L. Wang

Neutron stars (NSs) in low-mass X-ray binaries are considered promising candidate sources of continuous gravitational waves. These NSs are typically rotating many hundreds of times a second. The process of accretion can potentially generate and support non-axisymmetric distortions to the compact object, resulting in persistent emission of gravitational waves. We present a study of existing optical spectroscopic data for Sco X-1, a prime target for continuous gravitational-wave searches, with the aim of providing revised constraints on key orbital parameters required for a directed search with advanced Laser Interferometer Gravitational-Wave Observatory data. From a circular orbit fit to an improved radial velocity curve of the Bowen emission components, we derived an updated orbital period and ephemeris. Centre of symmetry measurements from the Bowen Doppler tomogram yield a centre of the disc component of 90 km s⁻¹, which we interpret as a revised upper limit to the projected orbital velocity of the NS K₁. By implementing Monte Carlo binary parameter calculations, and imposing new limits on K₁ and the rotational broadening, we obtained a complete set of dynamical system parameter constraints including a new range for K₁ of 40–90 km s⁻¹. Finally, we discussed the implications of the updated orbital parameters for future continuous-wave searches.

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 λ4686, Hβ, and the Bowen blend C iii/N iii 4630–50 Å. 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 N iii λ4640 Bowen transition exhibits a statistically significant (>4σ) 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 K₂. Combining our donor constraint with the well-constrained orbit of the neutron star leads to a determination of the binary mass ratio: q = |$0.123^{+0.012}_{-0.010}$|⁠. 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_⊙ (1.1 M_⊙ < M₁ < 3.1 M_⊙; 95 per cent). The 68/95 per cent confidence limits of M₂ 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 (M₂ ∼ 0.2 M_⊙).