Ratio of the EPIC pn spectrum of Ser X − 1 (Obs 0084020501) to its best-fit continuum model for the spectrum free of pile-up used in this paper (black). The red points show the ratio of the piled-up spectrum of the same observation, obtained when the full PSF is used, to the best-fit continuum model of the spectrum free of pile-up. The pile-up shows up mainly as a significant hardening of the spectrum at energies  ≳ 5 keV. This adds an “artificial” red-wing to the Fe line because of the different curvature of the spectrum.