Toward a Self‐Consistent Model of the Ionized Absorber in NGC 3783

Abstract

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 -1), a single turbulent velocity (300 km s -1), 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.