The torque reversals of 4U 1626-67

We investigated the detailed torque-reversal behavior of 4U 1626-67 in the framework of the recently developed comprehensive model of the inner disk radius and torque calculations for neutron stars accreting from geometrically thin disks. The model can reproduce the relation between the torque and X-ray luminosity across the torque reversals of 4U 1626-67. Our results imply that: (1) rotational equilibrium is reached when the inner disk radius equals the co-rotation radius, rco, while the conventional Alfvén radius is greater than but close to rco; (2) both spin-up and spin-down torques are operating on either side of torque reversal; and (3) with the increasing accretion rate, the spin-up torque associated with accretion onto the star gradually dominates the spin-down torque exerted by the disk. The torque reversals are the natural outcome of transitions between the well-defined weak-propeller and spin-up phases of the star with a stable, geometrically thin accretion disk.