Can AGN and galaxy clusters explain the surface brightness fluctuations
  of the cosmic X-ray background?
  Fluctuations of the surface brightness of cosmic X-ray background (CXB) carry
unique information about faint and low luminosity source populations, which is
inaccessible for conventional large-scale structure (LSS) studies based on
resolved sources. We used Chandra data of the XBOOTES field
($\sim9\,\mathrm{deg^2}$) to conduct the most accurate measurement to date of
the power spectrum of fluctuations of the unresolved CXB on the angular scales
of $\sim3\,$arcsec $-$ $\sim17\,$arcmin. We find that at sub-arcmin angular
scales, the power spectrum is consistent with the AGN shot noise, without much
need for any significant contribution from their one-halo term. This is
consistent with the theoretical expectation that low-luminosity AGN reside
alone in their dark matter halos. However, at larger angular scales we detect a
significant LSS signal above the AGN shot noise. Its power spectrum, obtained
after subtracting the AGN shot noise, follows a power law with the slope of
$-0.8\pm0.1$ and its amplitude is much larger than what can be plausibly
explained by the two-halo term of AGN. We demonstrate that the detected LSS
signal is produced by unresolved clusters and groups of galaxies. For the flux
limit of the XBOOTES survey, their flux-weighted mean redshift equals
$\left<z\right>\sim0.3$, and the mean temperature of their intracluster medium
(ICM), $\left<T\right>\approx 1.4$ keV, corresponds to the mass of $M_{500}
\sim 10^{13.5}\,\mathrm{M}_\odot$. The power spectrum of CXB fluctuations
carries information about the redshift distribution of these objects and the
spatial structure of their ICM on the linear scales of up to $\sim$Mpc, i.e. of
the order of the virial radius.
