Abstract
Cyclotron resonance scattering features are observed in the spectra of some X-ray pulsars and show significant changes in the line energy with the pulsar luminosity. In a case of bright sources, the line centroid energy is anti-correlated with the luminosity. Such a behaviour is often associated with the onset and growth of the accretion column, which is believed to be the origin of the observed emission and the cyclotron lines. However, this scenario inevitably implies large gradient of the magnetic field strength within the line-forming region, and it 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 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. The idea is based on the facts that a substantial part of column luminosity is intercepted by the neutron star surface and the reflected radiation should contain absorption features. The reflection model is developed and applied 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.
Highlights
X-ray pulsars are neutron stars in binary systems accreting matter usually from a massive companion
We argue 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
We show that a significant part of the radiation from the accretion column should be intercepted by the stellar surface because of the relativistic beaming (Section 2.1 and [12, 13]), and the absorption features should exist in the radiation reflected from the magnetised atmosphere (Section 2.2)
Summary
X-ray pulsars are neutron stars in binary systems accreting matter usually from a massive companion. The negative correlation of the CRSF energy with luminosity is usually explained with the onset and growth of the accretion column at high luminosities [9] In this scenario, the height of the column, and, the average displacement of the emission and the line-forming regions from the neutron star surface increase with luminosity, which to a shift of CRSF to lower energies. It is natural to assume that the line is formed in the atmosphere of the neutron star due to reflection of the intercepted radiation In this case the negative correlation between the luminosity and the cyclotron line energy is reproduced because of the changes in the illuminated part of a stellar surface: if the B-field decreases away from the magnetic poles, the relatively high column could illuminate regions with relatively low field strength (see Fig. 1)
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