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

Abstract

Results are presented for a semi-coherent search for continuous gravitational waves from the lowmass 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 256.06Hz, we report an upper limit on gravitational wave strain (at 95% confidence) of h 95% 0 = 6.16 × 10 −26 , assuming the orbital inclination angle takes its electromagnetically restricted value ι = 44 • . The upper limits on gravitational wave strain reported here are on average a factor of ∼ 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 ι = 44 • . I.

Search for continuous gravitational waves from ten H.E.S.S. sources using a hidden Markov model

“…[8][9][10][11]; (2) directed searches, where only the sky position of the source is known [e.g. [12][13][14][15][16][17][18]; and (3) all-sky searches where both the location and rotation parameters are unknown [e.g. [19][20][21][22][23][24][25][26].…”

Validating continuous gravitational-wave candidates from a semicoherent search using Doppler modulation and an effective point spread function

“…Here, δ qi (t N T ) is the likelihood of the path with the maximum probability ending in state q i (1 ≤ i ≤ N Q ) at step N T and δ q * (t N T ) is the likelihood of the optimal Viterbi path. In some applications of the HMM algorithm, e.g., [19,[22][23][24][25], the total log-likelihood along the Viterbi path, ln δ q * (t N T ), is directly used as the detection statistic. Transforming to the Viterbi score in Eq.…”

“…In this paper we focus on a computationally efficient, semicoherent search strategy based on a hidden Markov model (HMM) which is equipped to track a signal frequency that wanders stochastically and spins down secularly [14,15]. The tracking scheme has its origins in engineering applications, and has recently been used in many directed CW searches (e.g., [16][17][18][19][20][21][22][23][24][25]).…”

“…[21]. Some searches rely on an unnormalized log-likelihood instead of the Viterbi score to evaluate the significance of a candidate [19,[22][23][24][25].…”

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 (Owen et al 2022;Abbott et al 2022d;Ming et al 2022;Abbott et al 2021b;Zhang et al 2021). These searches are directed towards objects like supernova remnants, LMXBs or promising regions in the sky, and their computational cost is considerably higher than that of the searches for known pulsars.…”