Average observed spectra (data points) of the L − z subsamples with their average simulated continua (continuous line) and the one-sigma confidence limits (dashed lines). For comparison, in the bottom-left panel we represent with a dotted line the result of adding to the simulated continuum a Gaussian centered at 6.4 keV, with σ = 120 eV, as obtained from the simulations of the Fe line (Sect. 3.3).