Investigating pulse morphology in GX 1+4

Observational and theoretical evidence points to the existence of an unusually high magnetic field on GX 1 + 4. The pulsar is thus an ideal laboratory for studying two-photon cyclotron emission, an important source of photons of frequency significantly less than the cyclotron frequency in X-ray pulsars. Low-frequency approximations to the two-photon cyclotron emission transition probabilities are derived. These are used to calculate the theoretical opening angle of the double-humped pulse shape predicted by the two-photon cyclotron emission model. The theoretical pulse shape, incorporating the effects of gravitational light bending, is compared with observations of GX 1 + 4. Observed light curves have opening angles consistent with the theoretically predicted maximum value.