The angular correlation function of the soft X-ray background.

Several dozen observations with the ROSAT PSPC are used to measure fluctuations of the unresolved X-ray background using the autocorrelation function. At the lowest separation of 1-3 arc min a signal of (6.8+/-2.5)10^-3^ has been detected. At larger separations (3-27 arcmin) the data are dominated by statistical noise. The average amplitude of the correlation function in the range of 3-11 arc min amounts to (0.3+/-0.9)10^-3^. The tight upper limit on the angular correlation function severly constrains the clustering properties of sources producing the background. In particular, it can be shown that contribution of QSO to the residual background cannot exceed ~35 %. Low luminosity AGN (probably Seyfert galaxies) are likely candidates for the unresolved background. In this case they have to be weakly clustered and show relatively strong clustering evolution.