  We examine the published light curves (LCs) of 117 X-ray binary pulsars,
focusing on the dependence of their light curves on the observed energy bands.
It is found that the energy dependence of the LCs appears only when the X-ray
luminosity is larger than ~ 5 x 10^36 erg/s. Assuming that the behavior of
light curve is related to the radiative accretion column on the neutron star
surface, this energy threshold can be considered as the observational proof of
the accretion column formation proposed by Basko and Sunyaev. Once we can grasp
the existence of radiative column, we can also obtain several useful
informations on the neutron star properties. As an instance, we perform the
statistical analysis of the orientation angle of the magnetic axis, and we find
that the inclination angle of magnetic axis should be small in order to explain
the observed statistics.

Astrophysics