We study the surface brightness fluctuations of the cosmic X-ray background ( CXB ) using Chandra data of XBOOTES .
After masking out resolved sources we compute the power spectrum of fluctuations of the unresolved CXB for angular scales from \approx 2 \arcsec to \approx 3 ^ { \circ } .
The non-trivial large-scale structure ( LSS ) signal dominates over the shot noise of unresolved point sources at all scales above \sim 1 \arcmin and is produced mainly by the intracluster medium ( ICM ) of unresolved clusters and groups of galaxies , as shown in our previous publication .

The shot-noise-subtracted power spectrum of CXB fluctuations has a power-law shape with the slope of \Gamma = 0.96 \pm 0.06 .
Its energy spectrum is well described by the redshifted emission spectrum of optically-thin plasma with the best-fit temperature of T \approx 1.3 keV and the best-fit redshift of z \approx 0.40 .
They are in good agreement with theoretical expectations based on the X-ray luminosity function and scaling relations of clusters .
From these values we estimate the typical mass and luminosity of the objects responsible for CXB fluctuations , M _ { 500 } \sim 10 ^ { 13.6 } \mathrm { M _ { \sun } } h ^ { -1 } and L _ { 0.5 - 2.0 { keV } } \sim 10 ^ { 42.5 } \mathrm { erg s ^ { -1 } } .
On the other hand , the flux-weighted mean temperature and redshift of resolved clusters are T \approx 2.4 keV and z \approx 0.23 , confirming that fluctuations of unresolved CXB are caused by cooler ( i.e .
less massive ) and more distant clusters , as expected .
We show that the power spectrum shape is sensitive to the ICM structure all the way to the outskirts , out to \sim { few } \times R _ { 500 } .
We also look for possible contribution of the warm-hot intergalactic medium ( WHIM ) to the observed CXB fluctuations .

Our results underline the significant diagnostics potential of the CXB fluctuation analysis in studying the ICM structure in clusters .