06 46 0 v 2 3 1 Ju l 2 00 3 Toward a self consistent model of the ionized absorber in NGC 3783

We present a detailed model for the ionized absorbing gas evident in the 900 ksec Chandra HETGS spectrum of NGC 3783. The analysis was carried out with PHASE, a new tool designed to model X-ray and UV absorption features in ionized plasmas. The 0.5-10 keV intrinsic continuum of the source is well represented by a single power law (Γ = 1.53) and a soft black-body component (kT ∼ 0.1 keV). The spectrum contains over 100 features, which are well fit by PHASE with just six free parameters. The model consists of a simple two phase absorber with difference of ≈ 35 in the ionization parameter and difference of ≈ 4 in the column density of the phases. The two absorption components turned out to be in pressure equilibrium, and are consistent with a single outflow (≈ 750 km s) and a single turbulent velocity (300 km s), and with solar elemental abundances. The main features of the low ionization phase are an Fe M-shell unresolved transition array (UTA) and the O vii lines. The O vii features, usually identified with the O viii and a warm absorber, are instead produced in a cooler medium also producing O vi lines. The UTA sets tight constraints on the ionization degree of the absorbers, making the model more reliable. The high ionization phase is required by the O viii and the Fe L-shell lines, and there is evidence for an even more ionized component in the spectrum. A continuous range of ionization parameters is disfavored by the fits, particularly to the UTA. Our model indicates a severe blending of the absorption and emission lines, as well as strong saturation of the most intense O absorption lines. This is in agreement with the O vii (τλ = 0.33) and O viii (τλ = 0.13) absorption edges required to fit the spectrum. The low ionization phase can be decomposed into three subcomponents, based on the outflow velocity, FWHM, and H column densities found for three out of the four UV absorbers detected in NGC 3783. However, the ionization parameters are systematically smaller in our model than derived from UV data, indicating a lower degree of ionization. Finally, our model predicts a Ca xvi line for the feature observed at around 21.6 Å (a feature formerly identified as O vii), constraining the contribution from a zero redshift absorber. Subject headings: galaxies: absorption lines – galaxies: Seyferts – galaxies: active –