The response of the Fe Kα 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 Kα 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, Γ, and the incidence angle of the radiation, i. When Fx/Fdisc6 2 a neutral Fe Kα line is prominent for all but the largest values of Γ. At higher illuminating fluxes a He-like Fe Kα 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 Kα line when Γ 6 1.6. The Fe Kα 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 Kα EW and Γ 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 Kα line which take into account any narrow component would be able to test these predictions. Ionized Fe Kα 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.