Top panel: zoom in on the non-binned count spectrum of the longest, 124 ks RGS1 first–order exposure near the detected O VII Heα line. The red curve is the best-fit of the redshifted SPEX ‘slab’ model minimized to the data shown, yielding ion column log N_OVII (cm⁻²) = 16.5, i.e., a value consistent with the main analysis. The orange line marks the predicted O VII line centroid and the blue curve the subtracted background in each channel. The missing data bins were removed by the reduction software due to the bad response (see Sect. 2.1 for details). Below: The effective area of the RGS1 instrument in the corresponding wavelength range, showing two instrumental features.