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