Precision ephemerides for gravitational-wave searches -- III. Revised system parameters of Sco X-1

TL;DR

This paper refines the orbital parameters of Sco X-1 using optical spectroscopic data, providing updated constraints crucial for improving gravitational-wave searches from this neutron star system.

Contribution

It offers the first comprehensive revision of Sco X-1's orbital parameters, including a new range for the projected orbital velocity, aiding targeted gravitational-wave detection efforts.

Findings

Updated orbital period and ephemeris for Sco X-1.

Revised upper limit for the neutron star's projected orbital velocity.

Complete set of dynamical system parameter constraints.

Abstract

Neutron stars in low-mass X-ray binaries are considered promising candidate sources of continuous gravitational-waves. These neutron stars 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-LIGO 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 $\mathrm{s^{-1}}$, which we interpret as a revised upper limit to the projected orbital velocity of the NS $K_1$. By implementing Monte Carlo binary parameter calculations, and imposing new limits on $K_1$ and the rotational broadening, we obtained a complete set of dynamical system parameter constraints including a new range for $K_1$ of 40--90 km $\mathrm{s^{-1}}$. Finally, we discussed the implications of the updated orbital parameters for future continuous-waves searches.