Deep searches for X-ray pulsations from Scorpius X-1 and Cygnus X-2 in support of continuous gravitational wave searches

TL;DR

This study conducted a semi-coherent search for X-ray pulsations in Scorpius X-1 and Cygnus X-2 to aid gravitational wave detection efforts, setting new upper limits on pulsation amplitudes.

Contribution

It provides the first semi-coherent search for X-ray pulsations in these sources, improving upper limits and aiding gravitational wave search strategies.

Findings

No clear evidence for pulsations was found.

Upper limits on fractional pulse amplitude were set at 0.034% and 0.23%.

Upper limits improved previous constraints by factors of 8.2 and 1.6.

Abstract

Neutron stars in low mass X-ray binaries are hypothesised to emit continuous gravitational waves that may be detectable by ground-based observatories. The torque balance model predicts that a higher accretion rate produces larger-amplitude gravitational waves, hence low mass X-ray binaries with high X-ray flux are promising targets for gravitational wave searches. The detection of X-ray pulsations would identify the spin frequency of these neutron stars, and thereby improve the sensitivity of continuous gravitational-wave searches by reducing the volume of the search parameter space. We perform a semi-coherent search for pulsations in the two low mass X-ray binaries Scorpius X-1 and Cygnus X-2 using X-ray data from the \textit{ Rossi X-ray Timing Explorer} Proportional Counter Array. We find no clear evidence for pulsations, and obtain upper limits (at $90\%$ confidence) on the fractional pulse amplitude, with the most stringent being $0.034\%$ for Scorpius X-1 and $0.23\%$ for Cygnus X-2. These upper limits improve upon those of Vaughan et al. (1994) by factors of $\sim 8.2$ and $\sim 1.6$ respectively.