Limits on Mass and Radius for the ms-Period X-ray Pulsar SAX J1808.4-3658

SAX J1808.4-3658 has a 2.5 millisecond neutron star rotation period and exhibits X-ray pulsations due to its rotating hot spot. Here we present an analysis of the pulse shapes of SAX J1808 during its 1998 outburst. The modeling of the pulse shape includes several effects, including gravitational light-bending, doppler effects and two spectral components with different emissivity. In addition we include the new effects of light-travel time-delays and the neutron star's oblate shape. We also consider two different data sets, with different selection in time period (1 day versus 19 days of data combined) and different energy binning and time resolution. We find that including time-delays and oblateness results in stronger restriction on allowed masses and radii. A second result is that the choice of data selection strongly affects the allowed masses and radii. Overall, the derived constraints on mass and radius favor compact stars and a soft equation of state.