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Quasi-periodic X-ray brightness fluctuations in an accreting millisecond pulsar


The relativistic plasma flows onto neutron stars that are accreting material from stellar companions can be used to probe strong-field gravity as well as the physical conditions in the supra-nuclear-density interiors of neutron stars. Plasma inhomogeneities orbiting a few kilometres above the stars are observable as X-ray brightness fluctuations on the millisecond dynamical timescale of the flows1,2,3. Two frequencies in the kilohertz range dominate these fluctuations: the twin kilohertz quasi-periodic oscillations (kHz QPOs). Competing models for the origins of these oscillations (based on orbital motions) all predict that they should be related to the stellar spin frequency4,5,6,7,8,9,10, but tests have been difficult because the spins were not unambiguously known. Here we report the detection of kHz QPOs from a pulsar whose spin frequency is known. Our measurements establish a clear link between kHz QPOs and stellar spin, but one not predicted by any current model. A new approach to understanding kHz QPOs is now required. We suggest that a resonance between the spin and general relativistic orbital and epicyclic frequencies could provide the observed relation between QPOs and spin.

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Figure 1: The power-density spectra obtained for SAX J1808.4–3658 during its 2002 outburst.
Figure 2: The power-density spectrum obtained for SAX J1808.4–3658 using the combined data from 18–19 October and 22–26 October.


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We thank J. Swank and E. Smith for their efforts in scheduling our RXTE observations of SAX J1808.4–3658, and D. Psaltis for discussions. M.v.d.K. was supported by NWO.

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Correspondence to R. Wijnands.

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Wijnands, R., van der Klis, M., Homan, J. et al. Quasi-periodic X-ray brightness fluctuations in an accreting millisecond pulsar. Nature 424, 44–47 (2003).

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