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 $100$–$800\mathrm{H}\mathrm{z}$. 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 $h_{0,\mathrm{e}\mathrm{f}\mathrm{f}}^{95\mathrm{\%}}$ and corresponding neutron star ellipticity ${\epsilon }^{95\mathrm{\%}}$ are presented. The best strain sensitivity, $h_{0,\mathrm{e}\mathrm{f}\mathrm{f}}^{95\mathrm{\%}}=2.7\times {10}^{-26}$ at $211\mathrm{H}\mathrm{z}$, is achieved for the cluster NGC6544.