A cloud model of active galactic nuclei: the iron K-alpha line diagnostics

Abstract

The origin and the profile of the iron K-alpha line from active galactic nuclei is being studied. The model with a quasi-spherical distribution of clouds around a compact galactic nucleus is explored in which the intrinsic spectral feature around 6-7 keV is substantially affected by Comptonization of primary X-rays. The predicted spectral profiles are influenced mainly by motion of the clouds in strong gravitational field of the centre, and by orientation of the source with respect to the observer. The case of the Seyfert galaxy MCG-6-30-15 is examined in more detail and free parameters of the model are determined. A robust result is obtained: acceptable fits to the observed profile correspond to the spherical distribution of the clouds with strong self-obscuration effects and with rather high value of ionization parameter. This is a different situation from that considered in the model of a cold illuminated disc. Nevertheless, geometrically flat (disc-type) configurations are not rejected.