Astronomy and Astrophysics – Astrophysics
Scientific paper
Feb 1996
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1996rftu.proc..443c&link_type=abstract
International Conference on X-ray Astronomy and Astrophysics: Röntgenstrahlung from the Universe, p. 443 - 444
Astronomy and Astrophysics
Astrophysics
Active Galactic Nuclei: Plasma, Quasars: Plasma, Quasars: X Rays, Quasars: Uv Radiation, Active Galactic Nuclei: X Rays, Active Galactic Nuclei: Uv Radiation
Scientific paper
The authors examine models of realistic free-free emission: 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. They present detailed computations of the emitted spectra, the total energy budget (including the requirement for the mass accretion flow) and the description of the interaction with the hot optically thin medium responsible for hard X-ray emission. They also apply time variability constraints. This systematic approach allows to constrain viable cloud models to clouds optically thick for electron scattering, radiatively heated in Seyfert galaxies but perhaps with some contribution from mechanical heating in quasars. The successful scenario is the following: a quasi-spherical or thick disk-like distribution of accreting clouds is most probably embedded in its central part in hot plasma. Smaller covering factor for clouds, 0.5-0.9, and smaller optical depth for electron scattering of a single cloud, ≡50, are more appropriate for Seyfert galaxies while larger values (>0.9 and ≡500 respectively) are deduced for quasars. As a consequence, the authors expect higher Eddington ratios in quasars than in Seyfert galaxies. In their model UV radiation is due to pure emission, and observed X-rays are due to a combination of reflection and primary spectrum so absorption features are weak in the X-ray range in spite of the large covering factor. The primary emission may represent synchrotron emission; the alternative possibility of primary hard X-ray emission being the cloud emission Comptonized by hot plasma is postponed for future study. Although the cloud model seem to be able to fulfill observational constraints, it is not a unique possibility.
Collin-Souffrin Suzy
Czerny Bozena
Dumont Anne-Marie
Z˙Ycki P. T.
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