Discovery of 1122 Hz X-ray Burst Oscillations from the Neutron-star X-ray Transient Xte J1739-285

We report the discovery of oscillations at 1122 ± 0.3 Hz in an X-ray burst from the X-ray transient XTE J1739-285. The signal has a peak Leahy power of 42.8 and, after consideration of the number of trials, has a chance probability of occurrence of 4×10 −5 , equivalent to a 4.2σ detection. The oscillation frequency suggests that XTE J1739-285 contains the fastest rotating neutron star yet found. We also found millisecond quasiperiodic oscillations in the persistent emission with frequencies ranging from 757 Hz to 862 Hz. We detected seven X-ray bursts and derive an upper limit on the source distance of about 10.6 kpc from the brightest burst. 1. INTRODUCTION Weakly magnetized neutron stars can be spun up to rates of several 100 Hz by accretion in low-mass X-ray bi-naries (Alpar et al. 1982). The first direct measurements of millisecond spin rates in actively accreting neutron stars in low-mass X-ray binaries (LMXBs) came from the discovery of oscillations in thermonuclear X-ray bursts occurring on the neutron star surface (Strohmayer et al. 1996). The burst oscillation frequencies were later found to be nearly equal to those of coherent pulsations (in 't Zand et al. 2001; Strohmayer & Markwardt 2002; Chakrabarty et al. 2003; Strohmayer et al. 2003) implying that the burst oscillation frequency indicates the neutron star spin rate. This X-ray technique has no known biases against the detection of very high spin rates, unlike radio pulsation searches, and the sample of X-ray burst oscillation frequencies has been exploited to constrain the neutron star spin rate distribution. Analysis of a sample of 11 X-ray measured spin frequencies in the range 270-619 Hz suggests a limiting spin rate near 760 Hz if the distribution is uniform and bounded (Chakrabarty et al. 2003). This is below the expected maximum spin frequency possible without centrifugal breakup and has been interpreted as evidence that some physical process, possibly gravitational radiation, limits the maximum possible spin rate. However, the discovery of a radio pulsar spinning at 716 Hz, a frequency above any previously measured in X-rays, suggests that the true maximum spin rate is higher (Hessels et al. 2006). Here, we describe observations made with the Rossi X-Ray Timing Explorer (RXTE; Bradt, Rothschild, & Swank 1993) following the detection of X-ray bursts from the transient source XTE J1739-285 as part of a program to search for millisecond oscillations in both X-ray bursts and persistent emission …