MODELING OBSERVED X-RAY PULSAR PROFILES

Polar cap emission models for X-rays emitted by accreting neutron stars are constructed and compared to observed pulse profiles. This work is a generalization of the work of Leahy (1990). The intensity distribution from the neutron star is computed using a source region consisting of two annular polar-cap emission regions. These annuli are centred on the magnetic poles which are allowed to be offset from a single axis passing through the centre of the neutron star. Model profiles are generated from the calculated intensity distribution, then are least-squares fitted to the observed pulse profiles of 20 X-ray pulsars. Parameters such as orientation of neutron star spin and magnetic axes, polar cap size, and magnetic axis offset are derived. Results from the modelling include: nearly half the pulsars require an offset; only one pulsar requires the two polar caps to be of different size; the mean angle between spin and magnetic axes is almost-equal-to 27-degrees; no correlation is found between polar-cap size and pulse period or luminosity.