Search for continuous gravitational waves directed at sub-threshold radiometer candidates in O3 LIGO data

Abstract: We present results of a follow-up search for continuous gravitational waves (CWs) associated with sub-threshold 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 $1/32\,\,\mathrm{Hz}$ frequency band and ${\sim}13\,\,\mathrm{deg}^2$ sky pixel. Assuming they represent possible CW sources, we analyze all $515$ ASAF candidates using a semi-coherent, $\mathcal{F}$-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 $\pm10^{-9}\,\,\mathrm{Hz/s}$ 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 $h_0^{95\%} = 8.8\times 10^{-26}$ ($9.4\times 10^{-26}$) at a frequency of $222.6\,\,\mathrm{Hz}$. While this study focuses on ASAF sub-threshold candidates, the method presented could be applied to follow up candidates from future all-sky CW searches, complementing currently existing methods.