1996A&A...310..371R

We report results from the spectral analysis of pointed ROSAT PSPC observations of six PG quasars and the weak-bump quasar PHL 1657. Disregarding other frequency bands, the PSPC data are represented best by simple power-law source spectra (dN/dE∝E^{GAMMA}) with a slope {GAMMA}=-2.5±0.4 and do not show evidence for a more complex structure. However, within the limits given by statistics and systematical errors, a superposition of two power-law spectra, which allows a connection to the observed UV fluxes and to the generally flatter hard-X-ray spectra observed by EXOSAT and Ginga, is found to be indistinguishable from a simple power-law in the PSPC energy band. For the PG quasars, a direct connection to the observed UV flux is obtained with a steep soft-X-ray slope {GAMMA}_sx=-3.1±0.3, fixing the hard-X-ray slope. For PHL 1657, the soft-X-ray spectrum seen by ROSAT is much steeper than the Ginga spectrum, rising towards the EUV in νF_ν, and it can not be extrapolated down to meet the exceptionally weak UV flux. A Wien-shaped thermal UV/soft-X-ray bump with a temperature of ∼50eV connects the UV and soft-X-ray spectra in all cases, but gives only a poor representation of the high signal-to-noise PSPC spectra. We discuss implications of our results for models of the X-ray emission in quasars. Three more pointed PSPC observations, covering the radio galaxies 3C433, 3C83.1 and the quasar PG2214+139 (Mkn304), did not yield enough counts for a spectral analysis. For these sources, and four high redshift quasars from the Hewitt&Burbidge catalogue located in the observation fields, we present brief results or limits for their soft-X-ray fluxes.