Ubiquitous Variability of X-Ray-absorbing Column Densities in Seyfert 2 Galaxies

By G. Risaliti, M. Elvis, F. Nicastro. 2001

271 citations recorded by the source corpus.

We present a study of the variations in the absorbing column density of 25 X-ray-defined Seyfert 2 galaxies, as inferred from hard X-ray observations, on timescales from months to several years. We show that a significant variation of NH (from 20% to 80%) is observed in almost all (22 of 25) of the sources with multiple X-ray observations, although X-ray absorption never vanishes. For a subsample of 11 sources observed at least five times, the typical variation time, as defined by a structure function, is less than 1 yr for both heavily absorbed (NH ~ 1023 cm-2) and moderately absorbed (NH ~ 1022 cm-2) sources. These variations rule out the simplest version of the unified models, based on a homogeneous obscuring torus, and suggest the presence of clumpy circumnuclear material on a scale well below a parsec. We propose a modification of the torus model in which an overabundance of slightly dusty broad emission-line region (BELR) clouds obscures the BELR. The BELR needs, like the torus, to have an axisymmetric structure. This model is closely related to that of Elvis for type 1 active galactic nuclei (AGNs). For lightly obscured AGNs (NH ~ 1022 cm-2), the structure function shows an increase at a timescale of ~5 yr, indicating a second absorber, most probably on a 5-10 pc scale associated with the host galaxy.

Subfields: Astronomy and Astrophysics, Nuclear and High Energy Physics

View Paper: https://www.semanticscholar.org/paper/c3212c80323a2729b1b0773d8d9399896f9ce5d0

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Citations in the corpus

Resolving the mid-infrared cores of local Seyferts (2009, background): “NGC 4388 Combined analysis from several missions shows a simple PL above a few keV absorbed by a columnNH=2.7×1023 cm−2, but with highly variable fluxes and obscuring column (Beckmann et al. 2004; Cappi et al. 2006; Iwasawa et al. 2003; Risaliti et al. 2002).”
TESTING THE UNIFICATION MODEL FOR ACTIVE GALACTIC NUCLEI IN THE INFRARED: ARE THE OBSCURING TORI OF TYPE 1 AND 2 SEYFERTS DIFFERENT? (2011, background): “Observational evidence in the X-rays and the MIR indicates that the strong AGN continuum source must be absorbed by obscuring material over a wide solid angle (see e.g., Antonucci & Miller 1985; Maiolino et al. 1998; Risaliti et al. 2002).”
Polycyclic Aromatic Hydrocarbons as a Tracer of Star Formation? (2004, result): “This range is in good agreement with the mean Seyfert 2 absorbing column, as measured directly from X-ray data (Risaliti et al. 2002), and with the results of Clavel et al. (2000).”
Three-dimensional radiative transfer models of clumpy tori in Seyfert galaxies (2008, background): “Risaliti et al. (2002) claim that the observed variability of these measurements on timescales from months to several yeas can be explained by a clumpy structure of the torus.”
Evolution of broad-line emission from active galactic nuclei (2013, background): “Direct observational evidence for the clumpy nature of the BLR comes from studies of X-ray variability (Risaliti et al. 2002) and from a detailed study of a set of absorption lines in a quasar outflow (Hall et al. 2007).”
ON THE DISAPPEARANCE OF THE BROAD-LINE REGION IN LOW-LUMINOSITY ACTIVE GALACTIC NUCLEI (2009, background): “Additionally, Risaliti et al. (2002) find that X-ray variability caused by the passage of absorbing clouds across the line of sight has a continuous behavior across the timescales generated by the motions of dust-free BLR clouds close to the AGN and the more distant, dusty torus clouds.”
The incidence of obscuration in active galactic nuclei (2013, background): “In the nearby Universe, the presence of small-scale (sub-parsec) absorbers has been inferred in a number of cases (Risaliti et al. 2002; Maiolino et al. 2010), with NGC 4151 being the prototypical example, well known since more than 30 years (Ives, Sanford & Penston 1976).”
AGN Dusty Tori. II. Observational Implications of Clumpiness (2008, background): “The other evidence is the finding by Risaliti et al. (2002) that the X-ray-absorbing columns in Seyfert 2 galaxies display time variations caused by cloud transit across the line of sight.”
Rapid Compton-thick/Compton-thin Transitions in the Seyfert 2 Galaxy NGC 1365 (2005, methodology): “Dramatic X-ray–absorbing column density changes (factors of 110) over a few years have been seen in several type 2 (narrow permitted line) Seyfert galaxies (Risaliti et al. 2002), ruling out a homogeneous absorber.”
Active galactic nuclei: what’s in a name? (2017, background): “…have shown that the X-ray and optical spectral properties of AGN can vary on relatively short timescales as a result of changes in the absorbing column density along the line of sight (e.g. Risaliti et al. 2002, 2005; Matt et al. 2003; MacLeod et al. 2016; Ruan et al. 2016; see also Sect.”
The Nuclear Reddening Curve for Active Galactic Nuclei and the Shape of the Infrared to X-Ray Spectral Energy Distribution (2003, background): “There is support for this idea from the frequently seen variations in X-ray column densities on timescales of less than a year (e.g., Risaliti et al. 2002), and the temporal variations in the position angles of the polarizations of broad lines in AGNs (e.g., Goodrich 1989; Martel 1998), which imply…”
Smooth and clumpy dust distributions in AGN: a direct comparison of two commonly explored infrared emission models (2012, unknown): “As there is evidence favouring both a clumpy distribution (e.g. Risaliti et al. 2002; Wittkowski et al. 2004; Tristram et al. 2007) and a smooth distribution (e.g. Ibar & Lira 2007), both smooth and clumpy models are still equally widely © 0000 RAS, MNRAS 000, used to explain the observed SEDs of…”
The AGN-obscuring Torus: The End of the “Doughnut” Paradigm? (2006, unknown): “From time variations of X-ray obscuration that indicate cloud motions across the line of sight, Risaliti et al. (2002, 2005) ﬁnd that some of the obscuring clouds are at radii considered typical of the AGN broad-line region (BLR).”
New Spectral Model for Constraining Torus Covering Factors from Broadband X-Ray Spectra of Active Galactic Nuclei (2018, background): “…is a possible, if not likely, scenario in AGN is abundant from detailed spectroscopy (e.g., Arévalo et al. 2014, Baloković et al. 2014, Annuar et al. 2015) and studies of line-of-sight absorption variability (e.g., Risaliti et al. 2002, Torricelli-Ciamponi et al. 2014, Markowitz et al. 2014).”