Distribution on the BPT diagram (Baldwin et al. 1981) of 1347 SDSS X-ray-selected galaxies. Empirical lines (dashed blue and solid red) that separate LINERs from Seyfert galaxies are from Kewley et al. (2006). The grey background circles show all the 204 895 SDSS-DR17 galaxies that satisfy our basic optical quality cuts: all line fluxes have fractional uncertainties of < 0.13 dex (33%) and are ‘narrow’ σ_line < 200 km s⁻¹. The colour bar represents X-ray/Hα flux ratio (6a) and redshift z (6b) and corresponds only to 1347 SDSS X-ray-selected galaxies.