A. M. Soltan, M. J. Chodorowski
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.
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http://arxiv.org/abs/1212.5891
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