Search for continuous gravitational waves directed at subthreshold radiometer candidates in O3 LIGO data

We present results of a follow-up search for continuous gravitational waves (CWs) associated with subthreshold candidates from the LIGO-Virgo-KAGRA (LVK) all-sky all-frequency (ASAF) directed radiometer analysis, using Advanced LIGO data from the third observing run (O3). Each ASAF candidate corresponds to a 1/32 Hz frequency band and similar to 13 deg2 sky pixel. Assuming they represent possible CW sources, we analyze all 515 ASAF candidates using a semi-coherent, F-statistic-based matched filter search. The search algorithm incorporates a hidden Markov model (HMM), expanding the signal model to allow frequency spin-wandering, as well as unmodeled frequency evolution of less than 10-5 Hz per day that is not captured by the searched range of I10-9 Hz/s in frequency derivative. Significance thresholds with a 5% probability of false alarm per ASAF candidate are determined empirically by searching detector noise at various off-target sky positions. We obtain 14 outliers surviving a set of vetoes designed to eliminate instrumental artifacts. Upon further investigation, these outliers are deemed unlikely to represent astrophysical signals. We estimate the sensitivity of our search to both isolated and binary sources with orbital period greater than one year by recovering simulated signals added to detector data. The minimum detectable strain amplitude at 95% confidence for isolated (long-period binary) sources is h95% 8.8 x 10-26 (9.4 x 10-26) at a frequency of 222.6 Hz. While this study focuses on ASAF subthreshold candidates, the method presented could be applied to follow up candidates from future all-sky CW searches, complementing currently existing methods.