Absorption at the dust sublimation radius and the dichotomy between X-ray and optical classification in the Seyfert galaxy H0557-385★

Abstract

In this work, the analysis of multi-epoch (1995)(1996)(1997)(1998)(1999)(2000)(2001)(2002)(2003)(2004)(2005)(2006)(2007)(2008)(2009)(2010) X-ray observations of the Seyfert 1 galaxy H0557-385 is presented. The wealth of data presented in this analysis show that the source exhibits dramatic spectral variability, from a typical unabsorbed Seyfert 1 type spectrum to a Compton-thin absorbed state, on time scales of ∼ 5 years. This extreme change in spectral shape can be attributed to variations in the column density and covering fraction of a neutral absorbing medium attenuating the emission from the central continuum source. Evidence for Compton reflection of the intrinsic nuclear emission is present in each of the spectra, though this feature is most prominent in the low-state spectra, where the associated Fe emission line complex is clearly visible. In addition to the variable absorbing medium, a warm absorber component has been detected in each spectral state. Optical spectroscopy concurrent with the 2010 XMM-Newton observation campaign have detected the presence of broad optical emission lines during an X-ray absorption event. From the analysis of both X-ray and optical spectroscopic data, it has been inferred that the X-ray spectral variability is a result of obscuration of the central emission region by a clumpy absorber covering 80 per cent of the source with an average column density of N H ∼ 7×10 23 cm −2 , and which is located outside the broad line region at a distance from the central source consistent with the dust sublimation radius of the AGN.

XMM-Newtonreveals a Seyfert-like X-ray spectrum in thez= 3.6 QSO B1422+231

“…It is worth considering, however, that it is common in the local Universe as well to observe type I Seyferts that are absorbed in X-rays (see, for example, NGC 4151 and of NGC 4051, Warwick et al 1993;and Ponti et al 2006, respectively). In recent years, moreover, long-term monitoring of sources that have been displaying characteristics of type I objects from optical-UV to X-rays for decades showed that these sources may suddenly change their X-ray properties to those typical of type II objects and maintain them for years (see, for example, H0557-385 and NGC 5548, Coffey et al 2014;Kaastra et al 2014).…”

“…In some Seyfert galaxies, clouds, probably the same as are forming the broad line regions, have been observed to cross the line of sight and produce eclipses (Risaliti et al 2007). Similarly, in other objects, the changes in absorbing column have been associated with outflowing clouds (Coffey et al 2014;Kaastra et al 2014;Arav et al 2015). The changes in the absorption properties are thought to be responsible for at least part of the well-known AGN variability seen in X-rays (see, for example, NGC 1365, Risaliti et al 2005Risaliti et al , 2009; see also Miller et al 2008Miller et al , 2010Markowitz et al 2014).…”

Anatomy of the AGN in NGC 5548

“…Despite being recorded here for the first time in this source, this kind of "obscuration event" may not be unique to NGC 5548. Albeit with less quality data, other fast and variable obscuration events have been recorded in the past in a number of intermediate-type or type 1 sources, such as NGC 3227 (Lamer et al 2003), NGC 1365 (Risaliti et al 2005), NGC 4388 (Elvis et al 2004), NGC 4151 (Puccetti et al 2007), PG1535+547 (Ballo et al 2008), NGC 7582 ), H0557-385 (Longinotti et al 2009;Coffey et al 2014), Mrk 766 (Risaliti et al 2011), SwiftJ2127.4+5654 (Sanfrutos et al 2013), Mrk 335 (Longinotti et al 2013), NGC 5506 (Markowitz et al 2014), NGC 985 (Ebrero et al 2016a,b), Fairall 51 (Svoboda et al 2015), and ESO 323-G77 (Sanfrutos et al 2016), not to mention the systematic surveys by Malizia et al (1997), Markowitz et al (2014), andTorricelli-Ciamponi et al (2015). Based on various arguments, such as the ones above, and including the spectral properties and variability timescales of the absorbers, most authors have associated the origin of the absorbing clouds as either broad line region (BLR) clouds, a clumpy torus, or the inner boundary of a dusty torus.…”

Anatomy of the AGN in NGC 5548

“…In recent years, evidence for variability due to cold X-ray absorption in both type 1 and type 2 AGN has increased. In many cases, changes in the absorber column density and/or covering fraction, on timescales of a few hours or a few years, have been reported, e.g., in NGC 4388 (Elvis et al 2004), NGC 4151 (Puccetti et al 2007), and 1H 0557-385 (Coffey et al 2014). On these same timescales, the X-ray absorbing column density may even drastically switch from Compton thick to Compton thin as observed in several type 2 AGN, e.g., NGC 7582 (Piconcelli et al 2007), UGC 4203 (Risaliti et al 2010), and NGC 454 (Marchese et al 2012).…”