X-ray Observations of Optically-Selected, Radio-Quiet Quasars I: The ASCA results

Abstract

We present the result of 27 ASCA observations of 26 radio-quiet quasars (RQQs) from the PG survey. All the RQQs were detected except 2 BALQSOs. We find the variability characteristics of the sources to be consistent with Seyf-1s. A powerlaw offers an acceptable description of the time-averaged spectra in the 2-10 keV (RQQ-frame) band for all but 1 dataset. The photon indices vary from object to object over the range 1.5~<Gamma(2-10 keV)~<3 with a mean Gamma(2-10keV)~2 and dispersion ~=0.25. The distribution of Gamma(2-10 keV is therefore similar to that observed in other RQ AGN, and seems unrelated to X-ray luminosity. No single model adequately describes the full 0.6-10 keV (obs-frame) continuum of all the RQQs. ~50% of the sources can be described by a single powerlaw (or with only very subtle deviations). All but one of the remaining datasets have convex spectra (flattening to higher energies). The exception is PG 1411+442, in which a substantial column density obscures ~98% of the continuum. We find only 5 (or 6) of 14 objects with z<~0.25 to have a 'soft excess' at energies <~1 keV, but find no universal shape for these spectral components. The spectrum of PG 1244+026 contains an emission feature at ~1 keV. The detection rate of absorption due to ionized material in these RQQs is lower than in Seyf-1s. 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 Seyf-1s. We find evidence of Fe K-shell emission in at least 8 RQQs. These are all low-luminosity objects, and the line parameters are consistent with other low-luminosity RQ AGN. However we find a trend whereby the profile and strength of the Fe K-shell emission changes as a function of luminosity.