Alexander A. Mushtukov

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

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

A reflection model for the cyclotron lines in the spectra of X-ray pulsars

10^{40}\,{\rm erg/s}

Positive correlation between the cyclotron line energy and luminosity in sub-critical X-ray pulsars: Doppler effect in the accretion channel

for the magnetar-like magnetic field (

Compton scattering S-matrix and cross section in strong magnetic field

B\gtrsim 10^{14}\,{\rm G}

Relativistic kinetic equation for Compton scattering of polarized radiation in a strong magnetic field

) and long spin periods (

On the radiation beaming of bright X-ray pulsars and constraints on neutron star mass–radius relation

P\gtrsim 1.5\,{\rm s}

Propagating mass accretion rate fluctuations in X-ray binaries under the influence of viscous diffusion

). The relative narrowness of an area of feasible NS parameters which are able to provide higher luminosities leads to the conclusion that

On the origin of cyclotron lines in the spectra of X-ray pulsars

L\simeq 10^{40}\,\,{\rm erg/s}

The critical accretion luminosity for magnetized neutron stars

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.

Propeller effect in action in the ultraluminous accreting magnetar M82 X−2

Cyclotron resonance scattering features observed in the spectra of some X-ray pulsars show significant changes of the line energy with the pulsar luminosity. At high luminosities, these variations are often associated with the onset and growth of the accretion column, which is believed to be the origin of the observed emission and of the cyclotron lines. However, this scenario inevitably implies a large gradient of the magnetic field strength within the line-forming region, which makes the formation of the observed line-like features problematic. Moreover, the observed variation of the cyclotron line energy is much smaller than could be anticipated for the corresponding luminosity changes. We argue here that a more physically realistic situation is that the cyclotron line forms when the radiation emitted by the accretion column is reflected from the neutron star surface, where the gradient of the magnetic field strength is significantly smaller. Here we develop a reflection model and apply it to explain the observed variations of the cyclotron line energy in a bright X-ray pulsar V 0332+53 over a wide range of luminosities.