Search for millisecond X-ray pulsations from the Galactic bulge X-ray source GX 9+1

The recent discoveries of millisecond radio pulsars with small period derivatives and quasi-periodic oscillations in bright Galactic X-ray sources have given rise to theories which predict that low-mass X-ray binaries should have low-amplitude X-ray pulsations with short, possibly millisecond, periods. Most previous searches for these pulsations suffer from one or more of the following deficiencies: very short periods are not searched; the searches are not sufficiently sensitive; Doppler smearing of the pulsar due to its binary motion is ignored. A 2.5 hr uninterrupted observation of GX 9 + 1 with 0.25 ms time resolution was obtained on 1985 September 21 with the Medium Energy Experiment on board the EXOSAT Observatory. We have used an optimum Coherence Recovery Technique to correct the data for Doppler smearing. A complete search covering the phase space of all likely binary orbits was performed on a Connection Machine CM-2, a data parallel computer well-suited for tasks of this type. The complete search covered pulse periods between 0.5 ms and 17 min, binary periods greater than 3.6 hr, and companion star masses less than 0.9 M-circle dot. Approximately 3000 independent orbits were searched in each of nine independent data segments; more than 5 x 10(9) spectral channels were examined. We obtain a 3 sigma upper limit to the pulse fraction of GX 9+1 of 0.011 for periods greater than 1 ms, and 0.016 for periods between 0.5-1 ms.