Radiative Column and Light Curve of X-Ray Binary Pulsars

TL;DR

This study analyzes the energy-dependent light curves of 117 X-ray binary pulsars, linking their behavior to the formation of radiative accretion columns on neutron stars and deriving implications for neutron star properties.

Contribution

It provides observational evidence for radiative accretion column formation at high luminosities and explores neutron star magnetic axis orientations.

Findings

Energy dependence of light curves appears above ~5 x 10^36 erg/s luminosity.

The inclination angle of the magnetic axis is inferred to be small.

Supports the model of radiative accretion columns on neutron stars.

Abstract

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.