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 ~1keV is determined at separations 0.1deg<θ<40deg. We detect a significant signal for θ<6deg, 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 _<_ 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 0.5-2.0keV band is L=~2.5x10^43^h^-2^erg/s and their characteristic radius a =~10h^-1^Mpc (H_0_=100hkm/s/Mpc). 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 ~30% of the X-ray background fluctuations on scales above ~1deg