Flux (0.2–2.3 keV) of the X-ray sources versus the optical magnitude (V) of their counterparts from the SIMBAD data base. Galactic sources are labeled with a red cross. The AGNs and quasars are shown with blue crosses. Stellar objects for which the distances are not precisely estimated are shown with green crosses. The dashed line presents the X-ray flux threshold, which is caused by optical loading (Merloni et al. 2024). The sources with the optical loading problem are labelled with an extra black ring and their list is presented in Table 1.