Gravitational light bending and the lightcurves of HMXBs Original Research Article

We calculate the magnitude of gravitational light bending for a set of neutron stars described by various equations of state. We analyze the size of the parameter space for the visibility of one or two magnetic polar caps, corresponding to single and double peaked lightcurves. We show that the expected fraction of single peaked lightcurves should be less than 20%, while the observations seem to show a larger fraction. We also discuss the volume of the parameter space for accretion flow eclipses (or dips) such as these seen in e.g. A0535+262, GX1+4, and LS992. We conclude that the data favors a model where the magnetic axis tends to alignment with the rotation axis.