Sub-Torque-Balance Upper Limits on Continuous Gravitational Waves from Scorpius X-1

We present the results of a search for continuous gravitational waves from the low-mass X-ray binary Scorpius X-1 using LIGO data from the first part of the fourth LIGO-Virgo-KAGRA observing run. By applying the resampling version of the cross-correlation pipeline to search for signal frequencies $f_0$ between $25$ and $200\text{un}Hz$ (corresponding to neutron star spin frequencies of $12.5$ to $100\text{un}Hz$ for GW due to triaxiality, or $\sim 15-20$ to $\sim 120-150\text{un}Hz$ for GW due to $r$-modes), we set upper limits below the standard torque balance level, independent of neutron star spin inclination, for $50\text{un}Hz\lesssim f_0\lesssim 200\text{un}Hz$. While uncertainties in the modelling of torque and equation of state limit the strength of our inference, our results nonetheless argue against torque balance in this spin range for a neutron star described by a hadronic equation of state. The most sensitive upper limits on the gravitational wave amplitude $h_0$, at the upper end of the frequency band searched, approach $5\times {10}^{-26}$ marginalized over inclination angle and $2\times {10}^{-26}$ assuming the most favorable inclination. The marginalized upper limits correspond to a sensitivity depth of $70-75\text{un}{Hz}^{-1\mathrm{/}2}$, improving sensitivity considerably over previous searches. Expressed as constraints on the triaxial deformation of the neutron star, the limits correspond to an ellipticity of $3\times {10}^{-5}$ if the GW frequency $f_0$ is $75\text{un}Hz$ and $3\times {10}^{-6}$ if $f_0=200\text{un}Hz$, approaching deformations which could be supported by ordinary nuclear matter. Outliers from the search were ruled out as potential signals by a combination of hierarchical followup and analysis of additional data from later in the observing run.