1996A&A...314..393C

We examine four models for the optical/UV/soft X-ray emission of AGN: a mechanically heated plasma, either optically thin or thick for electron scattering, and a radiatively heated plasma, either optically thin or thick for electron scattering. We present detailed computations of the emitted spectra, and we discuss the total energy budget (including the requirement for the mass accretion flow), and the interaction with the hot optically thin medium responsible for hard X-ray emission. We show that in no case the spectrum can be considered as due to a pure free-free process: lines and/or free-bound continuum are always important. We can eliminate mechanical heating on the basis of variability time scales and of a detailed fit of the spectrum in the optical/UV/X-ray range. If the system of clouds is radiatively heated, the energy budget requires that it is optically thick to electron scattering. Finally, we conclude that a system of radiatively heated clouds, optically thick for electron scattering and having a covering factor close to unity, is a viable model for Seyfert galaxies. The same model is applicable for quasars although one cannot rule out in their case a small contribution from mechanical heating. Therefore we propose a new model for the emission of the optical to X-ray spectrum in AGN, made of a quasi-spherical (or a thick disk) distribution of mildly optically thick clouds. Smaller covering factor, 0.5-0.9, and smaller optical depth for electron scattering, ∼50, are more appropriate for Seyfert galaxies while larger values (>0.9 and ∼500 respectively) are deduced for quasars. As a consequence, we expect higher Eddington ratios in quasars than in Seyfert galaxies. One peculiarity of the model is that UV is due to pure emission, and X-rays to a combination of reflection and primary spectrum and that absorption features are weak in the X-ray range in spite of the large covering factor. Although it is attractive, this cloud scenario is not unique for the UV-X emission of AGN, with two others being: reprocessing clouds radiating as black bodies and accretion disks with hot coronae. We discuss the prospects of differentiation between these possibilities.