The large scale structure of the soft X-ray background. I: Clusters of galaxies

Large-scale fluctuations of the X-ray background are investigated using the ROSAT All-Sky Survey. The autocorrelation function of the extragalactic background at 1 keV is determined at separations 0.1 - 40 deg. We detect a significant signal for separations < 6 deg, which is larger than previous upper limits. We analyse the cross-correlation between the angular positions of Abell clusters of galaxies and the intensity of the X-ray background. The cross-correlation extends to several degrees for clusters of distance class .leq. 4. We conclude that Abell clusters are associated with extended, low surface brightness X-ray haloes. The mean luminosity of the diffuse sources in the 0.5-2.0 keV band is equal to approx 2.5 * 10^{43} h^{-2} erg s^{-1} and their characteristic radius a = 10 h^{-1} Mpc (H_0 = 100 h km s^{-1} Mpc^{-1}). It is shown that the higher density of normal galaxies and AGNs around clusters cannot be responsible for the increased X-ray emission. This is because it would produce a signal in the autocorrelation function measurable by the Ginga LAC experiment. Apparent extended emission could be produced either by a number of discrete sources associated with groups of galaxies concentrated around rich clusters or by a hot diffuse gas. It is shown that sources associated with Abell clusters account for approx. 30 % of the X-ray background fluctuations on scales above 1 deg.