The Disk-Magnetosphere Interaction in the Accretion-Powered Millisecond Pulsar SAX J1808.4-3658
Abstract
The recent discovery of the first known accretion-powered millisecond pulsar with the Rossi X-Ray Timing Explorer allows the first direct test of models for the interaction of the accretion disk and the magnetic field of weakly magnetic (B<10^10 G) neutron stars in X-ray binaries. The simple Mdot^(3/7) scaling law for the Keplerian frequency at the magnetic interaction radius, widely used to model disk accretion onto magnetic stars, is not consistent with observations of SAX J1808.4-3658 for most proposed equations of state for stable neutron stars. We argue that the usually neglected effects of multipole magnetic moments, radiation drag forces, and general relativity must be considered when modeling such weakly magnetic systems. Using only very general assumptions, we also obtain a robust estimate of mu=(1-10)x10^(26) G cm^3 for the dipole magnetic moment of SAX J1808.4-3658, implying a surface dipole field of 10^8-10^9 G at the stellar equator. Finally, we discuss the implications of our results for the origin of the millisecond radio pulsars and for the properties of the non-pulsing weakly-magnetic neutron stars in low-mass X-ray binaries.Keywords
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