Analysing X-ray pulsar profiles

Geometry and beam pattern of 4U 0115+63 and V 0332+53

¹ Institut für Astronomie und Astrophysik, Universität Tübingen, Sand 1, 72076 Tübingen, Germany
e-mail: sasaki@astro.uni-tuebingen.de
² Institut für Physik, Universität Hildesheim, Marienburger Platz 22, 31141 Hildesheim, Germany
³ ISDC Data Centre for Astrophysics, Univeristy of Geneva, Chemin d’Écogia 16, 1290 Versoix, Switzerland

Received: 13 December 2010
Accepted: 17 January 2012

Abstract

Aims. By analysing the asymmetric pulse profiles of the transient Be/X-ray binaries 4U 0115+63 and V 0332+53 we aim to identify the origin of the pulsed emission to understand the geometry of the accretion onto the neutron star.

Methods. We have applied the pulse-profile decomposition method, which enabled us to find two symmetric pulse profiles for the two magnetic poles of the neutron star. We derived beam patterns for different energy bands and luminosity states. This allowed us to identify the components that are responsible for the emission. The analysis and the models used for the interpretation of the results take relativistic light deflection into account.

Results. We find that the magnetic field of the neutron star is distorted in both 4U 0115+63 and V 0332+53. The beam patterns are interpreted in terms of a model for an accretion column that includes the formation of a halo at the bottom of the accretion column and scattering in the upper accretion stream.

Conclusions. In both systems, an accretion column forms while the accretion rate is high. If the accretion decreases and the sources become fainter, the emission from the halo and the accretion column disappears. In 4U 0115+63 there seems to be significant scattering of photons in the still existing accretion stream even at the end of the outburst. In V 0332+53, the scattering in the upper stream also disappears at the end and we apparently observe the emission from the hot spots on the neutron star.