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

Authors: George, I.M. 1,2 , Turner, T.J., 1,3 , Yaqoob, T. 1,2 , Netzer, H. 4 , Laor, A. 5 , Mushotzky, R.F. 1, Nandra, K. 1,2 , Takahashi, T. 6
Affiltns: 1 LHEA, NASA/GSFC, 2 USRA, 3 UMBC, 4 Tel Aviv University, 5 Technion, 6 ISAS
Journal: ApJ
Publication Date: 2000 Mar 01

Abstract

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 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 1 dataset. The best-fitting values of photon index vary from object to object over the range 1.5 <~ Gamma(2-10) <~ 3 with a mean < Gamma(2-10) > ~ 2 and dispersion sigma(Gamma(2-10)) ~0.25. The distribution of Gamma(2-10) 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 (observed-frame) continuum of all the RQQs. Approximately 50% of the sources can be adequately described by a single power law or a power law with only very subtle deviations. All but one of the remaining datasets 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,z} ~ 2x1023 cm-2) obscures ~98% of the continuum. We find only 5 (maybe 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 a rather narrow emission feature centered at an energy ~1 keV (quasar-frame). The detection rate of absorption due to ionized material in these RQQs is lower in that seen in Seyfert 1 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 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 the construction of the mean data/model ratios for various luminosity ranges reveals a trend whereby the profile and strength of the Fe K-shell emission changes as a function of luminosity.

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