Radiative column and light curve of X-ray binary pulsars

Abstract

We examine the published light curves (LCs) of 117 X-ray binary pulsars, while 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 × 1036 erg s -1. 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 observational proof of the accretion column formation proposed by Basko and Sunyaev. Once we can grasp the existence of a radiative column, we can also obtain several useful pieces of information about the neutron-star properties. As an instance, we performed a statistical analysis of the orientation angle of the magnetic axis, and we found that the inclination angle of the magnetic axis should be small in order to explain the observed statistics. © 2007. Astronomical Society of Japan.