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تسجيل مستخدم جديدCan AGN and galaxy clusters explain the surface brightness fluctuations of the cosmic X-ray background?
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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 $-0.8pm0.1$ 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 $left<zright>sim0.3$, and the mean temperature of their intracluster medium (ICM), $left<Tright>approx 1.4$ 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.
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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 ~2 arcsec
to ~3 deg. The non-trivial large-scale structure (LSS) signal dominates over the shot-noise of unresolved point sources at all 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 $Gamma = 0.96 pm 0.06$. 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, $M_{500} sim 10^{13.6},{rm M}_{odot}/h$ and $L_{0.5-2.0,{rm keV}} sim 10^{42.5}$ erg/s. 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 $sim{rm few}times R_{500}$. 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.
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