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

摘要

Context. The pulse profiles of the transient Be/X-ray binaryEXO2030+375 show strong dependence on energy, as well as on itsluminosity state, and are asymmetric in shape. Aims. We want to identify the emission components of the twomagnetic poles in the pulsed emission to understand the geometry of theneutron star and its beam pattern. Methods. We utilize a pulse-profile decomposition method thatenables us to find two symmetric pulse profiles from the magnetic polesof the neutron star. The symmetry characteristics of these single-polepulse profiles give information about the position of the magneticpoles of the neutron star relative to its rotation axis. Results. We find a possible geometry for the neutron star inEXO2030+375 through the decomposition of the pulse profiles,which suggests that one pole gets closer to the line of sight than theother and that, during the revolution of the neutron star, both polesdisappear behind the horizon for a short period of time. A considerablefraction of the emission arises from a halo while the pole is facingthe observer and from the accretion stream of the other pole while itis behind the neutron star, but the gravitational line bending makesthe emission visible to us. Key words: stars: neutron - X-rays: binaries