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Astron. Astrophys. 338, 386-398 (1998)

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5. Conclusions

Our results suggest that the quasar spectra can be explained within the frame of the optically thick cloud model (OTCM), i.e. as originating in a distribution of irradiated clouds optically thick for electron scattering. One of the important points is that the spectrum in the soft X-ray band is attributed mostly to the reflection from the illuminated partially ionized sides of the clouds. We are not able to definitively distinguish at present if the primary emission comes from the unspecified central source in the form of synchrotron emission or if it originates in the central hot plasma cloud by upscattering of soft photons coming from the clouds. However, we favor the second possibility as the presented results already lead to more self-consistent description for that case.

An alternative picture based on the accretion disk and a corona requires very hot plasma to form the hard X-ray emission and moderately hot but optically thicker plasma to explain the soft X-ray quasar spectra. These two pictures differ with respect to the presence of spectral features since the reflection is accompanied by emission lines and absorption edges which may be used for the purpose of diagnostics. These features are expected to be smeared to some extent by the cloud motion which may help to distinguish them from features arising in a distant warm absorber.

Within the frame of OTCM, the overall shape of the spectrum is very sensitive to the value of the ionization parameter [FORMULA]. Interestingly, the value favored by the comparison of the model to the mean quasar spectrum of Laor et al. (1997) is [FORMULA], in agreement with the energy of the Fe [FORMULA] line in moderately bright quasars (Nandra et al. 1997b). Present results allow too low a contribution (by a factor of a few) from the primary emission to account for the observed equivalent width of [FORMULA] but we expect that more advanced computations may remove this problem (Abrassart et al., in preparation).

The problem which remains is the prediction of the strong spectral features in the optical/UV band. Observations seem to indicate that no such features are observed in quasar spectra while both OTCM and advanced accretion disk models predict such atomic features although kinematic effects connected with the cloud motion decrease them to some extent. We will address this problem in the future.

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© European Southern Observatory (ESO) 1998

Online publication: September 14, 1998