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Abstract

We present the results of a search in LIGO O2 public data for continuous
gravitational waves from the neutron star in the low-mass X-ray binary Scorpius
X-1. We search for signals with $\approx$ constant frequency in the range
40-180 Hz. Thanks to the efficiency of our search pipeline we can use a long
coherence time and achieve unprecedented sensitivity, significantly improving
on existing results. This is the first search that has been able to probe
gravitational wave amplitudes that could balance the accretion torque at the
neutron star radius. Our search excludes emission at this level between 67.5 Hz
and 131.5 Hz, for an inclination angle $44^\circ \pm 6^\circ$ derived from
radio observations (Fomalont et al. 2001), and assuming that the spin axis is
perpendicular to the orbital plane. If the torque arm is $\approx $ 26 km -- a
conservative estimate of the \alfven\ radius -- our results are more
constraining than the indirect limit across the band. This allows us to exclude
certain mass-radius combinations and to place upper limits on the strength of
the star's magnetic field. We also correct a mistake that appears in the
literature in the equation that gives the gravitational wave amplitude at the
torque balance (Abbott et al. 2017b, 2019a) and we re-interpret the associated
latest LIGO/Virgo results in light of this.