Tuning into Scorpius X-1: adapting a continuous gravitational-wave search for a known binary system

TL;DR

This paper adapts the TwoSpect method for targeted searches of continuous gravitational waves from the binary system Sco X-1, demonstrating improved sensitivity and parameter estimation capabilities for known sources.

Contribution

It introduces a tuned, directed search approach for Sco X-1, enhancing sensitivity and parameter recovery compared to all-sky methods, and discusses its application to future gravitational wave data.

Findings

Sensitivity improves with known sky location and period.

Successfully recovers GW strain, frequency, and semi-major axis in simulations.

Sets upper limits on GW strain for undetected signals.

Abstract

We describe how the TwoSpect data analysis method for continuous gravitational waves (GWs) has been tuned for directed sources such as the Low Mass X-ray Binary (LMXB), Scorpius X-1 (Sco X-1). A comparison of five search algorithms generated simulations of the orbital and GW parameters of Sco X-1. Where that comparison focused on relative performance, here the simulations help quantify the sensitivity enhancement and parameter estimation abilities of this directed method, derived from an all-sky search for unknown sources, using doubly Fourier- transformed data. Sensitivity is shown to be enhanced when the source sky location and period are known, because we can run a fully-templated search, bypassing the all-sky hierarchical stage using an incoherent harmonic sum. The GW strain and frequency as well as the projected semi-major axis of the binary system are recovered and uncertainty estimated, for simulated signals that are detected. Upper limits on GW strain are set for undetected signals. Applications to future GW observatory data are discussed. Robust against spin-wandering and computationally tractable despite unknown frequency, this directed search is an important new tool for finding gravitational signals from LMXBs.