2003ApJ...597..832K

We present a detailed model for the ionized absorbing gas evident in the 900 ks 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 blackbody component (kT∼0.1 keV). The spectrum contains over 100 features, which are well fitted by PHASE with just six free parameters. The model consists of a simple two-phase absorber with a difference of ~35 in the ionization parameter and a 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), a single turbulent velocity (300 km/s), and 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 that also produces 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 (τ_λ=0.33) and O VIII (τ_λ=0.13) absorption edges required to fit the spectrum. The low-ionization phase can be decomposed into three subcomponents on the basis of the outflow velocity, FWHM, and H column densities found for three of the four UV absorbers detected in NGC 3783. However, the ionization parameters are systematically smaller in our model than those 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.