The response of the FeKα 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 Kalpha line are shown to depend strongly on the ratio of illuminating flux to disc flux, F (x) /F (disc) , the photon index of the irradiating power law, Gamma, and the incidence angle of the radiation, i . When F (x) /F (disc) less than or equal to2 a neutral Fe Kalpha line is prominent for all but the largest values of Gamma. At higher illuminating fluxes an He-like Fe Kalpha line at 6.7 keV dominates the line complex. With a high-energy cut-off of 100 keV, the thermal ionization instability seems to suppress the ionized Fe Kalpha line when Gammaless than or equal to1.6. The Fe Kalpha line flux correlates with F (x) /F (disc) , but the dependence weakens as iron becomes fully ionized. The EW is roughly constant when F (x) /F (disc) 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 Kalpha 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 Kalpha line, which take into account any narrow component, would be able to test these predictions. Ionized Fe Kalpha lines at 6.7 keV are predicted to be common in a simple magnetic flare geometry. A model that includes multiple ionization gradients on the disc is postulated to reconcile the results with observations.