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S. van Straaten, M. van der Klis (University of Amsterdam), R. Wijnands (University of St. Andrews)
We studied the correlated X-ray spectral and timing behaviour of the atoll sources 4U 0614+09 and 4U 1728-34 and demonstrate that the frequencies of the variability components of these two sources follow a universal scheme of correlations relatively independent of source luminosity. Most frequencies, from 0.1 to 1200 Hz, vary in a strongly correlated fashion, but one is nearly constant near 150 Hz. The transient source 4U 1608-52, previously classified as an atoll source but recently reported to trace a Z track in the color diagram, is found by us to exactly conform to this same universal atoll frequency correlation scheme, suggesting a very similar accretion flow configuration. A similar study of the millisecond X-ray pulsar SAX J1808.4-3658 is currently in progress, which indicates that some discrepancies occur in that source, perhaps related to a stronger magnetic field or a different spin in the pulsar as compared to the other atoll sources.
Bulletin of the American Astronomical Society,
35#2
© 2003. The American Astronomical Soceity.