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

By Y. Krongold, F. Nicastro, N. Brickhouse, M. Elvis, D. Liedahl, S. M. H. C. F. Astrophysics, Lawrence Livermore National Laboratory, D. O. Astronomy, O. University. 2003

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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 (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 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.

Subfields: Astronomy and Astrophysics, Instrumentation

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Citations in the corpus

The nature and origin of Seyfert warm absorbers (2004, background): “…multi-phase ionisation structure is concerned, there is currently no consensus as to whether this takes the form of discrete ionisation phases (i.e. Krongold et al. 2003), a continuous ionisation parameter distribution (e.g. Krolik & Kriss 2001; Różańska et al. 2004), or some combination of…”
The Compact, Conical, Accretion-Disk Warm Absorber of the Seyfert 1 Galaxy NGC 4051 and Its Implications for IGM-Galaxy Feedback Processes (2007, methodology): “A fourth parameter, the turbulent velocity of the gas, was set to 300 km s−1 (see Krongold et al. 2003 for details).”
THE SPIN OF THE SUPERMASSIVE BLACK HOLE IN NGC 3783 (2011, methodology): “Our analysis of the same HETG data confirms the Krongold et al. (2003) result.”
Simultaneous X-ray and UV spectroscopy of the Seyfert galaxy NGC 5548 - II. Physical conditions in the X-ray absorber (2005, background): “Krongold et al. (2003) claim that there are only two ionization components in such an equilibrium in the Seyfert 1 galaxy NGC 3783.”