A quasi-spherical inner accretion flow in Seyfert galaxies ?

Abstract

We study a phenomenological model for the continuum emission of Seyfert galaxies. In this quasi-spherical accretion scenario, the central X-ray source is constituted by a hot spherical plasma region surrounded by spherically distributed cold dense clouds. The cold material is radiatively coupled with the hot thermal plasma. Assuming energy balance, we compute the hard X-ray spectral slope Gamma and reflection amplitude R. This simple model enables to reproduce both the range of observed hard X-ray spectral slopes, and reflection amplitude R. It also predicts a correlation between R and Gamma which is very close to what is observed. Most of the observed spectral variations from source to source, would be due to differences in the cloud covering fraction. If some internal dissipation process is active in the cold clouds, darkening effects may provide a simple explanation for the observed distributions of reflection amplitudes, spectral slopes, and UV to X-ray flux ratios.