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3.3.2 The Accretion Disk Model

The standard Shakura-Sunyaev disk model is difficult to reconcile with the almost simultaneous variability in UV, optical and X-rays in NGC 4151 and NGC 5541, indicating that illumination by the hard X-ray source may give rise to a large fraction of the UV-optical flux through reprocessing. 99, 107However, it could be that these sources are atypical examples, as pointed out before. Indeed, there seems to be no correlation between variations in UV-optical and X-rays in most AGN. Ulrich et al. 98 suggested instead that the varying part of the optical flux is the low-energy tail of the UV-bump, thereby producing simultaneous UV-optical variability. They also argued that the latter may be due to disk instabilities in the inner region.

However, one advantage of the illumination model is that the LIL may be explained this way. Collin-Souffrin and collaborators (e.g., Ref. 108) have developed such a model, whose basic assumption is an extended accretion disk (to at least R ~ 104rg). One problem with the model has been the condition of negligible self-gravity, which imposes an upper limit on dotm. In F9 this would translate into a highly supersonic turbulence, which seems unphysical. A recent version 32, 103 avoided this problem by adopting a small dotm-value. The LIL emission region was confined to R ~ 102-104rg, since line saturation for R ltapprox 102rg and inadequate heating for R gtapprox 104rg inhibit line formation in other regions. The result seems able to meet quite a number of observational constraints. Thus, the line widths may be explained in terms of kinematics, since vK(103rg) ~ 103 km s-1, where vK is the Keplerian rotational velocity. The large column density and covering factor required to explain the intensity of the LIL and the formation and confinement problems may also be accounted for. Furthermore, the constancy of line ratios may be explained by quasi-thermalization of the lines.

The model requires that a sizeable fraction (~ 1/2) of the bolometric luminosity is emitted in hard X-rays or gamma-rays. The bulk of the UV luminosity should then be emitted by the accretion disk, of which a part is due to reprocessing. By combining the fitting procedures for the UV continuum and line emission from the disk, the obtained accretion rate was much lower than the one estimated from the bolometric luminosity. One explanation may be that the ``real'' accretion rate is higher, but that only part of the liberated gravitational energy comes out in optical-UV. The rest may be dissipated in a hot, optically thin phase. as in the Haardt and Maraschi model discussed above.

The applicability of these results to other AGN such as OSOs remains an open issue. Even though rapid broad line variability has been detected in QSOs, the under-sampling usually precludes definite conclusions. The BLR lines also seem to respond to a continuum change in a very complicated fashion. Furthermore, some sources show rapid continuum variability, but no line variability. Such null-results may yield interesting constraints on, e.g., the BLR geometry.

To summarize, a picture of a much smaller BLR and brighter continuum source than previously thought has emerged. The most promising method to map out the BLR may involve further developments of the cross-correlation technique, such as the echo mapping (1D, 2D or 1D + time) proposed by Welsh and Horne 109 and further discussed by Horne.110 The number of unsettled issues is still large, some of which include:

- how are the clouds formed and destroyed?
- if confinement is required, how is that achieved?
- what determines the kinematics, and what motion dominates the velocity field?
- is the overall geometry spherical (as in the standard model), or flattened (as in the accretion disk model), or a combination of both?
- how is the intermediate torus related to the narrow-line region (NLR), the BLR and the accretion disk?
- are jets and the BLR clouds somehow connected?

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