Soft X-ray background fluctuations and large scale structure in the Universe

We have studied the fluctuations of the soft (0.9-2 keV) X-ray background intensity for ~10 arcmin and ~2 arcmin beam sizes, using 80 high galactic latitude medium-deep images from the ROSAT PSPC. These fluctuations are dominated (and well reproduced) by confusion noise produced by sources unresolved with the beam sizes we used. We find no evidence for any excess fluctuations which could be attributed to source clustering. The 95 per cent confidence upper limits on excess fluctuations DeltaIclus are: (DeltaIclus/I)_{ 10 arcmin}1. Limits on the 2-10 keV excess fluctuations are also considered, showing that X-ray sources in that band have to be at redshifts z>1 unless Omega>0.4. Finally, if the spatial correlation function of the sources that produce these excess fluctuations is instead a power law, the density contrast delta rho/rho implied by the excess fluctuations reveals that the Universe is smooth and linear on scales of tens of Mpc, while it can be highly non-linear on scales ~1 Mpc