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Neutron stars in low mass X-ray binaries are hypothesised to emit continuousgravitational waves that may be detectable by ground-based observatories. Thetorque balance model predicts that a higher accretion rate produceslarger-amplitude gravitational waves, hence low mass X-ray binaries with highX-ray flux are promising targets for gravitational wave searches. The detectionof X-ray pulsations would identify the spin frequency of these neutron stars,and thereby improve the sensitivity of continuous gravitational-wave searchesby reducing the volume of the search parameter space. We perform asemi-coherent search for pulsations in the two low mass X-ray binaries ScorpiusX-1 and Cygnus X-2 using X-ray data from the \textit{ Rossi X-ray TimingExplorer} Proportional Counter Array. We find no clear evidence for pulsations,and obtain upper limits (at $90\%$ confidence) on the fractional pulseamplitude, 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 etal. (1994) by factors of $\sim 8.2$ and $\sim 1.6$ respectively.

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