Search for continuous gravitational waves from Fomalhaut b in the second Advanced LIGO observing run with a hidden Markov model

Abstract

Results are presented from a semicoherent search for continuous gravitational waves from a nearby neutron star candidate, Fomalhaut b, using data collected in the second observing run of Advanced LIGO. The search is based on a hidden Markov model scheme, capable of tracking signal frequency evolution from the star's secular spin down and stochastic timing noise simultaneously. The scheme is combined with a frequency domain matched filter (F-statistic), calculated coherently over five-day time stretches. The frequency band 100-1000 Hz is searched. After passing the above-threshold candidates through a hierarchy of vetoes, one candidate slightly above the 1% false alarm probability threshold remains for further scrutiny. No strong evidence of continuous waves is found. We present the frequentist strain upper limits at 95% confidence.

Application of a hierarchical MCMC follow-up to Advanced LIGO continuous gravitational-wave candidates

“…Reference [80] reports on a Viterbi search for CWs directed at Fomalhaut b, an astrophysical object whose exact nature is still surrounded by debate [94][95][96][97]. This search complements a previous one performed on Advanced LIGO O1 data using an F -statistic search assuming the standard deterministic evolution of a CW [98].…”

“…The search looks for CW emission at once, twice and 4=3 of the rotational frequency of the targeted pulsars in order to address several emission mechanisms [4]. After assessing the subdominant role of spin-wandering on frequency evolution, a biased random walk is implemented in a similar manner to [80], selecting the maximum T coh allowed by the spindown rate of each pulsar so that the frequency evolution is within the range of the HMM.…”

Geometric Approach to Analytic Marginalisation of the Likelihood Ratio for Continuous Gravitational Wave Searches

“…Figure 7 compares the distribution of κ computed from the Monte-Carlo samples using Equation (11) with the assumed prior of Equation (18). The Monte-Carlo distribution is a good fit to the prior for cos ι = 0, and a poor fit for cos ι ∼ 1; for the two signal populations where cos ι was randomly drawn, the fit is intermediate between the two extremes.…”

“…Marginalisation of the likelihood ratio, in the form of Equation ( 12), over κ with the prior of Equation (18) gives the analytic expression…”

“…Continuous gravitational waves are, at best, weak signals relative to the sensitivity of current-generation interferometric detectors [1][2][3]. Searches of data from the LIGO and Virgo observatories, most recently from their second [4][5][6][7][8][9][10][11][12][13][14][15][16][17][18][19][20] and third observing runs [21,22], have yet to make a first detection. Theoretical modelling of rapidly-rotating, non-axisymmetric neutron stars-the most likely source of continuous waves-predict a wide range of possible signal strengths [23][24][25][26][27][28][29][30].…”