The response of the Fe K 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 Ka line are shown to depend strongly on the ratio of illuminating flux to disc flux, F x /F d i s c , the photon index of the irradiating power law, Γ, and the incidence angle of the radiation, i. When F x /F d i s c ≤ 2 a neutral Fe Ka line is prominent for all but the largest values of Γ. At higher illuminating fluxes an He-like Fe Ka 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 Ka line when Γ ≤ 1.6. The Fe Ka line flux correlates with F x /F d i s c , but the dependence weakens as iron becomes fully ionized. The EW is roughly constant when F x /F d i s c 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 Ka 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 Ka line, which take into account any narrow component, would be able to test these predictions. Ionized Fe Ka 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.