System mass constraints for the accreting millisecond pulsar XTE J1814-338 using Bowen fluorescence

We present phase-resolved spectroscopy of the millisecond X-ray pulsar XTE J1814-338 obtained during its 2003 outburst. The spectra are dominated by high-excitation emission lines of He II lambda 4686, H beta, and the Bowen blend C III/N III 4630-50 angstrom. We exploit the proven Bowen fluorescence technique to establish a complete set of dynamical system parameter constraints using bootstrap Doppler tomography, a first for an accreting millisecond X-ray pulsar binary. The reconstructed Doppler map of the NIII lambda 4640 Bowen transition exhibits a statistically significant (>4 sigma) spot feature at the expected position of the companion star. If this feature is driven by irradiation of the surface of the Roche lobe filling companion, we derive a strict lower limit to the true radial velocity semi-amplitude K-2. Combining our donor constraint with the well-constrained orbit of the neutron star leads to a determination of the binary mass ratio: q = 0.123(-0.010)(+0.012). The component masses are not tightly constrained given our lack of knowledge of the binary inclination. We cannot rule out a canonical neutron star mass of 1.4 M-circle dot (1.1 M-circle times < M-1 < 3.1 M-circle dot; 95 per cent). The 68/95 per cent confidence limits of M-2 are consistent with the companion being a significantly bloated, M-type main-sequence star. Our findings, combined with results from studies of the quiescent optical counterpart of XTE J1814-338, suggest the presence of a rotation-powered millisecond pulsar in XTE J1814-338 during an X-ray quiescent state. The companion mass is typical of the so-called redback pulsar binary systems (M-2 similar to 0.2 M-circle dot).