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T.E. Strohmayer, J.H. Swank (NASA/GSFC), C.B. Markwardt (NRC/GSFC), RXTE/PCA Team
We have modelled the frequency evolution of X-ray brightness oscillations seen during thermonuclear X-ray bursts. We show that a simple exponential chirp model describes well the evolution of pulse frequency and phase during the cooling tails of X-ray bursts from 4U 1728-34 and 4U 1702-429. We show that the pulse trains present in the cooling tails of bursts are highly coherent, with Q values Q \equiv \nu / \delta\nufwhm ~4000. In 4U 1728-34 and 4U 1702-429 we find universal spin-up in the cooling tails, while for 4U 1636-53 a burst with an extremely long cooling tail shows an episode of both modest spin up followed by stronger spin down. We also present evidence for initial spin-down at the rising edge of some bursts. We use our frequency evolution models to add the signals from separate bursts coherently to search for harmonics and sub-harmonics of the strongest oscillation frequency in bursts from 4U 1728-34 and 4U1 702-429. We discuss the frequency evolution in terms of rotational modulation of an angular momentum conserving thermonuclear shell.
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