ASCA observations of Seyfert 1 galaxies. III. The evidence for absorption and emission due to photoionized gas

We present the results from a detailed analysis of the 0.6-10 keV spectra of 23 ASCA observations of 18 objects. We find that in most cases the underlying continuum can be well represented by a power law with a photon index Gamma similar to 2. However, we find strong evidence for photoionized gas in the line of sight to 13/18 objects. We present detailed modeling of this gas based upon the ION photoionization code. Other studies have been made of the "warm absorber" phenomenon, but this paper contains the first consideration of the importance of the covering fraction of the ionized gas and a direct comparison between models of attenuation by ionized versus neutral material. We find the X-ray ionization parameter for the ionized material is strongly peaked at U-X similar to 0.1. The column densities of ionized material are typically in the range N-H,N-z similar to 10(21)-10(23) cm(-2), although highly ionized (and hence pseudotransparent) column densities up to 10(24) cm(-2) cannot be excluded in some cases. We also investigate the importance of the emission spectrum from the ionized gas, finding that it significantly improves the fit to many sources with an intensity consistent with material subtending a large solid angle at the central source. Allowing a fraction of the continuum to be observed without attenuation also improves the fit to many sources and is definitely required in the case of NGC 4151. A deficit of counts is observed at similar to 1 keV in the sources exhibiting the strongest absorption features. We suggest this is likely to be the signature of a second zone of (more highly) ionized gas, which might have been seen previously in the deep Fe K-shell edges observed in some Ginga observations. We find evidence that the ionized material in NGC 3227 and MCG -6-30-15 contains embedded dust, while there is no such evidence in the other sources. We discuss these results in the context of previous studies and briefly explore the implications in other wave bands.