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

Abstract

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.

The thermal instability of the warm absorber in NGC 3783

“…Multi-waveband campaigns A&A 589, A76 (2016) provided the detailed broad-band spectrum and the timedependent behavior of X-ray outflows in a number of bright Seyfert-1 galaxies: NGC 7469 , NGC 5548 Kaastra et al 2004;Steenbrugge et al 2005), Mrk 509 Steenbrugge et al 2011;Kaastra et al 2011Kaastra et al , 2012Ebrero et al 2011;Arav et al 2012), IRAS 13449+2438 (Lee et al 2013;Di Gesu et al 2013, NGC 3783 (Kaspi et al 2002;Gabel et al 2003;Netzer et al 2003).…”

K-shell energy levels and radiative rates for transitions in Si ix

“…Recently, more and more observations showed that the K-shell lines, namely 1s−2p transitions, from these low-ionization ions of Si can also be found in high-resolution X-ray spectra. Kaspi et al (2002) and Netzer et al (2003) detected the Si vii to Si xiv K lines in spiral galaxy Full Tables 3 and 4 are only available at the CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or via http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/566/A105 NGC 3783. Holczer et al (2010) reported Si vi to Si xiv absorption lines in the slow warm absorber of bright Seyfert 1 galaxy MCG 6-30-15.…”

The outflow in Mrk 509

“…There are several sources for which the variability approach has been successful, but all have difficulties or large changes in flux. The Chandra data of NGC 3783 (Netzer et al 2003;Krongold et al 2005) were spread out over several months (with distance limits between 0.2 and 25 pc), while the XMM-Newton observations of this source ) occurred during a gradual rising phase without clear flux extrema, with no changes in the absorber properties leading to a minimum distance of 0.5-2 pc from the source, depending on the ionization component. In NGC 4051 the flux dropped by a factor of five (Steenbrugge et al 2009) leading to a 0.02-1 pc upper limit on the location of the absorbing gas.…”