Search for gravitational waves from Scorpius X-1 with a hidden Markov model in O3 LIGO data

Abstract

Results are presented for a semicoherent search for continuous gravitational waves from the low-mass xray binary Scorpius X-1, using a hidden Markov model (HMM) to allow for spin wandering. This search improves on previous HMM-based searches of Laser Interferometer Gravitational-Wave Observatory data by including the orbital period in the search template grid, and by analyzing data from the latest (third) observing run. In the frequency range searched, from 60 to 500 Hz, we find no evidence of gravitational radiation. This is the most sensitive search for Scorpius X-1 using a HMM to date. For the most sensitive subband, starting at 256.06 Hz, we report an upper limit on gravitational wave strain (at 95% confidence) of h 95% 0 ¼ 6.16 × 10 −26 , assuming the orbital inclination angle takes its electromagnetically restricted value ι ¼ 44°. The upper limits on gravitational wave strain reported here are on average a factor of ∼3 lower than in the second observing run HMM search. This is the first Scorpius X-1 HMM search with upper limits that reach below the indirect torque-balance limit for certain subbands, assuming ι ¼ 44°.

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

“…Since the cross-correlation analysis of this paper, as in Abbott et al (2022a), does not explicitly consider a signal with stochastically varying frequency ("spin wandering")-although it is somewhat robust to it (Whelan et al 2015)-there is information to be gained from reanalysis of the O3 data using a hidden Markov model as in Abbott et al (2022b).…”

“…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 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.…”

Continuous Gravitational Waves from Galactic Neutron Stars: Demography, Detectability, and Prospects

“…If the sky position of the object is known, but no information is available on the phase parameters, one can set up a directed search and explore the free parameter space. This may be constrained by information on the object, such as its age (Zhang et al 2021;Abbott et al 2021bAbbott et al , 2022dMing et al 2022;Owen et al 2022). These searches are directed toward objects such as supernova remnants, low-mass X-ray binaries (LMXBs), or promising regions in the sky, and their computational cost is considerably higher than that of the searches for known pulsars.…”