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.

We present a detailed investigation of the variability of 428 C iv and 235 Si iv Broad Absorption Line (BAL) troughs identified in multi-epoch observations of 291 quasars by the Sloan Digital Sky Survey-I/II/III. These observations primarily sample rest-frame timescales of 1-3.7 yr over which significant rearrangement of the BAL wind is expected. We derive a number of observational results on, e.g., the frequency of BAL variability, the velocity range over which BAL variability occurs, the primary observed form of BAL-trough variability, the dependence of BAL variability upon timescale, the frequency of BAL strengthening vs. weakening, correlations between BAL variability and BALtrough profiles, relations between C iv and Si iv BAL variability, coordinated multi-trough variability, and BAL variations as a function of quasar properties. We assess implications of these observational results for quasar winds. Our results support models where most BAL absorption is formed within an order-of-magnitude of the wind-launching radius, although a significant minority of BAL troughs may arise on larger scales. We estimate an average lifetime for a BAL trough along our line-of-sight of a few thousand years. BAL disappearance and emergence events appear to be extremes of general BAL variability, rather than being qualitatively distinct phenomena. We derive the parameters of a random-walk model for BAL EW variability, finding that this model can acceptably describe some key aspects of EW variability. The coordinated trough variability of BAL quasars with multiple troughs suggests that changes in "shielding gas" may play a significant role in driving general BAL variability.

Broad absorption lines (BALs) in quasar spectra indicate high‐velocity outflows that may be present in all quasars and could be an important contributor to feedback to their host galaxies. Variability studies of BALs help illuminate the structure, evolution and basic physical properties of the outflows. Here we present further results from an ongoing BAL monitoring campaign of a sample of 24 luminous quasars at redshifts 1.2 < z < 2.9. We directly compare the variabilities in the C ivλ1549 and Si ivλ1400 absorption to try to ascertain the cause(s) of the variability. We find that Si iv BALs are more likely to vary than C iv BALs. When looking at flow speeds >−20 000 km s−1, 47 per cent of quasars exhibited Si iv variability while 31 per cent exhibited C iv variability. Furthermore, ∼50 per cent of the variable Si iv regions did not have corresponding C iv variability at the same velocities, while nearly all occurrences of C iv variability had corresponding changes in Si iv. We do not find any correlation between the absolute change in strength in C iv and in Si iv, but the fractional change in strength tends to be greater in Si iv than in C iv. When both C iv and Si iv varied, those changes always occurred in the same sense (either getting weaker or stronger). We also include our full data set so far in this paper, which includes up to 10 epochs of data per quasar. The multi‐epoch data show that the BAL changes were not generally monotonic across the full ∼5–8 yr time span of our observations, suggesting that the characteristic time‐scale for significant line variations, and (perhaps) for structural changes in the outflows, is less than a few years. Coordinated variabilities between absorption regions at different velocities in individual quasars seem to favour changing ionization of the outflowing gas as the cause of the observed BAL variability. However, variability in limited portions of broad troughs fits naturally in a scenario where movements of individual clouds, or substructures in the flow, across our lines of sight cause the absorption to vary. The actual situation may be a complex mixture of changing ionization and cloud movements. Further discussion of the implications of variability, e.g. in terms of the size and location of the outflowing gas, will be presented in a forthcoming paper.

Associated absorption lines (AALs) are valuable probes of the gaseous environments near quasars. Here we discuss high-resolution (6.7 km s~1) spectra of the AALs in the radio-loud quasar 3C 191 (redshift z \ 1.956). The measured AALs have ionizations ranging from Mg I to N V and multicomponent proÐles that are blueshifted by D400 to D1400 km s~1 relative to the quasarÏs broad emission lines. These data yield the following new results : (1) The strengths of excited-state Si II* AALs indicate a density of D300 cm~3 in the Si`gas. (2) If the gas is photoionized, this density implies a distance of D28 kpc from the quasar. Several arguments suggest that all of the lines form at approximately this distance. (3) The characteristic Ñow time from the quasar is thus D3 ] 107 yr. (4) Strong Mg I AALs identify neutral gas with very low ionization parameter and high density. We estimate n HZ 5 ] 104 cm~3 in this region, compared to D15 cm~3 where the N V lines form. (5) The total column density is cm~2 in the neutral gas and cm~2 in the moderately ionized] 1020 regions. These column densities are consistent with 3C 191Ïs strong soft X-ray Ñux and the implied absence of soft X-ray absorption. (6) The total mass in the AAL outÑow is M D 2 ] 109 assuming M _ , a global covering factor (as viewed from the quasar) of D10%. (7) The absorbing gas only partially covers the background light source(s) along our line(s) of sight, requiring absorption in small clouds or Ðlaments less than 0.01 pc across. The ratio implies that the clouds have radial (line-of-sight) N H /n H thicknesses pc. These properties might characterize a subclass of AALs that are physically related [0.2 to quasars but form at large distances. We propose a model for the absorber in which pockets of dense neutral gas are surrounded by larger clouds of generally lower density and higher ionization. This outÑowing material might be leftover from a blowout associated with a nuclear starburst, the onset of quasar activity, or a past broad absorption line (BAL) wind phase.