The torque reversals of 4U 1626–67

¹ Sabancı University, Orhanlı, Tuzla, 34956 İstanbul, Turkey
e-mail: gencali@sabanciuniv.edu
² Faculty of Science, Department of Physics, İstanbul University, 34134 Vezneciler, İstanbul, Turkey
³ Istanbul Technical University, Faculty of Science and Letters, Physics Engineering Department, 34469 İstanbul, Turkey
⁴ Faculty of Science, Department of Astronomy and Space Sciences, Ege University, 35100 Bornova, İzmir, Turkey
⁵ Institut für Astronomie und Astrophysik, Universität Tübingen, Sand 1, 72076 Tübingen, Germany
⁶ Cumhuriyet mah., Zümrüt sok., Üsküdar, 34699 İstanbul, Turkey

Received: 11 July 2021
Accepted: 24 October 2021

Abstract

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, r_co, while the conventional Alfvén radius is greater than but close to r_co; (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.