Soft X-ray properties of a high redshift sample of QSOs observed with ROSAT

In order to study systematically the soft X-ray emission of active galactic nuclei (AGNs) at medium to high redshifts, we have analysed ROSAT PSPC and HRI data of QSOs at 0.26 less than or equal to z less than or equal to 3.43 selected from the second deepest ROSAT PSPC survey carried out in 1991-1993 by McHardy et al. Our sample of 22 type 1 QSOs is nearly complete above a flux limit of 1.4 x 10(-14) erg cm(-2) s(-1) in the 0.1-2 keV band. Of these, nine QSOs show long-term (similar to2 yr) X-ray variability by a factor of 1.5-3.5. Significant excess absorption above the Galactic column is seen in three QSOs. The soft X-ray photon index of the QSOs ranges from 1 4 to 3.7. Three QSOs have steep soft X-ray spectra ( Gamma(X) > 3.0), one of which is a narrow-line QSO - a high-luminosity version of narrow-line Seyfert 1 galaxies. The average photon index ([Gamma(X)]) is 2.40+/-0.09 (with a dispersion of 0.57) in the 0.1-2 keV band. The average QSO spectra in four redshift bins flatten from an average photon index of [Gamma(X)] similar to2.53 at 0.25 less than or equal to z less than or equal to 1 to [Gamma(X)] similar to2 at 2 less than or equal to z less than or equal to 3.4. The flattening of the average photon index can be understood in terms of the redshift effect of the mean intrinsic QSO spectrum consisting of two components - a soft X-ray excess and a power-law component. We have also studied optical spectra of 12 of the 22 QSOs.