Abstract
In order to study systematically the soft -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 1.4 × 10−14 erg cm−2 s−1 in the 0.1–2 keV band. Of these, nine QSOs show long-term (∼2 yr) -ray variability by a factor of 1.5–3.5. Significant excess absorption above the Galactic column is seen in three QSOs. The soft -ray photon index of the QSOs ranges from 1.4 to 3.7. Three QSOs have steep soft -ray spectra (ΓX > 3.0), one of which is a narrow-line QSO – a high-luminosity version of narrow-line Seyfert 1 galaxies. The average photon index (〈ΓX〉) 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 〈ΓX〉∼ 2.53 at 0.25 ⩽z⩽ 1 to 〈ΓX〉∼ 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 -ray excess and a power-law component. We have also studied optical spectra of 12 of the 22 QSOs.
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