We analyze the properties of the pulsed emission from the accreting millisecond pulsar SAX J1808.4-3658in 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 \pm 0.06 ) to a soft spectrum ( index 3.39 \pm 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 .