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Session 78 - X-ray Binaries.
Oral session, Thursday, June 11

[78.04] On the Stability of Millisecond Oscillations During X-ray Bursts

T. E. Strohmayer, J. H. Swank, W. Zhang, N. E. White (NASA/GSFC), A. B. Giles, K. M. Hill (Univ. Tasmania)

We discuss the frequency stability of millisecond oscillations observed during thermonuclear X-ray bursts with the Rossi X-ray Timing Explorer (RXTE). We show that bursts from 4U1728-34 separated in time by more than 1.5 yr. have highest observed oscillation frequencies consistent with each other. The largest observed offset in frequency from burst to burst corresponds to a doppler shift of 20 km/sec, well within the magnitude expected from binary orbital motion for this source. This corresponds to a characteristic time to change the frequency of > 23,000 yr. We argue that this result favors a highly stable clock, such as stellar rotation, as the mechanism for the oscillations. For 4U1636-53, which has a known orbital period of 3.8 hrs., frequencies observed in different bursts can be used to determine whether binary motion of the system can be consistent with any observed frequency differences during bursts. We discuss the constraints on the X-ray mass function which could be derived with this technique.

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