Application of hidden Markov model tracking to the search for long-duration transient gravitational waves from the remnant of the binary neutron star merger GW170817

The nature of the postmerger remnant of GW170817, the first binary neutron star coalescence observed by the Advanced Laser Interferometer Gravitational Wave Observatory (Advanced LIGO) and Advanced Virgo, is unknown. Searches have been carried out for short (-1 s), intermediate (-500 s), and long (∼ days) signals using various algorithms without yielding a detection. We describe an efficient frequency tracking scheme based on a hidden Markov model to search for long-duration transient signals from a neutron star remnant with spin-down timescale in the range ∼102 s-104 s. The method was used to the search for a signal from GW170817. We validate the method and estimate its sensitivity through Monte Carlo simulations on the same data set as used in the GW170817 search. We describe the search configuration and follow-up procedure step by step. The search achieves an astrophysical reach of ∼1 Mpc and hence cannot detect a source like GW170817 (40-14+8 Mpc), given the current sensitivities of Advanced LIGO and Advanced Virgo. The methodology of the hidden Markov model is described fully to ensure that future analyses of this kind can be reproduced by an independent party.