Difference spectrum between the highest and lowest α_ox subgroups from the left panel of Fig. 4 highlighting the spectral shape of the emerging soft excess component. The binned yellow data follow a line in this log-log plot corresponding to a Γ = 3.2 power law (dashed purple line), but fall below beyond 1 keV. The horizontal lines mark where the flux (E²N) of the difference spectrum amounts to 100%, or 10% of the original normalized spectra at 4 keV, where the spectra are normalized. The yellow data points are above the 1x line below 0.5 keV, and below the 0.1x line above 2 keV. Above 3 keV, the difference falls below 1%.