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General Relativity and Quantum Cosmology, gr-qc, Astrophysics, High Energy Astrophysical Phenomena, astro-ph.HE
Abstract:
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, the most
stringent upper limits to date 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. At optimal
strain sensitivity, achieved at 165 Hz, the 95% confidence level
random-polarization upper limit on dimensionless strain $h_0$ is approximately
$1.8 \times 10^{-24}$. 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. The TwoSpect method will be used in
upcoming searches of Advanced LIGO and Virgo data.
