Search for gravitational waves from Scorpius X-1 in the second Advanced LIGO observing run with an improved hidden Markov model

B. P. Abbott et al. (LIGO Scientific Collaboration and Virgo Collaboration)

Phys. Rev. D 100, 122002 – Published 4 December 2019; Erratum Phys. Rev. D 104, 109903 (2021)

Abstract

We present results from a semicoherent search for continuous gravitational waves from the low-mass x-ray binary Scorpius X-1, using a hidden Markov model (HMM) to track spin wandering. This search improves on previous HMM-based searches of LIGO data by using an improved frequency domain matched filter, the $J$ -statistic, and by analyzing data from Advanced LIGO’s second observing run. In the frequency range searched, from 60 to 650 Hz, we find no evidence of gravitational radiation. At 194.6 Hz, the most sensitive search frequency, we report an upper limit on gravitational wave strain (at 95% confidence) of $h_{0}^{95 %} = 3.47 \times 10^{- 25}$ when marginalizing over source inclination angle. This is the most sensitive search for Scorpius X-1, to date, that is specifically designed to be robust in the presence of spin wandering.

Received 28 June 2019

DOI:https://doi.org/10.1103/PhysRevD.100.122002

Physics Subject Headings (PhySH)

Gravitational waves

Gravitation, Cosmology & Astrophysics

Erratum

Erratum: Search for gravitational waves from Scorpius X-1 in the second Advanced LIGO observing run with an improved hidden Markov model [Phys. Rev. D 100, 122002 (2019)]

B. P. Abbott et al. (The LIGO Scientific Collaboration and the Virgo Collaboration)

Phys. Rev. D 104, 109903 (2021)

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Vol. 100, Iss. 12 — 15 December 2019

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Images

Figure 1
Flowchart of the $J$ -statistic search pipeline for each subband. Note that the $F$ -statistic atoms are computed once per block and per subband, then the $J$ -statistic is recalculated for each $(a_{0}, ϕ_{a})$ pair. The gray ovals are the start and end of the algorithm, the green rectangles are procedures, the blue (red) parallelograms are intermediate (input) data, the yellow diamonds are decision points, and the gray dashed line represents a loop repeated once for each choice of parameter. The rectangles with a dashed boundary were run on graphical processing units, while those with a solid boundary were run on central processing units.
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Figure 2
Tail of the PDF of the Viterbi score $S$ in noise. The purple histogram shows the empirical PDF derived from $1 0^{2}$ realizations of the noise analyzed in the seven 0.61 Hz subbands starting at 55, 155, 255, 355, 455, 555, and 650 Hz. The green curve is an exponential fitted to the histogram.
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Figure 3
Sensitivity of a search for Sco X-1 across the frequency band searched in this work. The horizontal axis shows the search frequency. The vertical axis shows the wave strain $h_{0}^{95 %}$ needed for a 95% detection efficiency, on the assumption $\cos ι = 1$ , namely, a circularly polarized signal. The blue dashed curve is based on simulations of Gaussian noise, while the red solid curve is corrected for the non-Gaussian statistics of the noise and the interferometer duty cycle, through multiplying by $κ_{freq} (f)$ (see Sec. 3d). The diamonds show $h_{0}^{95 %}$ derived through injections into subbands, again assuming a circularly polarized signal (see Sec. 3d).
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Figure 4
Candidates identified by the search. The left-hand panel plots the detection score $S$ (indicated by color; see color bar) as a function of final frequency $f_{0} (t_{N_{T}})$ (horizontal axis) and orbital semimajor axis $a_{0}$ (vertical axis) recovered by the HMM. The right-hand panel plots the candidates with $T_{asc}$ on the vertical axis. Undecorated candidates are eliminated by the known line veto, candidates marked by blue circles are eliminated by the single-interferometer veto, candidates marked by orange squares are eliminated by the $T_{obs} / 2$ veto, and the candidates marked by green triangle survive the veto process.
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Figure 5
Wave strain upper limits at 95% confidence as a function of signal frequency, corresponding to the frequentist upper limit in each subband (width $Δ f_{band}$ ), for three scenarios: a flat prior on $\cos ι$ (blue crosses), the orientation $ι \approx 44 °$ derived from observations of the radio jet (filled purple circles), and the most optimistic case of circularly polarized waves (open green diamonds). These are compared to the indirect torque-balance upper limit, where the accretion torque is applied at the Alfvén radius (solid orange curve) or the stellar radius (solid red curve).
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