Constraints on Compact Star Parameters from Burst Oscillation Light Curves of the Accreting Millisecond Pulsar XTE J1814-338

Detailed modeling of the millisecond brightness oscillations from low mass X-ray binaries during thermonuclear bursts can provide us with important information about compact star parameters. Until now the implementation of this idea has not been entirely successful, largely because of the negligible amount of harmonic content in burst oscillation lightcurves. However, the recent discovery of unique, non-sinusoidal burst oscillation lightcurves from the accreting millisecond pulsar XTE J1814-338 has changed this situation. We, therefore, for the first time, make use of this opportunity to constrain compact star structure parameters effectively. In our detailed study of the lightcurves of 22 bursts we fit the burst oscillation lightcurves with fully general relativistic models that include light-bending and frame-dragging for lightcurve calculation, and compute numerically the structure of compact stars using realistic equations of state. We find that the 90% confidence interval of the dimensionless radius to mass ratio of the compact star in XTE J1814-338 is Rc^2/GM = 3.9-4.9. Our study also suggests that the observer inclination angle (measured from the upper rotational pole of the compact star) of the source is greater than 50 degree with very high probability. This, combined with some other known binary parameters, leads us to conclude that the secondary companion is a significantly bloated (probably due to X-ray heating) hydrogen main sequence star. We also find that the bursting hot spot on the surface of the compact star always remains large (angular radius typically greater than 25 degree with 90% confidence level). Our study, for the first time, also supports the natural expectation that the photons from the thermonuclear flash come out through the layers of accreted matter under conditions of Thomson scattering