X-ray illuminated accretion discs: a model for the iron line in NGC 3227

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Abstract
Substantial amounts of optically thick, ‘cold’ gas close to the central regions of Active Galactic Nuclei reprocess the underlying non-thermal continuum. In the X-ray band, features due to bound–free absorption, fluorescence, and electron scattering, as well as thermal re-emission are imprinted on the observed spectrum. Iron K-shell features are particularly evident, and provide a strong diagnostic of the geometry and kinematics of the innermost regions of the accretion flow. Here we consider a specific geometry of the cold material, namely that of a Keplerian accretion disc illuminated by an external X-ray source. The results from Monte Carlo simulations of the composite spectrum are fitted to Ginga data from NGC 3227, a Seyfert galaxy with an established emission feature. We find that the form of the X-ray spectrum of NGC 3227 is consistent with such a model if it is viewed almost face-on. The consequences of our findings for this and other sources are discussed briefly.

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