Iron Kα lines from ionized discs in Z-type X-ray binaries

The properties of the iron Kα line emitted by a photoionized accretion disc in a Z-type X-ray binary system are determined for different values of the mass accretion rate and disc inclination angle. We consider a geometry where the accretion disc is illuminated by X-rays emitted from a spherical, optically thick Comptonized cloud, representing the magnetosphere on the horizontal branch, and the magnetosphere plus puffed accretion disc on the normal and flaring branches. We find that line profiles can be complex due to Doppler, relativistic, shadowing, line-blending, resonant trapping and other effects. The iron Kα line equivalent width, intrinsic width and centroid energy vary in a tortuous manner as a function of the accretion rate, cloud radius and inclination angle. We examine the reasons for this behaviour and discuss how it will affect inferences about the physical states of Z-sources based on their iron line properties. We compare our results with available data, in particular on Cygnus X-2, and indicate how tests of Z-source models could be made using high-energy-resolution measurements like those expected from ASCA.