An accretion column model for the accreting pulsar Her X-1

Abstract

A model is developed here to reproduce the pulse shape of Her X-1. The 35-d cycle of pulse shape changes during the 35-d Her X-1 cycle 'high-low-short-high-low' is caused by varying obscuration of the emission region by the accretion disc. The observed sequence of pulse shape changes imply a pencil beam from the near pole and a fan beam from the far pole. A newly developed code for modelling accretion column emission, which includes accurate treatment of gravitational light-bending effects and allows for various emissivity functions and for various geometrical asymmetries, is used to calculated model pulse profiles. The model profiles can reproduce the main features of the observed pulse profiles of Her X-1 in various energy bands. The resulting properties of the accretion column geometry and the emissivity function are given.