ROSAT PSPC observations of six PG quasars and PHL 1657

Original · EN

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 ∝ EΓ) with a slope Γ= -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 Γsx= -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 νFν, 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.