Abstract
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 ≈ 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 and 131.5 Hz, for an inclination angle 44° ± 6° derived from radio observations, and assuming that the spin axis is perpendicular to the orbital plane. If the torque arm is ≈26 km—a conservative estimate of the Alfvén 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 with a different probe than ever used before. We also correct a mistake that appears in the literature in the equation that gives the gravitational wave amplitude at the torque balance and we re-interpret the associated latest LIGO/Virgo results in light of this.
Highlights
Fast spinning neutron stars are promising sources of continuous gravitational waves in the frequency range 20 Hz - 2 kHz
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
∼ 90 Hz the values of the ellipticity that we are exploring are a few ×10−5 and smaller. Deformations which are this large may be sustained by a neutron star crust (Johnson-McDaniel & Owen 2013), very recent work suggests that the maximum deformations may be smaller (Gittins et al 2020). This search has placed upper limits on stable GW emission that are tighter than the level predicted by torque balance models for Scorpius X-1 (Sco X-1), for ι ∼ 44◦
Summary
Fast spinning neutron stars are promising sources of continuous gravitational waves in the frequency range 20 Hz - 2 kHz. Uncertainty about the accretion process and the stellar response makes it hard to compute firm estimates for the expected size of the resulting ellipticities, but they could be large enough for the resulting gravitational wave emission to be detectable with the current generation of detectors What effect might such a gravitational wave torque have on an accreting neutron star?
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