2013A&A...557A..39S

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 information on matter distribution on very large scales. We test the possibility of detecting the variation in the X-ray source number counts over the celestial sphere. A large number of Chandra pointings spread over both galactic hemispheres are investigated. We searched for all the point-like sources in the soft band of 0.5-2keV and statistically assessed the population of sources below the detection threshold. A homogeneous sample of the number counts at fluxes above ∼10^–15erg/s/cm2 was constructed for more than 300 ACIS fields. The sources were counted within a circular area of 15arcmin diameter. The count correlations between overlapping fields were used to assess the accuracy of the computational methods used in the analysis. The average number of sources in the investigated sample amounts to 46 per field. It is shown that the source number counts vary between fields at a level exceeding the fluctuation amplitude expected for the random (Poissonian) distribution. The excess fluctuations are attributed to the cosmic variance generated by the large-scale structures. The rms variations of the source counts due to the cosmic variance within the 15arcmin circle reach ∼8% of the average number counts. An amplitude of the potential correlations of the source counts on angular scales larger than the size of a single pointing remains below the noise level.