Continuum model and Galactic absorption fit to Mrk 509 RGS spectrum. 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 continuum model of Mrk 509 SED absorbed by the Milky Way is marked with continuous red and blue lines. The lower panel shows residuals against the best-fit model. The wavelengths are in the observed frame of reference. For the purpose of this plot the data are binned by five channels.