2018MNRAS.473.4653K

We study 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 ~2 arcsec to ≃3°. The non-trivial large-scale structure (LSS) signal dominates over the shot noise of unresolved point sources on angular scales above ∼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 Γ = 0.96 ± 0.06. Their energy spectrum is well described by the redshifted emission spectrum of optically thin plasma with the best-fitting temperature of T ≃ 1.3 keV and the best-fitting redshift of z ≃ 0.40. These numbers 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, M₅₀₀ ∼ 10^13.6 M_☉ h^–1 and L_0.5-2.0 keV_ ∼ 10^42.5 erg s^–1. On the other hand, the flux-weighted mean temperature and redshift of resolved clusters are T ≃ 2.4 keV and z ≃ 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 ∼few x R₅₀₀. We also searched for possible contribution of the warm-hot intergalactic medium (WHIM) to the observed CXB fluctuations. Our results underline the significant diagnostic potential of the CXB fluctuation analysis in studying the ICM structure in clusters.