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

Abstract

We describe how the TwoSpect data analysis method for continuous gravitational waves (GWs) has been tuned for directed sources such as the lowmass 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. Whereas 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 for GW strain are set for undetected signals. Applications to future GW observatory data are discussed. Robust against spin-wandering and computationally tractable despite an unknown frequency, this directed search is an important new tool for finding gravitational signals from LMXBs.

Hidden Markov model tracking of continuous gravitational waves from a binary neutron star with wandering spin. II. Binary orbital phase tracking

“…[25] was published, e.g. the tuned TwoSpect method [14]. When performance data are published for the refined algorithms, some of the entries in Table VI will require updating.…”

“…[25] Stage II of the MDC is currently being prepared and is planned to include spin wandering. The MDC establishes a standard to compare the J -statistic HMM tracker against the Bessel-weighted F-statistic [34], CrossCorr [20][21][22][23], TwoSpect [12][13][14], Radiometer [18], Sideband [15][16][17] and Polynomial [24] pipelines.…”

Recent searches for continuous gravitational waves

“…One previous fully coherent search 126 restricted its coherence length to 10 days, to be conservative. Semi-coherent and cross-correlation methods 104,116,118,122 should be more robust against wandering, an expectation to be tested soon in a new mock data challenge.…”

Searches for continuous gravitational waves from Scorpius X-1 and XTE J1751-305 in LIGO’s sixth science run

“…GW detector data have been analyzed using various strategies to search for continuous waves [10][11][12][13][14][15]. The method used here, TwoSpect [16], has been used opportunistically in a previous all-sky analysis to search for Sco X-1 in LIGO Science Run 6 (S6) and the second and third Virgo Science Runs (VSR2 and 3) [17], but it has been subsequently improved (by a factor of 9.5/4.0 = 2.375 for random polarization [18]) and those improvements demonstrated in a Mock Data Challenge (MDC) of simulated signals [19]. This method searches for evidence of orbital modulation of a continuous narrowband signal in strain data as seen in the time-frequency domain.…”