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Nuclear-powered millisecond pulsars and the maximum spin frequency of neutron stars

Abstract

Millisecond pulsars are neutron stars that are thought to have been spun-up by mass accretion from a stellar companion1. It is not known whether there is a natural brake for this process, or if it continues until the centrifugal breakup limit is reached at submillisecond periods. Many neutron stars that are accreting mass from a companion star exhibit thermonuclear X-ray bursts that last tens of seconds, caused by unstable nuclear burning on their surfaces2. Millisecond-period brightness oscillations during bursts from ten neutron stars (as distinct from other rapid X-ray variability that is also observed3,4) are thought to measure the stellar spin2,5, but direct proof of a rotational origin has been lacking. Here we report the detection of burst oscillations at the known spin frequency of an accreting millisecond pulsar, and we show that these oscillations always have the same rotational phase. This firmly establishes burst oscillations as nuclear-powered pulsations tracing the spin of accreting neutron stars, corroborating earlier evidence5,6. The distribution of spin frequencies of the 11 nuclear-powered pulsars cuts off well below the breakup frequency for most neutron-star models, supporting theoretical predictions that gravitational radiation losses can limit accretion torques in spinning up millisecond pulsars7,8,9.

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Figure 1: Dynamic power spectra of millisecond oscillations in an X-ray burst on 18 October 2002 from SAX J1808.4–3658.
Figure 2: Relative phase of the burst tail oscillations in SAX J1808.4–3658, and the persistent accretion-powered pulsations just before the burst, for all four bursts.

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Acknowledgements

We thank L. Bildsten, F. Lamb, D. Psaltis and S. Thorsett for discussions. This work was supported in part by NASA and the Alfred P. Sloan Foundation.

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Correspondence to Deepto Chakrabarty.

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Chakrabarty, D., Morgan, E., Muno, M. et al. Nuclear-powered millisecond pulsars and the maximum spin frequency of neutron stars. Nature 424, 42–44 (2003). https://doi.org/10.1038/nature01732

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