Cumulative distribution function of the spatial broadening profile of M 87. We built the spatial broadening profiles for both two observations (black and red) of the RGS 99%-xpsf source extraction regions. For the second observation (0200920101), a 1.5 arcmin offset is applied so that the extraction region is centred on the peak of the X-ray emission. Moreover, for the second observation, we also built a spatial broadening profile with a narrower 95%-xpsf extraction region (blue) that is less strongly affected by central contamination (Fig. B.2).