Accretion onto Fast X‐Ray Pulsars

The recent emergence of a new class of accretion-powered, transient,millisecond X-ray pulsars presents some difficulties for the conventionalpicture of accretion onto rapidly rotating magnetized neutron stars and theirspin behavior during outbursts. In particular, it is unclear from the standardparadigm how these systems manage to accrete over such a wide range in dM/dt(i.e., >~ a factor of 50), and why the neutron stars exhibit a high rate ofspindown in at least a number of cases. Following up on prior suggestions, wepropose that `fast' X-ray pulsars can continue to accrete, and that theiraccretion disks terminate at approximately the corotation radius. Wedemonstrate the existence of such disk solutions by modifying theShakura-Sunyaev equations with a simple magnetic torque prescription. Thesolutions are completely analytic, and have the same dependence on dM/dt andalpha (the viscosity parameter) as the original Shakura-Sunyaev solutions; but,the radial profiles can be considerably modified, depending on the degree offastness. We apply these results to compute the torques expected during theoutbursts of the transient millisecond pulsars, and find that we can explainthe large spindown rates that are observed for quite plausible surface magneticfields of ~10^9 G.Comment: 10 pages, 5 figures, submitted to Ap