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

Neutron stars in low mass X-ray binaries (LMXBs) 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 LMXBs 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 LMXBs Scorpius X-1 (Sco X-1) and Cygnus X-2 (Cyg X-2) using X-ray data from the Rossi X-ray Timing Explorer Proportional Counter Array. We find no clear evidence for pulsations, and obtain upper limits (at 90 per cent confidence) on the fractional pulse amplitude, with the most stringent being 0.034 per cent for Sco X-1 and 0.23 per cent for Cyg X-2. These upper limits improve upon those in previous work by factors of ∼8.2 and ∼1.6, respectively.