Density diagnostics of ionized outflows in active galactic nuclei

Abstract

Context. Ionized outflows in active galactic nuclei (AGNs) are thought to influence their nuclear and local galactic environment. However, the distance of the outflows with respect to the central engine is poorly constrained, which limits our understanding of their kinetic power as a cosmic feedback channel. Therefore, the impact of AGN outflows on their host galaxies is uncertain. However, when the density of the outflows is known, their distance can be immediately obtained from their modeled ionization parameters. Aims. We perform a theoretical study of density diagnostics of ionized outflows using absorption lines from metastable levels in Be-like to C-like cosmic abundant ions. Methods. With the new self-consistent PhotoIONization (PION) model in the SPEX code, we are able to calculate detailed level populations, including the ground and metastable levels. This enables us to determine under what physical conditions the metastable levels are significantly populated. We then identify characteristic lines from these metastable levels in the 1−2000 Å wavelength range. Results. In the broad density range of n H ∈ (10 6 , 10 20 ) m −3 , the metastable levels 2s2p ( 3 P 0−2 ) in Be-like ions can be significantly populated. For B-like ions, merely the first excited level 2s 2 2p ( 2 P 3/2 ) can be used as a density probe. For C-like ions, the first two excited levels 2s 2 2p 2 ( 3 P 1 and 3 P 2 ) are better density probes than the next two excited levels 2s 2 2p 2 ( 1 S 0 and 1 D 2 ). Different ions in the same isoelectronic sequence cover not only a wide range of ionization parameters, but also a wide range of density values. On the other hand, within the same isonuclear sequence, those less ionized ions probe lower density and smaller ionization parameters. Finally, we reanalyzed the high-resolution grating spectra of NGC 5548 observed with Chandra in January 2002 using a set of PION components to account for the ionized outflow. We derive lower (or upper) limits of plasma density in five out of six PION components based on the presence (or absence) of the metastable absorption lines. Once atomic data from N-like to F-like ions are available, combined with the next generation of spectrometers that cover both X-ray and UV wavelength ranges with higher spectral resolution and larger effective areas, tight constraints on the density and thus the location and kinetic power of AGN outflows can be obtained.

Anatomy of the AGN in NGC 5548

“…The absorption features caused by the warm absorber are accounted for using six PION components. In both the 2013-2014 and 2016 spectra, the hydrogen column densities (N H ), outflow velocities (v out ), and microscopic turbulence velocities (v mic ) of the PION components are fixed to values given in Mao et al (2017), which are obtained by fitting the high quality 2002 Chandra grating spectra of NGC 5548. Nonetheless, the ionization parameters (log ξ) can differ for these three epochs, due to the variability of the obscurer, as well as the changes of the intrinsic SED (Cappi et al 2016).…”

“…Ebrero et al (2016) estimate the distance of the warm absorber components based on variability, with component A and B at least 10 pc away from the central engine and components C to F within 5 pc from the central engine. Nonetheless, based on a spectral analysis using the density sensitive metastable absorption lines, Mao et al (2017) found the warm absorber component B is even closer (<0.23 pc, 3σ upper limit) than the optical NELR, rather than further away. Ebrero et al (2016) constrain the lower limit of the distance of the warm absorber component B based on the non-detection of variability on a timescale of 500 days.…”

“…A different approach for the photoionized plasma modeling is used here (see also Mao et al 2017). We use SPEX (v3.03.02 Kaastra et al 1996) to fit the entire observed spectra on the fly.…”

R-matrix electron-impact excitation data for the C-like iso-electronic sequence

“…Plasma models built on extensive and accurate atomic databases are essential to determine plasma parameters that span several orders of magnitudes in the parameter space. For instance, the density of photoionized outflows in the vinicity of black holes can vary from ∼ 10 3−5 cm −3 (C iii, Gabel et al 2005;Arav et al 2015) to 10 6−14 cm −3 (Si ix and Fe xxi, Miller et al 2008;King et al 2012;Mao et al 2017) . Currently, the status of level-resolved electron-impact excitation data of C-like ions is rather poor.…”

X-ray spectra of the Fe-L complex

“…As a result, the population of the ground state decreases, and the related spectral features, e.g., lines from ground excitation, become weaker. On the other side, the transitions from the excited states become substantially more important (Mao et al 2017).…”