The thermal instability of the warm absorber in NGC 3783

Abstract

Context. The X-ray absorption spectra of active galactic nuclei frequently show evidence of winds with velocities in the order of 10 3 km s −1 extending up to 10 4 km s −1 in the case of ultra-fast outflows. At moderate velocities, these winds are often spectroscopically explained by assuming a number of absorbing clouds along the line of sight. In some cases it was shown that the absorbing clouds are in pressure equilibrium with each other. Aims. We assume a photo-ionized medium with a uniform total (gas+radiation) pressure. The irradiation causes the wind to be radiation pressure compressed (RPC). We attempt to reproduce the observed spectral continuum shape, ionic column densities, and X-ray absorption measure distribution (AMD) of the extensively observed warm absorber in the Seyfert galaxy NGC 3783. Methods. We compare the observational characteristics derived from the 900 ks Chandra observation to radiative transfer computations in pressure equilibrium using the radiative transfer code titan. We explore different values of the ionization parameter ξ of the incident flux and adjust the hydrogen-equivalent column density, N 0 H , of the warm absorber to match the observed soft X-ray continuum. From the resulting models we derive the column densities for a broad range of ionic species of iron and neon and a theoretical AMD that we compare to the observations. Results. We find an extension of the degeneracy between ξ and N 0 H for the constant pressure models previously discussed for NGC 3783. Including the ionic column densities of iron and neon in the comparison between observations and data we conclude that a range of ionization parameters between 4000 and 8000 erg cm s −1 is preferred. For the first time, we present theoretical AMDs for a constant pressure wind in NGC 3783 that correctly reproduces the observed level and is in approximate agreement with the observational appearance of an instability region. Conclusions. Using a variety of observational indicators, we confirm that the X-ray outflow of NGC 3783 can be described as an RPC medium in pressure equilibrium. The observed AMD agrees with a uniformly hot or a uniformly cold thermal state. The measured ionic column densities suggest that the wind tends to the uniformly cold thermal state. The occurrence of thermal instability in the warm absorber model may depend on the computational method and the spatial scale on which the radiative transfer is solved.

Evidence for radiation pressure compression in the X-ray narrow-line region of Seyfert galaxies

“…Similar troughs are observed in the absorption measure distributions of warm absorbers (e.g. Holczer et al 2007;Behar 2009;Adhikari et al 2015;Goosmann et al 2016).…”

Thermally driven wind as the origin of warm absorbers in AGN

“…They interpret a distinct dip in this distribution for the warm absorber gas in several objects as evidence for the thermal instability (see Behar 2009;Holczer & Behar 2012). The thermal instabilities are always triggered in material with large enough column density (radiation pressure confinement: Stern et al 2014;Goosmann et al 2016;Adhikari et al 2019). Fig.14 shows the temperature versus column density into an irradiated slab in pressure balance for this SED and black hole mass.…”

Multi-wavelength campaign on NGC 7469

“…This information derived from X-ray and UV spectra is valuable for matching the observations with the theoretical outflow mechanisms. The AMD shape can be explained by absorbing clouds in photoionisation equilibrium (Różańska et al 2006;Adhikari et al 2015;Goosmann et al 2016). However, A72, page 11 of 14 A&A 615, A72 (2018) a gap is often observed in the middle of the AMD, producing a bi-modal distribution (e.g.…”