Relativistic Effects in the Pulse Profile of the 2.5 msec X-Ray Pulsar   SAX J1808.4-3658
  We analyze the properties of the pulsed emission from the accreting millisecond pulsar SAX J1808.4-3658 in observations of its April 1998 outburst by the Rossi X-Ray Timing Explorer. Pulse phase spectroscopy shows that the emission evolves from a hard spectrum (power law with photon index 2.39+-0.06) to a soft spectrum (index 3.39+-0.24). This softening is also observable as a phase lag in the fundamental of low-energy photons with respect to high-energy photons. We show that this lag is roughly constant over ten days of the outburst. We fit these data with a model where the pulse emission is from a hot spot on the rotating neutron star and the flux as a function of phase is calculated including the effects of general relativity. The energy-dependent lags are very well described by this model. The harder spectra at earlier phases (as the spot approaches) are the result of larger Doppler boosting factors which are important for this fast pulsar. 
Eric C. Ford (U Amsterdam)