We study properties of luminous X-ray pulsars using a simplified model of the accretion column. The maximal possible luminosity is calculated as a function of the neutron star (NS) magnetic field and spin period. It is shown that the luminosity can reach values of the order of 10^40erg/s for the magnetar-like magnetic field (B≳10^14G) and long spin periods (P≳1.5s). The relative narrowness of an area of feasible NS parameters which are able to provide higher luminosities leads to the conclusion that L≃10^40erg/s is a good estimate for the limiting accretion luminosity of a NS. Because this luminosity coincides with the cut-off observed in the high mass X-ray binaries luminosity function which otherwise does not show any features at lower luminosities, we can conclude that a substantial part of ultra-luminous X-ray sources are accreting neutron stars in binary systems.