What X-ray source counts can tell about large-scale matter distribution
  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 - 2 keV and statistically assessed
the population of sources below the detection threshold. A homogeneous sample
of the number counts at fluxes above ~10^{-15} erg s^{-1} cm^{-2} was
constructed for more than 300 ACIS fields. The sources were counted within a
circular area of 15 arcmin 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 15$ arcmin 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.
