2002MNRAS.337..693D

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 ≤z ≤ 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 1.4x10^–14 erg cm ^–2 s ^–1 in the 0.1-2 keV band. Of these, nine QSOs show long-term (∼2 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 (Γ_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 (<Γ_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 <Γ_X>∼ 2.53 at 0.25 ≤z ≤ 1 to <Γ_X >∼ 2 at 2 ≤z ≤ 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.