We have detected quasi-periodic oscillations ( QPOs ) near 1 kHz from the low mass X-ray binary 4U 0614+091 in observations with XTE .
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 % .
At high count rates , two high frequency QPOs are detected simultaneously .
The difference of their frequency centroids is consistent with 323 Hz in all observations .
During one interval a third signal is detected at 328 \pm 2 Hz .
This suggests the system has a stable ‘ clock ’ which is most likely the neutron star with spin period 3.1 msec .
Thus , our observations and those of another neutron star system by Strohmayer et al .
( 1996 ) 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 ( Alpar et al .
1982 ) .
We suggest that the kinematics of the magnetospheric beat-frequency model ( Alpar and Shaham 1985 ) applies to these QPOs .
In this interpretation the high frequency signal is associated with the Keplerian frequency of the inner accretion disk and the lower frequency ‘ beat ’ signal arises from 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 _ { \odot } and a maximum radius of 17 km for the neutron star .