessment of the population of sources below the detection
threshold is carried out. A homogeneous sample of the number counts at fluxes
above ~10^{-16} erg/s/cm^2 for more than 300 ACIS fields was constructed. The
counts correlations between overlapping fields were used to assess the accuracy
of the computational methods used in the analysis. It is shown that the source
number counts vary between fields at the level only slightly larger than the
fluctuation amplitude expected for the random (Poissonian) distribution.
Nevertheless, small asymmetry between galactic hemispheres is present. The
average number of sources in the northern hemisphere is larger than in the
southern at the 2.75 sigma level. Also the autocorrelation function of the
source density in both hemispheres are substantially different. Possible
explanations for the observed anisotropies are considered. If the effect is
unrelated to the observational selection, a large scale inhomogeneities in the
distribution of X-ray sources are required. Correlations of the source number
counts observed in the southern hemisphere could be generated by a coherent
structure extending over 1200 Mpc.