A reflection model for the cyclotron lines in the spectra of X-ray
  pulsars
  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 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. We develop here the 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.
