X-ray illumination of accretion disks is an invaluable diagnostic of the structure of these disks because of the associated iron K \alpha emission .
Here we point out that the resulting reflected spectra depend very sensitively on the geometry of the X-ray source , and that this fact can be efficiently used to test these models observationally .
In particular , we discuss three different accretion disk geometries : the “ lamppost model ” , accretion disks with magnetic flares , and the model with a full corona overlying a cold thin disk .
We show that in the case of the lamppost model , unless the X-ray luminosity of the central source is larger than that of the cold disk by a factor of 10 or more , a significant fraction of iron in the ionized skin of the disk is in the hydrogen and helium-like ions .
Because these ions have large fluorescence yields , the resulting reflected spectra look strongly ionized , with Equivalent Width ( EW ) of the line increasing with X-ray luminosity L _ { x } up to the maximum of \sim 500 eV .
This situation contrasts to the magnetic flare model , where the large X-ray flux near flares completely ionizes the skin of the disk and thus the resulting spectra appear to be that from a neutral material .
The line EW in this model anti-correlates with X-ray luminosity , and becomes arbitrarily small when L _ { x } is a good fraction of the Eddington luminosity .
Finally , in the full corona case , due to the additional pressure and weight of the corona , the gas pressure ( and its density ) below the corona is always large enough to make the gas very cool and effectively neutral .
No highly ionized skin forms in such a model .
If the corona is Thomson thin , then EW of the line does not depend on the accretion disk or corona luminosities for the full corona model .