Evidence from quasi-periodic oscillations for a millisecond pulsar in the low-mass X-ray binary 4U 0614+091

We have detected quasi-periodic oscillations (QPOs) near 1 kHz from the low-mass X-ray binary 4U 0614+091 in observations with the Rossi X-Ray Timing Explorer. The observations span several months and sample the source over a large range of X-ray luminosity. In every interval, QPOs are present above 400 Hz with fractional rms amplitudes from 3% to 12% over the full Proportional Counter Array energy band, At high count rates, two high-frequency QPOs are detected simultaneously. The difference in their frequency centroids is consistent with a constant value of 323 +/- 4 Hz in all observations. During one interval, a third signal is detected at 328 +/- 2 Hz. This suggests that the system has a stable ''clock'' that is most likely the neutron star with spin period 3.1 ms. Thus, our observations of 4U 0614+091, and those of 4U 1728-34 and KS 1731-260, provide the first evidence for millisecond pulsars within low-mass X-ray binary systems and reveal the ''missing-link'' between millisecond radiopulsars and the late stages of binary evolution in low-mass X-ray binaries. The constant difference in the high-frequency QPOs suggests a beat-frequency interpretation. In this model, the high-frequency QPO is associated with the Keplerian frequency of the inner accretion disk, and the lower frequency QPO is a ''beat'' between the differential rotation frequency of the inner disk and the spinning neutron star. Assuming the high-frequency QPO is a Keplerian orbital frequency for the accretion disk, we find a maximum mass of 1.9 M. and a maximum radius of 17 km for the neutron star.