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

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1. Introduction

It is broadly accepted now that active galactic nuclei are powered by accretion onto a supermassive black hole and that the flow is a multi-phase (at least two-phase) medium. A hot optically thin phase is responsible for the X-ray and [FORMULA]-ray emission while a cool phase of considerable optical depth is responsible for the thermal optical/UV emission as well as for the preprocessing of a significant fraction of the radiation produced by the hot phase (for a review, see e.g. Mushotzky et al. 1993). There is mounting evidence based on the shape of [FORMULA] line profile as well as on the variability in soft and hard X-rays, that the release of the major part of the X-ray energy, of the thermal EUV/soft X-ray emission and the reprocessing of X-ray by cold material takes place in a very compact region, of order of [FORMULA] Schwarzschild radii and the cold matter is approximately in Keplerian motion (see e.g. Nandra et al. 1997a).

However, the geometrical arrangement of the two phases is still under debate. The three most viable models are: a disk with a hot corona, very optically thick clouds embedded in a hot medium, and moderately optically thick clouds also coexisting with a hot medium. In order to differentiate between these models detailed studies of all of them are necessary. The disk model is the best studied so far (e.g. Ross & Fabian 1993, Stern et al. 1995, Sincell & Krolik 1997). Optically thick clouds were studied for example by Sivron & Tsuruta (1993; see also the references therein). Clouds of moderate optical depth were broadly advertised as a 'free-free' emission model (e.g. Antonucci & Barvainis 1988). However, clouds which are optically thin for electron scattering are not a viable model for an AGN (Barvainis 1993, Collin-Souffrin et al. 1996, Kuncic et al. 1997; see also Lightman & White 1988). On the other hand clouds, which are optically thick for electron scattering but optically thin for absorption in the UV and moderately optically thick in the optical band (hereafter OTCM model), can reproduce the required fraction of the X-ray emission and roughly produce the correct multi-wavelength spectra, as well as provide enough material to support the nuclear activity (Collin-Souffrin et al. 1996; hereafter Paper I). As the model may have an advantage over clouds optically thick for absorption it deserves a detailed study of its observational consequences.

In this paper we concentrate on the comparison of the predictions of the OTCM with the broad band mean spectrum for radio quiet quasars determined by Laor et al. (1997). As direct analysis of the X-ray emission does not give any reliable answer to the question of the nature of the X-ray and [FORMULA]-ray emission due to the still poor determination of the spectra of radio quiet AGN in the [FORMULA]MeV range (see e.g. Gondek et al. 1997 for combined Seyfert 1 spectra from GRO) we analyze three representative cases of the sources of emission external to the clouds. As is customary and convenient, we call this incident radiation for the clouds the "primary" radiation although this is unjustified when the X-ray emission is itself the emission of the clouds comptonized by a hot medium.

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© European Southern Observatory (ESO) 1998

Online publication: September 14, 1998