A directed search for gravitational waves from Scorpius X-1 with initial LIGO

TL;DR

This study conducted a semi-coherent search for continuous gravitational waves from Scorpius X-1 using initial LIGO data, setting new upper limits on gravitational wave strain and demonstrating the effectiveness of Bayesian methods.

Contribution

It presents the first semi-coherent search for gravitational waves from Scorpius X-1 with initial LIGO data, applying Bayesian upper limits with different priors.

Findings

Median strain upper limits of 1.3e-24 at 150 Hz for standard prior.

Median strain upper limits of 8e-25 at 150 Hz for angle-restricted prior.

Improved upper limits by factors of ~1.4 and 2.3 over previous results.

Abstract

We present results of a search for continuously-emitted gravitational radiation, directed at the brightest low-mass X-ray binary, Scorpius X-1. Our semi-coherent analysis covers 10 days of LIGO S5 data ranging from 50-550 Hz, and performs an incoherent sum of coherent $\mathcal{F}$-statistic power distributed amongst frequency-modulated orbital sidebands. All candidates not removed at the veto stage were found to be consistent with noise at a 1% false alarm rate. We present Bayesian 95% confidence upper limits on gravitational-wave strain amplitude using two different prior distributions: a standard one, with no a priori assumptions about the orientation of Scorpius X-1; and an angle-restricted one, using a prior derived from electromagnetic observations. Median strain upper limits of 1.3e-24 and 8e-25 are reported at 150 Hz for the standard and angle-restricted searches respectively. This proof of principle analysis was limited to a short observation time by unknown effects of accretion on the intrinsic spin frequency of the neutron star, but improves upon previous upper limits by factors of ~1.4 for the standard, and 2.3 for the angle-restricted search at the sensitive region of the detector.