The response of the Fe K\alpha line to changes in the X-ray illumination   of accretion discs
  X-ray reflection spectra from photoionized accretion discs in active galaxies are presented for a wide range of illumination conditions. The energy, equivalent width (EW) and flux of the Fe K line are shown to depend strongly on the ratio of illuminating flux to disc flux, Fx/Fdisc, the photon index of the irradiating power-law, \Gamma, and the incidence angle of the radiation, i. When Fx/Fdisc \leq 2 a neutral Fe K line is prominent for all but the largest values of \Gamma. At higher illuminating fluxes a He-like Fe K line at 6.7 keV dominates the line complex. With a high-energy cutoff of 100 keV, the thermal ionization instability seems to suppress the ionized Fe K line when \Gamma \leq 1.6. The Fe K line flux correlates with Fx/Fdisc, but the dependence weakens as iron becomes fully ionized. The EW is roughly constant when Fx/Fdisc is low and a neutral line dominates, but then declines as the line progresses through higher ionization stages. There is a strong positive correlation between the Fe K EW and \Gamma when the line energy is at 6.7 keV, and a slight negative one when it is at 6.4 keV. This is a potential observational diagnostic of the ionization state of the disc. Observations of the broad Fe K line which take into account any narrow component would be able to test these predictions. Ionized Fe K lines at 6.7 keV are predicted to be common in a simple magnetic flare geometry. A model which includes multiple ionization gradients on the disc is postulated to reconcile the results with observations. 
D.R. Ballantyne (1), R.R. Ross (2) ((1) IoA, Cambridge (2) College of   the Holy Cross)