Search for continuous gravitational waves from neutron stars in five globular clusters with a phase-tracking hidden Markov model in the third LIGO observing run

A search is performed for continuous gravitational waves emitted by unknown neutron stars in five nearby globular clusters using data from the third Laser Interferometer Gravitational Wave Observatory (LIGO) observing run, over the frequency range 100--800 Hz. The search uses a hidden Markov model to track both the frequency and phase of the continuous wave signal from one coherent segment to the next. It represents the first time that a phase-tracking hidden Markov model has been used in a LIGO search. After applying vetoes to reject candidates consistent with non-Gaussian artifacts, no significant candidates are detected. Estimates of the strain sensitivity at 95% confidence ${h}_{0,\mathrm{eff}}^{95%}$ and corresponding neutron star ellipticity ${\ensuremath{\epsilon}}^{95%}$ are presented. The best strain sensitivity, ${h}_{0,\mathrm{eff}}^{95%}=2.7\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}26}$ at 211 Hz, is achieved for the cluster NGC6544.