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S. Nayakshin, D. Kazanas, T. Kallman (NASA/GSFC)
We present a simple analytical approach (which approximates our earlier detailed calculations) to the problem of the X-ray reflection and iron line formation in accretion disks. In particular, we calculate the Thomson depth of the Compton-heated skin on the top of the accretion disk as a function of the following parameters: (i) the dimensionless accretion rate; (ii) the magnitude of the incident X-ray flux; (iii) the Compton temperature, (which is easily determined once the spectral index of the ionizing radiation is known), and the radius in the disk. Using this analytical approach, we test several accretion disk models on whether each of them can explain the X-ray ``Baldwin'' effect. We find that, in general, for X-ray luminosities greater than a tenth of the Eddington value, the hot ionized skin in the innermost parts of the disk become Thomson thick (thus explaining X-ray Baldwin effect). We show examples of X-ray reflection spectra and list theoretical predictions about Fe line EW and the absorption edge. Whereas different accretion disk geometries may qualitatively reproduce the observations, they are quantitatively different, and hence one can hopefully distinguish between these based on future more detailed data on the behavior of the iron lines in high luminosity AGN.