EPIC-pn (black), MOS 1 (red), MOS 2 (green), and RGS (blue) spectra and the best-fit continuum model (red curve), from the stacking of all XMM-Newton observations. The continuum emission components are represented by dashed lines in orange (bb) and green (comt). The EPIC data are ignored in the 0.5 − 1.77 keV energy range to employ the full high spectral resolution of RGS and avoid model degeneracy. The EPIC data are grouped according to the optimal binning and, for a better visualisation, the RGS data are grouped by a factor of 9. The bottom panel shows the residuals of the best-fit model.