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F. K. Lamb, W. Yu (U. Illinois)
RXTE observations have shown that weak-field neutron stars in low-mass X-ray binary systems (LMXBs) produce three distinct types of millisecond X-ray oscillations. These are generated directly or indirectly by the star’s magnetic field and rotation and can therefore be used to determine the spin rates and estimate the magnetic fields of these stars. Periodic accretion-powered oscillations have been detected at the spin frequencies of 7 stars. Nearly periodic nuclear-powered oscillations have been detected at or close to the spin frequencies of more than a dozen stars. In some of these stars and in many others, pairs of accretion-powered kilohertz QPOs have been detected with a frequency separation equal to the spin frequency or half of it. These oscillations show that more than two dozen neutron stars in LMXBs have spin rates and magnetic fields in the range that will likely make them radio-emitting millisecond pulsars when accretion ceases, providing compelling evidence that they are progenitors of rotation-powered millisecond pulsars. The current P-\dot P distribution of the observed rotation-powered millisecond pulsars strongly supports the hypothesis that some were accreting at rates high enough to reach magnetic spin equilibrium before accretion ceased and they became rotation-powered millisecond pulsars. However, many are accreting at average rates too low to be spun up to frequencies much above a hundred Hertz, even in the absence of magnetic or gravitational radiation braking. These stars are not in magnetic spin equilibrium now and are unlikely ever to reach magnetic spin equilibrium before mass transfer ends, contrary to what has generally been assumed. This has important implications for the evolution of accretion-powered millisecond pulsars, the initial spin rates of rotation-powered millisecond pulsars, and the prospects for detecting gravitational radiation from them.
This work was supported in part by NSF grant AST 0098399, NASA grant NAG5-12030, and funds of the Fortner Endowed Chair at Illinois.
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Bulletin of the American Astronomical Society, 37 #4
© 2005. The American Astronomical Soceity.