THE ANGULAR-CORRELATION FUNCTION OF THE SOFT-X-RAY BACKGROUND

Several dozen observations with the ROSAT PSPC are used to measure flu ctuations 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 arc min) the data are dominated by statistical noise. The average amplitud e of the correlation function in the range of 3 - 11 arc min amounts t o (0.3 +/- 0.9). 10(-3). The tight upper limit on the angular correlat ion function severly constrains the clustering properties of sources p roducing the background. In particular, it can be shown that contribut ion of QSO to the residual background cannot exceed approximately 35 % . Low luminosity AGN (probably Seyfert galaxies) are likely candidates for the unresolved background. In this case they have to be weakly cl ustered and show relatively strong clustering evolution.