The response of the Fe K\alpha line to changes in the X-ray illumination of accretion discs

Abstract

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.