On the maximum accretion luminosity of magnetized neutron stars: connecting X-ray pulsars and ultraluminous X-ray sources

TL;DR

This paper models the maximum accretion luminosity of magnetized neutron stars, suggesting that many ultraluminous X-ray sources are likely neutron stars with strong magnetic fields, reaching luminosities around 10^40 erg/s.

Contribution

It provides a simplified model linking neutron star magnetic fields and spin periods to their maximum possible luminosity, explaining the observed cutoff in X-ray binary luminosities.

Findings

Maximum luminosity can reach ~10^40 erg/s for magnetar-like fields and long spin periods.

The luminosity cutoff explains the observed luminosity function in high mass X-ray binaries.

Many ultraluminous X-ray sources are likely accreting neutron stars.

Abstract

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 $10^{40}\,{\rm erg/s}$ for the magnetar-like magnetic field ($B\gtrsim 10^{14}\,{\rm G}$) and long spin periods ($P\gtrsim 1.5\,{\rm s}$). The relative narrowness of an area of feasible NS parameters which are able to provide higher luminosities leads to the conclusion that $L\simeq 10^{40}\,\,{\rm erg/s}$ 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.