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C. B. Markwardt (U. Maryland and GSFC)
Neutron star accretion disks are predicted to apply a torque to the central compact object at the Alfven radius in the disk. The amount depends on the mass accretion rate and magnetic field of the neutron star. The accretion torque should be directly measureable in the spin evolution of the neutron star if it is a pulsar. The recently discovered millisecond pulsar XTE J0929--314 has a reported, essentially constant, spin-down rate of ~9\times 10-14 Hz/s. We investigated the timing solutions of the two other accreting millisecond pulsars, SAX J1808.4--3658 (1998 outburst) and XTE J1751--305 (2002 outburst), using the Z2 technique. We find that for a portion of the outbursts, there are distinct spin-ups of ~ 3\times 10-13 Hz/s and 2\times 10-12 Hz/s for the two sources respectively. However, these spin-ups are not uniform over the entire outbursts, and there is some suggestion of glitch behavior. We will discuss the implications of these timing results.
Bulletin of the American Astronomical Society,
© 2003. The American Astronomical Soceity.