Scorpius X-1 ( Sco X-1 ) and X-ray transient XTE J1751-305 are Low-Mass X-ray Binaries ( LMXBs ) that may emit continuous gravitational waves detectable in the band of ground-based interferometric observatories .
Neutron stars in LMXBs could reach a torque-balance steady-state equilibrium in which angular momentum addition from infalling matter from the binary companion is balanced by angular momentum loss , conceivably due to gravitational-wave emission .
Torque-balance predicts a scale for detectable gravitational-wave strain based on observed X-ray flux .
This paper describes a search for Sco X-1 and XTE J1751-305 in LIGO Science Run 6 data using the TwoSpect algorithm , based on searching for orbital modulations in the frequency domain .
While no detections are claimed , upper limits on continuous gravitational-wave emission from Sco X-1 are obtained , spanning gravitational wave frequencies from 40 to 2040 Hz and projected semi-major axes from 0.90 to 1.98 light-seconds .
These upper limits are injection-validated , equal any previous set in initial LIGO data , and extend over a broader parameter range .
At optimal strain sensitivity , achieved at 165 Hz , the 95 % confidence level random-polarization upper limit on dimensionless strain h _ { 0 } is approximately 1.8 \times 10 ^ { -24 } .
The closest approach to the torque-balance limit , within a factor of 27 , is also at 165 Hz .
Upper limits are set in particular narrow frequency bands of interest for J1751-305 .
These are the first upper limits known to date on r -mode emission from this XTE source .
The TwoSpect method will be used in upcoming searches of Advanced LIGO and Virgo data .