Position of 1347 SDSS X-ray-selected galaxies on the [NII]/HαBPT diagram. The grey background circles show 204 895 SDSS-DR17 galaxies that satisfy the conditions for optical spectra σ _{log f} ≤ 0.13 and σ_line < 200 km s⁻¹. Empirical lines (dashed blue and solid red) that separate LINERs from Seyfert galaxies are from Kewley et al. (2006). The colour code represents the hardness ratios among the HR1, HR2, HR3, and HR4 bands.