Absorption-Dominated Models

Alternatively, absorption-dominated models have been proposed by some authors. The mo- tivation behind such models has arisen from the discovery of multiple components of com- plex absorption varying in ionisation, column density and kinematics from high-resolution UV observations of Seyfert galaxies (Crenshaw, Kraemer & George 2003). Since the highest- ionisation lines in high-accretion-rate sources are suggestive of an origin in a disc wind (Gaskell 1982; Murray & Chiang 1998), this, coupled with the fact that winds have been theoretically predicted to be driven off accretion discs (King & Pounds 2003, possibly extending close to the black hole; Proga, Stone & Kallman 2000), has motivated the search for such absorption signatures via X-ray spectroscopy. The application of photoionisation models alongside direct detection of absorption lines (e.g. with grating data) through such X-ray observations has confirmed that absorbing gas exists with higher ionisation states and column densities than those observed in the UV band (e.g. Blustin et al. 2005; McKernan, Yaqoob & Reynolds 2007) and may be linked to large-scale outflows.

In absorption-dominated scenarios, the changes in the spectral shape of variable sources can be accounted for by allowing the line-of-sight covering fraction of a “partial-covering” absorber to vary (e.g. Miller, Turner & Reeves 2009). When the covering fraction is high, a large proportion of the continuum photons are absorbed and scattered out of the line-of- sight, hence reducing the observed flux at lower energies and producing the observed hard spectral shape, while there is no overall change when integrated over all solid angles. In this case, the “red wing” of the Fe Kα emission line is simply an artifact of the variations of the partial-coverer and the constancy in flux of the associated narrow component of emission can then be accounted for if it arises in more distant material (e.g. the torus). In many cases, the absorption models require covering fractions . 1 with the variability accounted for by allowing for changes in the covering fraction of tens of per cent (e.g. Reeves et al. 2002;

a

a

b

c

d

c

b

Figure 1.11: Spectral model of Miller, Turner & Reeves (2008) fitted to the mean broad-band Suzaku spectrum of MCG–6-30-15. Component (a) is the primary power-law continuum (absorbed by three warm-absorber zones), (b) is the partially-covered power law, (c) is a low-ionisation distant reflection component and (d) is a component of cosmic X-ray background (CXB) emission. Figure taken from Miller, Turner & Reeves (2008).

Turner et al. 2007). This scenario is consistent with the simple two-component models borne out of PCA whereby the variability is driven primarily by changes in a soft power-law-like component (of constant slope) superimposed on a quasi-constant hard component (e.g. Mrk 766, Miller et al. 2007; NGC 4051, Miller et al. 2010b). Temporal changes in the ionisation structure and/or the opacity of the warm absorber (e.g. Krongold et al. 2005) have also been considered to explain the spectral variability of some sources but it is often found that the most extreme variations require changes in the line-of-sight covering fraction.

One such AGN to be modelled in this way is MCG–6-30-15 (see Figure 1.11). Contrary to the reflection-dominated models of Miniutti et al. (2003), Miller, Turner & Reeves (2008) developed an absorption-dominated model based upon the detection of a wealth of discrete absorption features in the Chandra grating data (also see Miller, Turner & Reeves 2009).

wind?

~ g

accretion disk

scattering gas and inner disk reflection

wind?

absorbing gas

extended continuum source

partially covered direct N (3) H N (1) + N (2) H H absorbing gas N (1) + N (2) H H scattered ( < 100 r )

Figure 1.12: A schematic representation of the inner regions of the Mrk 766 models described in (and taken from) Turner et al. (2007). Here, direct, partially-covered and scattered emission components (all of which are absorbed by more distant Compton-thin gas) may all fall into the observer’s sightline. Note that a distant reflector is also present in the models but omitted here for clarity.

They account for the variability by allowing for changes in the line-of-sight covering fraction of a single warm-absorber zone which they argue most likely originates in a clumpy disc wind. Such a scenario is supported by the observation of an eclipse-like event in the lightcurve (McKernan & Yaqoob 1998) which can be interpreted as an occultation by the wind.

In the case of partial covering, common arguments against it have been based upon probability since for an absorber to partially cover the source, it should have some structure on a similar size scale to the source that it is obscuring. If the absorber is made up of discrete clouds then these clouds should be expected to be of comparable size to the source (e.g. BLR-sized clouds). Then, if the absorbing clouds are distant from the source, it is implied that it is not only somewhat coincidental that the clouds have passed into the line-of-sight but also that there must be a large distribution of clouds across this region if there is to be a significant chance of one falling across the observer’s sightline. However, it could be that the clouds exist at radial locations close to the source and the timescales of variations in some sources may suggest that this is plausible (e.g. NGC 1365; Risaliti et al. 2009, NGC 3516; Turner et al. 2008).

were to exist as part of a clumpy disc wind as opposed to being in the form of discrete clouds. Figure 1.12 shows a schematic representation of such a disc-wind scenario based on models developed for Mrk 766 (Turner et al. 2007). Clumpy absorbers comprising wide ranges of scale sizes are predicted in the disc-wind models of Proga, Ostriker & Kurosawa (2008) and, as mentioned above, such winds have been theoretically predicted to be driven off accretion discs (King & Pounds 2003). If this scenario is correct, it would help to counter the argument about probability since an equatorial disc wind would have a preferred plane and hence the observed spectral variations due to varying covering fractions would simply depend upon the observer’s viewing angle to the disc.

Turner & Miller (2009) point out that when fitting partial-covering models to X-ray spectra, they are often indistinguishable from models whereby a fully covered, absorbed continuum source is observed plus a significant ‘Thomson-scattered’ component that has been energy-independently scattered into the line-of-sight. In some cases, such models may provide alternative explanations for the spectra of some type-1 AGN but with high scattering fractions.

1.4.2.3 Additional Variability and Evidence for Compton-Thick Gas in Type-1