Abstract
Abstract We present the results of a semicoherent search for continuous gravitational waves from the low-mass X-ray binary Scorpius X-1, using data from the first Advanced LIGO observing run. The search method uses details of the modeled, parametrized continuous signal to combine coherently data separated by less than a specified coherence time, which can be adjusted to trade off sensitivity against computational cost. A search was conducted over the frequency range 25– , spanning the current observationally constrained range of binary orbital parameters. No significant detection candidates were found, and frequency-dependent upper limits were set using a combination of sensitivity estimates and simulated signal injections. The most stringent upper limit was set at , with comparable limits set across the most sensitive frequency range from 100 to . At this frequency, the 95% upper limit on the signal amplitude h 0 is marginalized over the unknown inclination angle of the neutron star’s spin, and assuming the best orientation (which results in circularly polarized gravitational waves). These limits are a factor of 3–4 stronger than those set by other analyses of the same data, and a factor of ∼7 stronger than the best upper limits set using data from Initial LIGO science runs. In the vicinity of , the limits are a factor of between 1.2 and 3.5 above the predictions of the torque balance model, depending on the inclination angle; if the most likely inclination angle of 44° is assumed, they are within a factor of 1.7.
Highlights
Rotating neutron stars (NSs) are the primary expected source of continuous, periodic gravitational waves (GWs) for groundbased GW detectors
It has been suggested (Papaloizou & Pringle 1978; Wagoner 1984; Bildsten 1998) that an low-mass X-ray binary (LMXB) may be in an equilibrium state where the spin-up due to accretion is due to the spin-down due to GWs
For each range of frequencies, this table shows the minimum and maximum coherence time Tmax used for the search, across the different orbital parameter space cells, the threshold in signal-to-noise ratio (S/N) ρ used for follow-up, the total number of templates, and the number of candidates at various stages of the process. (See Section 4 for detailed description of the follow-up procedure.) a This is the number of candidates that would be expected in Gaussian noise, given the number of templates and the follow-up threshold. b This is actual number of candidates that crossed the S/N threshold and were followed up. c This is the number of candidates remaining after refinement
Summary
Rotating neutron stars (NSs) are the primary expected source of continuous, periodic gravitational waves (GWs) for groundbased GW detectors. Such an equilibrium scenario would produce a relation between the observed accretion-induced X-ray flux of the LMXB and the expected strength of the GWs. Scorpius X-1 (Sco X-1), the most luminous LMXB, is a promising potential source of GWs (Papaloizou & Pringle 1978; Wagoner 1984; Bildsten 1998). There are uncertainties (relevant to the present search) in the projected velocity amplitude K1 of the NS, the orbital period Porb, and the time Tasc at which the neutron star crosses the ascending node (moving away from the observer), measured in the solar system barycenter.
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