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 lowmass X-ray binary 4U 0614+091 in observations with XTE. The observations spanseveral months and sample the source over a large range of X-ray luminosity. Inevery interval QPOs are present above 400 Hz with fractional RMS amplitudesfrom 3 to 12%. At high count rates, two high frequency QPOs are detectedsimultaneously. The difference of their frequency centroids is consistent with323 Hz in all observations. During one interval a third signal is detected at328 +/- 2 Hz. This suggests the system has a stable `clock' which is mostlikely the neutron star with spin period 3.1 msec. Thus, our observations andthose of another neutron star system by Strohmayer et al. (1996) provide thefirst evidence for millisecond pulsars within low-mass X-ray binary systems andreveal the `missing-link' between millisecond radiopulsars and the late stagesof binary evolution in low mass X-ray binaries (Alpar et al. 1982). We suggestthat the kinematics of the magnetospheric beat-frequency model (Alpar andShaham 1985) applies to these QPOs. In this interpretation the high frequencysignal is associated with the Keplerian frequency of the inner accretion diskand the lower frequency `beat' signal arises from the differential rotationfrequency of the inner disk and the spinning neutron star. Assuming the highfrequency QPO is a Keplerian orbital frequency for the accretion disk, we finda maximum mass of 1.9 solar masses and a maximum radius of 17 km for theneutron star.Comment: 9 pages, submitted to the Astrophysical Journal Letter