Analyzing X-ray pulsar profiles: geometry and beam pattern of EXO 2030+375

¹ Institut für Astronomie und Astrophysik, Universität Tübingen, Sand 1, 72076 Tübingen, Germany e-mail: sasaki@astro.uni-tuebingen.de
² Institut für Physik und Technik, Universität Hildesheim, Marienburger Platz 22, 31141 Hildesheim, Germany
³ CEA Saclay, DSM/IRFU/SAp – UMR AIM (7158) CNRS/CEA/Université P. Diderot, 91191 Gif-sur-Yvette, France

Received: 14 December 2009
Accepted: 7 April 2010

Abstract

Context. The pulse profiles of the transient Be/X-ray binary EXO 2030+375 show strong dependence on energy, as well as on its luminosity state, and are asymmetric in shape.

Aims. We want to identify the emission components of the two magnetic poles in the pulsed emission to understand the geometry of the neutron star and its beam pattern.

Methods. We utilize a pulse-profile decomposition method that enables us to find two symmetric pulse profiles from the magnetic poles of the neutron star. The symmetry characteristics of these single-pole pulse profiles give information about the position of the magnetic poles of the neutron star relative to its rotation axis.

Results. We find a possible geometry for the neutron star in EXO 2030+375 through the decomposition of the pulse profiles, which suggests that one pole gets closer to the line of sight than the other and that, during the revolution of the neutron star, both poles disappear behind the horizon for a short period of time. A considerable fraction of the emission arises from a halo while the pole is facing the observer and from the accretion stream of the other pole while it is behind the neutron star, but the gravitational line bending makes the emission visible to us.