[Paper Review] Anatomy of the AGN in NGC 5548. VI. Long-term variability of the warm absorber

J. Ebrero, J. S. Kaastra+14 more|Leiden Repository (Leiden University)|Jan 11, 2016

Galaxies: Formation, Evolution, Phenomena58 references19 citations

TL;DR

This study analyzes long-term X-ray variability in the warm absorber (WA) of NGC 5548 using archival XMM-Newton and Chandra grating spectra from 1999–2007, modeling ionization changes via updated photoionization codes to infer gas density and distance constraints. It finds six stratified ionization phases at pc-scale distances, with a total kinetic luminosity of only 0.2% of the bolometric luminosity—too low to drive significant feedback in the host galaxy.

ABSTRACT

(Abridged) The archetypal Seyfert 1 galaxy NGC 5548 was observed in 2013-2014 in the context of an extensive multiwavelength campaign, which revealed the source to be in an extraordinary state of persistent heavy obscuration. We re-analyzed the archival grating spectra obtained by XMM-Newton and Chandra between 1999 and 2007 in order to characterize the classic warm absorber (WA) using consistent models and up-to-date photoionization codes and atomic physics databases and to construct a baseline model that can be used as a template for the WA in the 2013 observations. The WA in NGC 5548 is composed of 6 distinct ionization phases outflowing in 4 kinematic regimes in the form of a stratified wind with several layers intersected by our line of sight. If the changes in the WA are solely due to ionization or recombination processes in response to variations in the ionizing flux among the different observations, we are able to estimate lower limits on the density of the WA, finding that the farthest components are less dense and have a lower ionization. These limits are used to put stringent upper limits on the distance of the WA components from the central ionizing source, with the lowest ionization phases <50, <20, and <5 pc, respectively, while the intermediately ionized components lie at <3.6 and <2.2 pc from the center, respectively. The highest ionization component is located at ~0.6 pc or closer to the AGN central engine. The mass outflow rate summed over all WA components is ~0.3 Msun/yr, about six times the nominal accretion rate of the source. The total kinetic luminosity injected into the ISM is a small fraction (~0.03%) of the bolometric luminosity of the source. After adding the contribution of the UV absorbers, this value augments to ~0.2% of the bolometric luminosity, well below the minimum amount of energy required by current feedback models to regulate galaxy evolution.

Motivation & Objective

• To characterize the long-term variability of the warm absorber (WA) in NGC 5548 using consistent photoionization models and up-to-date atomic data.
• To establish a baseline physical model of the WA for use in interpreting the 2013–2014 XMM-Newton observations, during which the source was heavily obscured.
• To estimate lower limits on the density and upper limits on the distance of WA components by modeling ionization changes in response to flux variations.
• To assess the kinetic luminosity and mass outflow rate of the WA and evaluate its potential role in AGN feedback mechanisms.

Proposed method

• Used the latest version of the photoionization code CLOUDY and the SPEX fitting package to model X-ray grating spectra from XMM-Newton and Chandra.
• Applied consistent photoionization models and updated atomic physics databases to ensure reproducibility and accuracy across archival observations (1999–2007).
• Assumed that observed WA variability was driven solely by ionization/recombination processes in response to flux changes, holding outflow velocity and column density constant.
• Calculated lower bounds on gas density from ionic column density variations, using ions dominant in each WA component.
• Converted density limits into upper bounds on distance from the central engine using ionization parameter and flux variations.
• Summed mass outflow rates and kinetic luminosities across all WA components, comparing total values to bolometric luminosity and feedback model thresholds.

Experimental results

Research questions

• RQ1 What are the ionization and kinematic structures of the warm absorber in NGC 5548 across multiple archival observations (1999–2007)?
• RQ2 How do changes in the ionization state of the WA components correlate with variations in the ionizing flux from the AGN?
• RQ3 What are the lower limits on the density and upper limits on the distance of the WA components, assuming ionization/recombination is the sole driver of variability?
• RQ4 What is the total mass outflow rate and kinetic luminosity of the X-ray WA, and how does it compare to the bolometric luminosity and feedback model requirements?
• RQ5 Can the observed WA properties be reconciled with disk-driven or torus-driven wind models, and what does this imply for feedback mechanisms?

Key findings

• The warm absorber in NGC 5548 consists of six distinct ionization phases distributed across four kinematic regimes with outflow velocities of ~-250, ~-550, ~-800, and ~-1200 km s⁻¹.
• The lowest ionization components are located at distances of <50 pc, <20 pc, and <5 pc from the central source, while intermediate-ionization components lie within <3.6 pc and <2.2 pc.
• The highest ionization component is confined to within ~0.6 pc (and possibly as close as ~0.4 pc) of the central engine.
• The total mass outflow rate across all WA components is ~0.3 M☉ yr⁻¹, approximately six times the nominal accretion rate of the black hole.
• The kinetic luminosity of the X-ray WA alone is ~0.03% of the bolometric luminosity, rising to ~0.2% when UV absorbers are included.
• This total kinetic luminosity remains insufficient to drive significant feedback, being two orders of magnitude below the ~0.5–5% of bolometric luminosity required by current feedback models to affect host galaxy evolution.