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Session 113 - Old Worn-Out Stars.
Oral session, Thursday, January 16
Piers 2/3,

[113.02] Quasi-periodic Oscillations and Constraints on the Neutron Star Mass and Radius in the Low-mass X-ray Binary 4U 0614+091

P. Kaaret, E. Ford, M. Tavani (Columbia U.), D. Barret, P. Bloser, J. Grindlay (Harvard-Smithsonian CfA), B. A. Harmon (NASA/MSFC), W. S. Paciesas (U. Alabama), S. N. Zhang (USRA/MSFC)

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. 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. This suggests the system has a stable `clock' which is most likely the neutron star with spin period 3.1 msec. Thus, our observations provide 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. We suggest that the kinematics of the magnetospheric beat-frequency model applies to these QPOs: the higher frequency QPO is associated with the Keplerian frequency of the inner accretion disk and the lower frequency QPO is the `beat' signal arising from differential rotation of the inner disk and the spinning neutron star. Assuming the high frequency QPO is a Keplerian orbital frequency, we find a maximum mass of 1.9 M_ødot and a maximum radius of 17 km for the neutron star.

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