Average charge states of Ne, Si, and Fe, as well as the combined column densities of Fe XIX and Fe XX, plotted as a function of the ionizing luminosity. The average charges are defined in Sect. 2. Each data point corresponds to an individual observation and was obtained via fitting with the slab model. The values obtained from the pion and tpho models are shown with red and blue curves, respectively. The χ-square values between the models and the data are displayed as numbers in the lower-right corner.