The Ionized Gas and Nuclear Environment in NGC 3783. IV. Variability and Modeling of the 900 KilosecondChandraSpectrum

We present a detailed spectral analysis of the data obtained from NGC 3783during the period 2000-2001 using Chandra. This analysis leads us to thefollowing results. 1) NGC 3783 fluctuated in luminosity by a factor ~1.5 duringindividual observations (~170 ks duration). These fluctuations were notassociated with significant spectral variations. 2) On a longer time scale(20-120 days), we found the source to exhibit two very different spectralshapes. The main difference between these can be well-described by theappearance and disappearance of a spectral component that dominates theunderlying continuum at the longest wavelengths. The spectral variations arenot related to the brightening or the fading of the continuum at shortwavelengths in any simple way. 3) The appearance of the soft continuumcomponent is consistent with being the only spectral variation, and there is noneed to invoke changes in the opacity of the absorbers. 4) Photoionizationmodeling indicates that a combination of three ionized absorbers, each splitinto two kinematic components, can explain the strengths of almost all theabsorption lines and bound-free edges. All three components are thermallystable and seem to have the same gas pressure. 5) The only real discrepancybetween our model and the observations concerns the range of wavelengthsabsorbed by the iron M-shell UTA feature. This most likely arises as the resultof our underestimation of the poorly-known dielectronic recombination ratesappropriate for these ions. 6) The lower limit on the distance of the absorbinggas in NGC 3783 is between 0.2 and 3.2 pc. The assumption of pressureequilibrium imposes an upper limit of about 25 pc on the distance of theleast-ionized component from the central source. (abridged)Comment: 16 pages, 12 figures (9 in color), emulateapj5, accepted for publication in The Astrophysical Journa