Multi-epoch Analysis of Pulse Shapes from the Neutron Star SAX J1808.4-3658

The pulse shapes detected during multiple outbursts of SAX J1808 are analyzed in order to constrain the neutron star's mass and radius. We use a hot-spot model with a small scattered-light component to jointly fit data from two different epochs, under the restriction that the star's mass and radius and the binary's inclination do not change from epoch to epoch. All other parameters describing the spot location, emissivity, and relative fractions of blackbody to Comptonized radiation are allowed to vary with time. The joint fit of data from the 1998 "slow decay" and the 2002 "end of outburst maximum" epochs using the constraint i<90 degrees leads to the 3 sigma confidence constraint on the neutron star mass 0.8 M_sun < M < 1.7 M_sun and equatorial radius 5 km < R < 13 km. Inclinations as low as 41 degrees are allowed. The best-fit models with M > 1.0 M_sun from joint fits of the 1998 data with data from other epochs of the 2002 and 2005 outbursts also fall within the same 3 sigma confidence region. This 3 sigma confidence region allows a wide variety of hadronic equations of state, in contrast with an earlier analysis (Leahy et al 2008) of only the 1998 outburst data that only allowed for extremely small stars.