| Abstract: | We present the results of a model-based search for continuous gravitational
waves from the low-mass X-ray binary Scorpius X-1 using LIGO detector data from
the third observing run of Advanced LIGO, Advanced Virgo and KAGRA. This is a
semicoherent search which uses details of the signal model to coherently
combine data separated by less than a specified coherence time, which can be
adjusted to balance sensitivity with computing cost. The search covered a range
of gravitational-wave frequencies from 25Hz to 1600Hz, as well as ranges in
orbital speed, frequency and phase determined from observational constraints.
No significant detection candidates were found, and upper limits were set as a
function of frequency. The most stringent limits, between 100Hz and 200Hz,
correspond to an amplitude h0 of about 1e-25 when marginalized isotropically
over the unknown inclination angle of the neutron star’s rotation axis, or less
than 4e-26 assuming the optimal orientation. The sensitivity of this search is
now probing amplitudes predicted by models of torque balance equilibrium. For
the usual conservative model assuming accretion at the surface of the neutron
star, our isotropically-marginalized upper limits are close to the predicted
amplitude from about 70Hz to 100Hz; the limits assuming the neutron star spin
is aligned with the most likely orbital angular momentum are below the
conservative torque balance predictions from 40Hz to 200Hz. Assuming a broader
range of accretion models, our direct limits on gravitational-wave amplitude
delve into the relevant parameter space over a wide range of frequencies, to
500Hz or more. |
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