The large scale structure of the soft X-ray background .1. 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 similar to 1 keV is determined at separations 0 degrees 1. < theta < 40 degrees. We detect a significant signal for theta < 6 degrees, 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 less than or equal to 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 L approximate to 2.5 x 10(43) h(-2) ergs(-1) and their char acteristic radius a approximate to 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 similar to 30% of the X-ray background fluctuations on scales above similar to 1 degrees.