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Astron. Astrophys. 348, 71-76 (1999)

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4. Discussion and conclusion

The hard X-ray radiation, coming from the center, can illuminate the outer parts of the nonplanar disk, where optical emission lines will be emitted as a result of reprocessing. Profiles of such lines are modeled here (Fig. 4). Changing some basic parameters, such as the hard X-ray luminosity [FORMULA], the source height [FORMULA], [FORMULA], etc., a large variety of asymmetric profiles, displaced in frequency from the systemic velocity profiles can be produced.

Signatures of warped disk profiles can be seen in many objects (for instance 3C 227, Mkn 668, 3C 390.3, etc., Eracleous & Halpern 1994). It is possible that these objects contain an illuminated warped disk, but of course, other explanations can not be excluded. However, most of the AGN show symmetric profiles, without frequency shifts. On average, no more than roughly 10-15% of the broad line emission in AGN can be reproduced by an irradiated nonplanar disk. We note that this value is close to the covering factor of such a disk. One may conclude, therefore, that the bulk of the line emission in AGN arises from some more or less spherical or conical structure (a system of clouds or star atmospheres, a jet), with a covering factor which is close to unity, but not from a irradiated thin disk. In that case, the presence of an inclined disk structure will cause only asymmetry of lines or additional peaks, displaced in frequency from the main profile. There are several possibilities for objects where no such asymmetries or displaced peaks are observed.

  • i) The presence of a thin disk in AGN is quite unusual. The disk may also be thick (slim, advection dominated - Rees 1984; Blandford & Begelman 1998; Chakrabarti 1998; Park & Ostriker 1998).

  • ii) The central black hole is nonrotating and the disk is not warped. In this case, if the irradiation is possible, the profiles should be symmetric and double-peaked, which is also seldom observed, however. Similar profiles should be observed in case of a disk transparent to visual light and/or X-rays.

  • iii) The spins of the infalling gas and the hole are aligned and the disk is not warped. This is possible if the accreting matter is supplied from a single direction for a long period of time. As a result the black hole would align its spin with the spin of the accreting matter on relatively short time scale - [FORMULA] (Scheuer & Feiler 1996; Natarajan & Pringle 1998).

  • iv) The disk is warped, but the illuminating source is positioned far above the disk plane - at [FORMULA] or more (or a huge reflecting corona with a significant optical depth is present). In this case a small inclination of the disk would not affect significantly the line profiles.

Nevertheless, many objects reveal asymmetries and displaced peaks, which could be successfully reproduced by a warped accretion disk. These profiles depend strongly on the twisting structure, the illuminating source geometry and the line of sight to the observer. In principle they can be modeled with high signal-to-noise spectra of the object by varying the model parameters. Knowledge of the twisting structure may allow the determination of some very important characteristics of AGN, at first order the spin momentum of the black hole and the viscosity parameter of the accretion disk. Their determination by other methods is still unreliable.

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© European Southern Observatory (ESO) 1999

Online publication: July 16, 1999