Spotlight searches for continuous gravitational waves triggered on radiometer candidates in LIGO O4a data

Abstract

We report on a follow-up search for continuous gravitational waves (CWs) targeting sub-threshold candidates from the O4a LIGO-Virgo-KAGRA (LVK) all-sky all-frequency (ASAF) directed radiometer analysis. We analyze a total of $562$ ASAF candidates using Advanced LIGO data from the first part of the fourth LVK observing run. Each candidate comprises a narrow $1/32$ Hz frequency band paired with a ${\sim}13$ deg$^2$ sky pixel, defining the search parameter space for our analysis. Our search pipeline leverages the $F$-statistic matched filter equipped with a hidden Markov model to enable tracking of a potentially frequency-wandering CW signal. Of the 562 initial candidates, we obtain $21$ outliers with false alarm probability less than $5\%$ that survive all instrumental vetoes. None of the outliers are recovered at comparable statistical significance in the second part of the fourth observing run, from which we conclude that no convincing CW signals are detected. We estimate the sensitivity of our analysis by recovering simulated signals added to real detector data. Across the $20\text{--}1726$ Hz frequency band, our search achieves $95\%$ detection efficiency for strain amplitudes in the range of $h_0\sim (0.63\text{--}6.3)\times10^{-25}$ with respect to isolated neutron stars (NSs), and $h_0\sim (0.82\text{--}7.1)\times10^{-25}$ with respect to NSs in long-period ($>1$ yr) binary systems. The large range in sensitivity is due to the strong frequency dependence of detector noise, particularly at low frequencies.