Importance of electron-positron pairs on the maximum possible luminosity of the accretion columns in ULXs

TL;DR

This paper investigates how electron-positron pair creation affects the maximum luminosity of accretion columns in ULXs, showing that pair production limits luminosity despite magnetic field effects.

Contribution

It demonstrates that electron-positron pairs negate the luminosity increase expected from magnetic field effects in accretion columns of ULXs.

Findings

Maximum luminosity is limited to 10^{40}-10^{41} erg/s.

Pair creation compensates for opacity reduction at high magnetic fields.

Luminosity does not significantly increase with magnetic field strength.

Abstract

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.