Model-based Cross-correlation Search for Gravitational Waves from the Low-mass X-Ray Binary Scorpius X-1 in LIGO O3 Data

Abstract

We present the results of a model-based search for continuous gravitational waves from the low-mass X-ray binary Scorpius X-1 using LIGO detector data from the third observing run of Advanced LIGO and Advanced Virgo. This is a semicoherent search that uses details of the signal model to coherently combine data separated by less than a specified coherence time, which can be adjusted to balance sensitivity with computing cost. The search covered a range of gravitational-wave frequencies from 25 to 1600 Hz, as well as ranges in orbital speed, frequency, and phase determined from observational constraints. No significant detection candidates were found, and upper limits were set as a function of frequency. The most stringent limits, between 100 and 200 Hz, correspond to an amplitude h 0 of about 10−25 when marginalized isotropically over the unknown inclination angle of the neutron star’s rotation axis, or less than 4 × 10−26 assuming the optimal orientation. The sensitivity of this search is now probing amplitudes predicted by models of torque balance equilibrium. For the usual conservative model assuming accretion at the surface of the neutron star, our isotropically marginalized upper limits are close to the predicted amplitude from about 70 to 100 Hz; the limits assuming that the neutron star spin is aligned with the most likely orbital angular momentum are below the conservative torque balance predictions from 40 to 200 Hz. Assuming a broader range of accretion models, our direct limits on gravitational-wave amplitude delve into the relevant parameter space over a wide range of frequencies, to 500 Hz or more.

Search for Gravitational Waves from Scorpius X-1 in LIGO O3 Data with Corrected Orbital Ephemeris

“…The parameter-space ranges used for searches of O2 and O3 data (Abbott et al 2019b(Abbott et al , 2022b(Abbott et al , 2022aZhang et al 2021) were generated using the PEGS III ephemeris (Wang et al 2018). Killestein et al (2023) subsequently published the improved PEGS IV ephemeris, also documenting calibration errors in PEGS III.…”

“…The values of P orb and t asc in these ephemerides are summarized in Table 1, which shows that the t asc range originally published in Wang et al (2018) is inconsistent with current estimates of that parameter. In Figure 1, we show the plausible ranges of P orb and t asc ¢ (propagated to the middle of O3), along with the region of parameter space searched in the cross-correlation analysis (Abbott et al 2022a), and in the reanalysis presented in this paper. We see that the parameter-space region searched in the original O3 search is inconsistent with the PEGS IV ephemeris, but the region searched in the present reanalysis has about 1/3 the area in parameter space, allowing a search to be performed more quickly at the same sensitivity.…”

“…We present here the results of a reanalysis of the LIGO O3 data (Abbott et al 2023) using the cross-correlation search (Whelan et al 2015), with the revised PEGS IV ephemeris of Killestein et al (2023). Full details of the analysis pipeline are given in Abbott et al (2022a). We highlight here changes made in light of the revised ephemeris.…”

“…The low-mass X-ray binary (LMXB) Scorpius X-1 (ScoX1), which is presumed to consist of a neutron star (NS) of mass ≈1.4M e in a binary orbit with a companion star of mass ≈0.4M e (Steeghs & Casares 2002), is a very promising potential source of continuous gravitational waves (GWs), generated by the spin of the NS (Bildsten 1998;Watts et al 2008). As such, it has been the target of a number of searches (Abbott et al 2007(Abbott et al , 2017a(Abbott et al , 2017b(Abbott et al , 2017c(Abbott et al , 2019a(Abbott et al , 2019b(Abbott et al , 2021(Abbott et al , 2022b(Abbott et al , 2022aAasi et al 2015a;Meadors et al 2017;Zhang et al 2021) using data from the Advanced LIGO GW detectors (Aasi et al 2015b), which have conducted three observing runs (O1, O2, and O3), the last two in coordination with Advanced Virgo (Acernese et al 2015). As the spin frequency of ScoX1 is unknown, searches typically cover a wide range of the intrinsic GW signal frequency f 0 , which in the simplest model is twice the spin frequency.…”

“…The most recent update, known as PEGS IV (Killestein et al 2023), in addition to producing a more refined ephemeris also corrected errors in the previously published PEGS I (Galloway et al 2014) and PEGS III (Wang et al 2018) ephemerides. 11 This means that the parameter-space region searched by the analysis of O1 data in Abbott et al (2017c), which used elements of PEGS I and PEGS III, as well as the O2 and O3 analyses in Abbott et al (2019bAbbott et al ( , 2022bAbbott et al ( , 2022a, Zhang et al (2021), which used PEGS III, did not overlap with the likely regions of parameter space according to the PEGS IV ephemeris, nor with the revised PEGS I and PEGS III ephemerides published in Killestein et al (2023). 12 This paper presents a reanalysis of the LIGO O3 data (Abbott et al 2023) according to the method of Abbott et al (2022a), but with the parameter space determined by the PEGS IV ephemeris.…”