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

Detailed modeling of the millisecond brightness oscillations during thermonuclear bursts from low-mass X-ray binaries can provide important information about neutron star structure. Until now, the implementation of this idea has not been entirely successful, largely because of the negligible harmonic content in burst oscillation light curves. However, the recent discovery of nonsinusoidal burst oscillation light curves 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 neutron star parameters. In our detailed study of the light curves of 22 bursts, we fit the burst oscillation light curves with fully general relativistic models that include light bending and frame dragging for light curve calculation and numerically compute the structure of neutron stars using realistic equations of state. We find that for our model and parameter grid values, at the 90% confidence level, Rc²/GM > 4.2 for the neutron star in XTE J1814-338. We also find that the photons from the thermonuclear flash come out through the layers of accreted matter under conditions consistent with Thomson scattering and show that the secondary companion is a hydrogen-burning main-sequence star with possible bloating (probably due to X-ray heating).