Warm absorber on top of continuum modified by Galactic absorption with Mrk 509 RGS data points. RGS 1 and RGS 2 spectra are plotted in orange and cyan. RGS 2 is arbitrarily shifted by 0.02 counts s⁻¹ Å⁻¹. The total model consisting of continuum together with a single CTP WA is plotted in red and blue. The wavelengths are in the observed frame of reference. The lower panel shows residua. For the purpose of this plot the data are binned by five channels.