Precision Ephemerides for Gravitational-wave Searches -- IV: Corrected and refined ephemeris for Scorpius X-1

TL;DR

This paper provides a highly precise and corrected ephemeris for Sco X-1, improving gravitational-wave search sensitivity by reducing template numbers and offering detailed insights into the system's accretion disk dynamics.

Contribution

It presents a comprehensive, reanalyzed ephemeris for Sco X-1 based on 20 years of data, correcting previous models and enhancing gravitational-wave search accuracy.

Findings

Corrected and refined the ephemeris for Sco X-1.

Achieved a threefold reduction in search templates.

Revealed transient accretion disk structures.

Abstract

Low-mass X-ray binaries have long been theorised as potential sources of continuous gravitational-wave radiation, yet there is no observational evidence from recent LIGO/Virgo observing runs. Even for the theoretically 'loudest' source, Sco X-1, the upper limit on gravitational-wave strain has been pushed ever lower. Such searches require precise measurements of the source properties for sufficient sensitivity and computational feasibility. Collating over 20 years of high-quality spectroscopic observations of the system, we present a precise and comprehensive ephemeris for Sco X-1 through radial velocity measurements, performing a full homogeneous reanalysis of all relevant datasets and correcting previous analyses. Our Bayesian approach accounts for observational systematics and maximises not only precision, but also the fidelity of uncertainty estimates - crucial for informing principled continuous-wave searches. Our extensive dataset and analysis also enables us to construct the highest signal-to-noise, highest resolution phase-averaged spectrum of a low-mass X-ray binary to date. Doppler tomography reveals intriguing transient structures present in the accretion disk and flow driven by modulation of the accretion rate, necessitating further characterisation of the system at high temporal and spectral resolution. Our ephemeris corrects and supersedes previous ephemerides, and provides a factor three reduction in the number of templates in the search space, facilitating precision searches for continuous gravitational-wave emission from Sco X-1 throughout the upcoming LIGO/Virgo/KAGRA O4 observing run and beyond.