T. Bulik et al., GEOMETRY AND PULSE PROFILES OF X-RAY PULSARS - ASYMMETRIC RELATIVISTIC FITS TO 4U-1538-52 AND VELA-X-1, The Astrophysical journal, 444(1), 1995, pp. 405-414
Model calculations of inhomogeneous magnetized neutron star atmosphere
s are used to analyze the phase-dependent spectra of 4U 1538-52 and Ve
la X-l taking into account general relativistic effects. A chi(2) fitt
ing procedure is used on the Ginga data for these objects to determine
the geometry and the magnetic structure of the polar caps, as well as
their size and location on the surface of the stars. Other parameters
determined by the fit are the masses and radii of the neutron stars,
the accretion rates, and the rotation and magnetic inclination angles
including an azimuthal offset to reproduce the asymmetry of the pulse
shapes. We find that general relativistic effects play a significant r
ole in determining the derived cap sizes, the fluxes, and the observat
ional appearance of the pulses. There is evidence that the polar caps
are unequal and nonantipodal, suggesting either an off-center or bent
magnetic axis, or a strong nondipole component. The magnetic field str
ucture is approximated by a two-component model. The rotation axes are
inferred to be moderately close to the mean magnetic axes and seen at
large inclinations with respect to the line of sight.