We present a detailed spectral analysis of the data obtained from NGC 3783 during the period 2000-2001 using Chandra. The data were split in various ways to look for time-and luminosity-dependent spectral variations. This analysis, along with the measured equivalent widths of a large number of X-ray lines and photoionization calculations, lead us to the following results and conclusions. 1) NGC 3783 fluctuated in luminosity by a factor ∼ 1.5 during individual observations (most of which were of 170 ks duration). These fluctuations were not associated with significant spectral variations. 2) On a longer time scale (20-120 days), we found the source to exhibit two very different spectral shapes. The main difference between these can be well-described by the appearance (in the "high state") and disappearance (in the "low state") of a spectral component that dominates the underlying continuum at the longest wavelengths. Contrary to the case in other objects, the spectral variations are not related to the brightening or the fading of the continuum at short wavelengths in any simple way. NGC 3783 seems to be the first AGN to show this unusual behavior.3) The appearance of the soft continuum component is consistent with being the only spectral variation, and there is no need to invoke changes in the opacity of the absorbers lying along the line of sight. Indeed, we find all the absorption lines which can be reliably measured have the same equivalent widths (within the observational uncertainties) during high-and low-states. 4) Photoionization modeling indicates that a combination of three ionized absorbers, each split into two kinematic components, can explain the strengths of almost all the absorption lines and boundfree edges. These three components span a large range of ionization, and have total column of about 4×10 22 cm −2 . Moreover, all three components are thermally stable and seem to have the same gas pressure. Thus all three may co-exist in the same volume of space. This is the first detection of such a multi-component, equilibrium gas in an AGN. 5) The only real discrepancy between our model and the observations concerns the range of wavelengths absorbed by the iron M-shell UTA feature. This most likely arises as the result of our underestimation of the poorly-known dielectronic recombination rates appropriate for these ions. We also note a small discrepancy in the calculated column density of O VI and discuss its possible origin. 6) The lower limit on the distance of the absorbing gas in NGC 3783 is between 0.2 and 3.2 pc, depending on the component of ionized gas considered. The assumption of pressure equilibrium imposes an upper limit of about 25 pc on the distance of the least-ionized component from the central source.

We present results from a 900 ks exposure of NGC 3783 with the High-Energy Transmission Grating Spectrometer on board the Chandra X-ray Observatory. The resulting X-ray spectrum, which covers the 0.5-10 keV energy range, has the best combination of signal-to-noise and resolution ever obtained for an AGN. This spectrum reveals absorption lines from H-like and He-like ions of N, O, Ne, Mg, Al, Si, and S. There are also possible absorption lines from H-like and He-like Ar and Ca as well as H-like C. We also identify inner-shell absorption from lowerionization ions such as Si VII-Si XII and S XII-S XIV. The iron absorption spectrum is very rich; L-shell lines of Fe XVII-Fe XXIV are detected, as well as probable resonance lines from Fe XXV. A strong complex of M-shell lines from iron ions is also detected in the spectrum The absorption lines are blueshifted relative to the systemic velocity by a mean velocity of −590 ± 150 km s −1 . We resolve many of the absorption lines, and their mean FWHM is 820 ± 280 km s −1 . We do not find correlations between the velocity shifts or the FWHMs with the ionization potentials of the ions. Most absorption lines show asymmetry, having more extended blue wings than red wings. In O VII we have resolved this asymmetry to be from an additional absorption system at ∼ −1300 km s −1 . The two X-ray absorption systems are consistent in velocity shift and FWHM with the ones identified in the UV lines of C IV, N V, and H I. Equivalent width measurements for all absorption and emission lines are given and column densities are calculated for several ions. We resolve the narrow Fe Kα line at 6398.2 ± 3.3 eV to have a FWHM of 1720 ± 360 km s −1 , which suggests that this narrow line may be emitted from the outer part of the broad line region or the inner part of the torus. We also detect a "Compton shoulder" redward of the narrow Fe Kα line which indicates that it arises in cold, Compton-thick gas.