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R. Wijnands (University of St Andrews), M. van der Klis (University of Amsterdam), J. Homan (INAF-OAB), D. Chakrabarty (MIT), C.B. Markwardt (NASA/GSFC), E.H. Morgan (MIT)
In October-November 2002 the accretion driven millisecond X-ray pulsar SAX J1808.4-3658 exhibited its first completely observable outburst since its discovery as a pulsar in 1998. During this outburst, we have obtained approximately 700 ksec of on source data using the Rossi X-ray Timing Explorer satellite. We report the discovery of two simultaneous quasi-periodic oscillations (QPOs) with varying frequencies between 300 and 700 Hz. These are the first kHz QPOs seen from a pulsar. The frequency separation between the two QPOs is approximately 200 Hz, which is not equal or close to the 401 Hz spin frequency of the source, but it is consistent with half that frequency. This is reminiscent of what has been observed in non-pulsing sources in which the frequency separation is close to half the frequency of the oscillations seen during type-I X-ray bursts. It proves that there is a relation between kHz QPOs and neutron star spin, and strengthens the spin interpretation of these burst oscillations. This result is not easily accommodated within beat-frequency models for the kHz QPOs. We discuss further implications of our results for models proposed for kHz QPOs in neutron star X-ray binaries. We also report the discovery of a third, enigmatic kHz QPO seen so far only in SAX J1808.4-3658 and therefore possibly unique to pulsating kHz QPO sources. We discuss the possible nature of this QPO and its relationship to the other two kHz QPOs.
Bulletin of the American Astronomical Society,
© 2003. The American Astronomical Soceity.