On the torque reversals of 4U 1626--67

TL;DR

This paper models the torque reversals of neutron star 4U 1626--67 using a comprehensive disk model, explaining the transition between spin-up and spin-down phases and matching observed torque-luminosity relations.

Contribution

It introduces a detailed model of the inner disk radius and torque behavior that accurately reproduces the torque reversals and luminosity relations of 4U 1626--67.

Findings

Torque reversals occur at the transition between weak-propeller and spin-up phases.

Rotational equilibrium is when the inner disk radius equals the co-rotation radius.

Both spin-up and spin-down torques operate around the reversal point.

Abstract

We have 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 torque -- X-ray luminosity relation 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, $r_\mathrm{co}$, while the conventional Alfven radius is greater than and close to $r_\mathrm{co}$, (2) both spin-up and spin-down torques are operating on either side of torque reversal, (3) with 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.