  One of the models explaining the high luminosity of pulsing ultra-luminous
X-ray sources (pULXs) was suggested by Mushtukov et al. (2015). They showed
that the accretion columns on the surfaces of highly magnetized neutron stars
can be very luminous due to opacity reduction in the high magnetic field.
However, a strong magnetic field leads also to amplification of the
electron-positron pairs creation. Therefore, increasing of the electron and
positron number densities compensates the cross-section reduction, and the
electron scattering opacity does not decrease with the magnetic field
magnification. As a result, the maximum possible luminosity of the accretion
column does not increase with the magnetic field. It ranges between 10$^{40} -
10^{41}$ erg s$^{-1}$ depending only slightly on the magnetic field strength.
