The autocorrelation function of the soft X-ray background
  The first positive detection of the X-ray background fluctuations at small angular scales is reported. ROSAT PSPC archive pointed observations are used to measure fluctuations at scales of 0.03 - 0.4 deg. The pointings have been selected from an area free from galactic contamination. At separations below 0.1 deg clusters of galaxies become a substantial source of the background fluctuations. The autocorrelation function of the fluctuations in the power law approximation has a slope of ~1 for all the data but is substantially flatter (with slope of ~0.7) when pointings containing bright clusters are removed. At separations 0.3 - 0.4 deg where the ACF estimates based on the ROSAT pointings and All-Sky Survey are available, both data sets give consistent results. 
Andrzej M. Soltan, Michael J. Freyberg