Probing more deeply in an all-sky search for continuous gravitational waves in the LIGO O3 data set

We report results from an all-sky search of the LIGO data from the third LIGO-Virgo-KAGRA run (O3) for continuous gravitational waves from isolated neutron stars in the frequency band [30, 150] Hz and spindown range of $[-1\times {10}^{-8},+1\times {10}^{-9}]\mathrm{Hz}/s$. This search builds upon a previous analysis of the first half of the O3 data using the same powerflux pipeline. We search more deeply here by using the full O3 data and by using loose coherence in the initial stage with fully coherent combination of LIGO Hanford (H1) and LIGO Livingston (L1) data, while limiting the frequency band searched and excluding narrow, highly disturbed spectral bands. We detect no signal and set strict frequentist upper limits on circularly polarized and on linearly polarized wave amplitudes, in addition to estimating population-averaged upper limits. The lowest upper limit obtained for circular polarization is $\sim 4.5\times {10}^{-26}$, and the lowest linear polarization limit is $\sim 1.3\times {10}^{-25}$ (both near 144 Hz). The lowest estimated population-averaged upper limit is $\sim 1.0\times {10}^{-25}$. In the frequency band and spindown range searched here, these limits improve upon the O3a powerflux search by a median factor of $\sim 1.4$ and upon the best previous limits obtained for the full O3 data by a median factor of $\sim 1.1$.