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

By H. Netzer, S. Kaspi, Eda Behar, W. N. Brandt, D. Chelouche, I. George, D. Crenshaw, J. Gabel, Fred Hamann, S. Kraemer and 5 more. 2003

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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 of ~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 timescale (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 active galactic nucleus (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 that all the absorption lines that 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 bound-free edges. These three components span a large range of ionization and have total column of about 4 × 1022 cm-2. Moreover, all three components are thermally stable and seem to have the same gas pressure. Thus, all three may coexist in the same volume of space. This is the first detection of such a multicomponent, 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.

Subfields: Astronomy and Astrophysics

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Citations in the corpus

THE SPIN OF THE SUPERMASSIVE BLACK HOLE IN NGC 3783 (2011, methodology): “…a high-column density and multi-component warm absorber (WA) that has been studied well by every spectroscopic X-ray observatory, including a 900 ks campaign by Chandra using the High-Energy Transmission Grating Spectrometer (HETGS; Kaspi et al. 2002; Krongold et al. 2003; Netzer et al. 2003).”
The Compact, Conical, Accretion-Disk Warm Absorber of the Seyfert 1 Galaxy NGC 4051 and Its Implications for IGM-Galaxy Feedback Processes (2007, background): “…efforts to understand WAs have concentrated on the accurate spectral modeling of the hundreds of bound-bound and bound-free transitions directly visible in the time-averaged, high signal-to-noise (S/N), highresolution X-ray spectra of nearby Seyferts (e.g. Krongold et al. 2003; Netzer et al. 2003).”
The nature and origin of Seyfert warm absorbers (2004, background): “…0558-504 NLSy1 RLQ 0.137 ∼46 O’Brien et al. (2001b) IRAS 13349+2438 NLSy1 RQQ 0.10764 46.4b • • • Sako et al. (2001) ESO 141-G55 Sy1 0.036 ∼45 Gondoin et al. (2003) NGC 4593 Sy1 0.0084 43.5c • • • Steenbrugge et al. (2003a) NGC 3783 Sy1 0.00973 44.7d • • • Netzer et al. (2003) Markarian 509 Sy1.”
THE QUASAR OUTFLOW CONTRIBUTION TO AGN FEEDBACK: VLT MEASUREMENTS OF SDSS J0318-0600 (2009, background): “The hot phase of the gas (or X-ray, T = 106 K) has been shown in Seyfert galaxies (e.g., NGC 3783; Netzer et al. 2003) to contribute up to 50× more column density than the cold phase, therefore it is possible that the gas in the hot phase of the outflow in SDSS J0318−0600 could contribute…”
Quasar outflows and AGN feedback in the extreme UV: HST/COS observations of HE 0238−1904 (2013, methodology): “X-ray observations of Seyfert outflows, the so-called “warm absorbers”, routinely show the need for two or more ionization phases (e.g., Netzer et al. 2003; Steenbrugge et al. 2005; Holczer et al. 2007).”
A long hard look at MCG–6-30-15 with XMM-Newton (2002, result): “This result is consistent with the studies of the deep warm absorber in NGC 3783 usingChandra(Netzer et al. 2003) andXMM-Newton (Behar et al. 2003) which showed the X-ray absorption remained unchanged despite large changes in source luminosity. c© 2003 RAS, MNRAS000, 1–25 Furthermore, the spectral…”
Simultaneous X-ray and UV spectroscopy of the Seyfert galaxy NGC 5548 - II. Physical conditions in the X-ray absorber (2005, background): “As in previous papers the iron abundance is measured to be overabundant for lower ionization ions (Kaastra et al. 2002a; Blustin et al. 2002; Steenbrugge et al. 2003; Netzer et al. 2003).”