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Sources generating most of the X-ray background (XRB) are dispersed over awide range of redshifts. Thus, statistical characteristics of the sourcedistribution carry information on matter distribution on very large scales. Wetest the possibility of detecting the variation in the X-ray source numbercounts over the celestial sphere. A large number of Chandra pointings spreadover both galactic hemispheres are investigated. We searched for all thepoint-like sources in the soft band of 0.5 - 2 keV and statistically assessedthe population of sources below the detection threshold. A homogeneous sampleof the number counts at fluxes above ~10^{-15} erg s^{-1} cm^{-2} wasconstructed for more than 300 ACIS fields. The sources were counted within acircular area of 15 arcmin diameter. The count correlations between overlappingfields were used to assess the accuracy of the computational methods used inthe analysis. The average number of sources in the investigated sample amountsto 46 per field. It is shown that the source number counts vary between fieldsat a level exceeding the fluctuation amplitude expected for the random(Poissonian) distribution. The excess fluctuations are attributed to the cosmicvariance generated by the large-scale structures. The rms variations of thesource counts due to the cosmic variance within the 15$ arcmin circle reach 8%of the average number counts. An amplitude of the potential correlations of thesource counts on angular scales larger than the size of a single pointingremains below the noise level.

What X-ray source counts can tell about large-scale matter distribution