THE COMPLEX OPTICAL TO SOFT-X-RAY SPECTRUM OF THE LOW-REDSHIFT RADIO-QUIET QUASARS .1. THE X-RAY DATA

Eight high signal-to-noise ROSAT PSPC observations of six low-redshift (0.048 < z < 0. 1 55) radio-quiet quasars have been analyzed to study any soft excess. All the spectra can, at least roughly, be described in the 0.1-2.5 keV band by simple power laws reduced at low energies by Galactic absorption. The strong oxygen edges seen in the PSPC spectra of several Seyfert galaxies and quasars are not observed in this sample. The limits implied for the amount of absorbing gas intrinsic to the quasars are particularly tight: of the order of approximately 10(20) cm-2. The range of energy indices is broad: 1.3 < alpha(E) < 2.3. The energy indices are systematically steeper than those found in the same sources at higher energies [by DELTAalpha(E) approximately 0.5-1 with respect to Ginga or EXOSAT (2-10 keV) measurements, and by DELTAalpha(E) approximately 0.5 with respect to IPC (0.2-3.5 keV) measurements]. This suggests a break between the hard and soft components in the ''keV'' region and, therefore, that the PSPC spectra are strongly dominated by the soft components. In fact, a fit to the composite, high signal-to-noise spectrum reveals a significant excess above approximately 1 keV with respect to the simple power-law model. No evidence for strong emission lines is found in any of the quasars. This argues against emission from an ionized plasma as the main contributor to the soft X-ray component unless there is a distribution of temperatures. If the soft X-ray spectrum of these quasars is dominated by radiation reflected by the photoionized surface of an accretion disk, the absence of strong emission lines suggests high ionization parameters and therefore high accretion rates. We include in two Appendices a comparison of the two official PSPC resolution matrices, those released on 1992 March and on 1993 January, a discussion of the amplitude of the residual systematic uncertainties in the 1993 January matrix, and a comparison between the PSPC and IPC spectra of a sample of sources.