X-ray observations of optically selected, radio-quiet quasars.: I.: The ASCA results

We present the result of 27 ASCA observations of 26 radio-quiet quasars (RQQs) from the Palomar-Green (PG) survey. The sample is not statistically complete, but it is reasonably representative of RQQs in the PG survey. For many of the sources, the ASCA data are presented here for the first time. All the RQQs were detected except for two objects, both of which contain broad absorption lines in the optical band. We find the variability characteristics of the sources to be consistent with Seyfert 1 galaxies. A power law offers an acceptable description of the time-averaged spectra in the 2-10 keV (quasar frame) band for all but one data set. The best-fitting values of the photon index vary from object to object over the range 1.5 less than or similar to Gamma(2-10) less than or similar to 3, With a mean (Gamma(2-10)) similar or equal to 2 and dispersion sigma(Gamma(2-10)) similar or equal to 0.25. The distribution of Gamma(2-10) is therefore similar to that observed in other RQ active galactic nuclei (AGNs) and seems to be unrelated to X-ray luminosity. No single model adequately describes the full 0.6-10 keV (observed frame) continuum of all the RQQs Approximately 50% of the sources can be adequately described by a single power law or by a power law with only very subtle deviations. All but one of the remaining data sets were found to have convex spectra (flattening as one moves to higher energies). The exception is PG 1411 + 442, in which a substantial column density (N-H,N-z similar to 2 x 10(23) cm(-2)) obscures similar to 98% of the continuum. We find only five (maybe six) of 14 objects with z less than or similar to 0.25 to have "soft excesses" at energies less than or similar to 1 keV, but we find no universal shape for these spectral components. The spectrum of PC 1244+026 contains a rather narrow emission feature centered at an energy similar to 1 keV (quasar frame). The detection rate of absorption due to ionized material in these RQQs is lower than that seen in Seyfert I galaxies. In part, this may be due to selection effects. However, when detected, the absorbers in the RQQs exhibit a similar range of column density and ionization parameter as Seyfert 1 galaxies. We find evidence of Fe R-shell emission in at least eight RQQs. These are all low-luminosity objects, and the line parameters are consistent with those of other low-luminosity RQ AGNs. However the construction of the mean data/model ratios far various luminosity ranges reveals a trend whereby the profile and strength of the Fe K-shell emission changes as a function of luminosity.