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

TL;DR

This paper presents a highly sensitive semi-coherent search for continuous gravitational waves from Scorpius X-1 using a hidden Markov model in LIGO O3 data, improving previous methods by including orbital parameters and reaching below the torque-balance limit in some frequency bands.

Contribution

It introduces the first HMM-based search for Scorpius X-1 that incorporates orbital period and analyzes O3 data, setting new upper limits on gravitational wave strain.

Findings

No evidence of gravitational waves was detected.

Upper limits on strain are about three times lower than previous O2 results.

Some sub-bands reach below the torque-balance limit assuming an inclination angle of 44°.

Abstract

Results are presented for a semi-coherent search for continuous gravitational waves from the low-mass X-ray 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 (LIGO) data by including the orbital period in the search template grid, and by analyzing data from the latest (third) observing run (O3). 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 sub-band, starting at $256.06$Hz, we report an upper limit on gravitational wave strain (at $95 \%$ confidence) of $h_{0}^{95\%}=6.16\times10^{-26}$, assuming the orbital inclination angle takes its electromagnetically restricted value $\iota=44^{\circ}$. The upper limits on gravitational wave strain reported here are on average a factor of $\sim 3$ lower than in the O2 HMM search. This is the first Scorpius X-1 HMM search with upper limits that reach below the indirect torque-balance limit for certain sub-bands, assuming $\iota=44^{\circ}$.