Search for Millisecond X-Ray Pulsations from the Galactic Bulge X-Ray Source GX 9+1

Abstract

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 folowing 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. 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 solar mass. A 3-sigma upper limit to the pulse fraction of GX 9+1 of 0.011 is obtained for periods greater than 1 ms, and 0.016 for periods between 0.5-1 ms.