On the Torque Reversals of Accreting Neutron Stars

TL;DR

This paper extends an analytical model to explain torque reversals in accreting neutron stars, accounting for observed phenomena like accretion at low luminosities and transitions between spin-up and spin-down phases.

Contribution

The model now includes conditions for transitions between strong and weak propeller phases, explaining observed torque reversals without large changes in accretion rates.

Findings

Model explains accretion at low luminosities and transitions at lower accretion rates.

Torque magnitudes during reversals are similar across different systems.

Estimated torque reversal properties match observations of 4U 1626--67.

Abstract

We have extended the analytical model proposed earlier to estimate the inner disk radius of accreting neutron stars in the strong-propeller (SP) phase, and the conditions for the transitions between the strong and weak propeller (WP) phases (Ertan 2017, 2018) to the WP (accretion with spin-down) and the spin-up (SU) phases, and the torque reversals during the WP/SU transitions. The model can account for some basic observed properties of these systems that are not expected in conventional models: (1) accretion on to the star at low X-ray luminosities and the transitions to the SP phase (no accretion) at critical accretion rates much lower than the rate required for the spin-up/spin-down transition, (2) ongoing accretion throughout a large range of accretion rates while the source is spinning down (WP phase), and (3) transitions between the spin-up and spin-down phases with comparable torque magnitudes, without substantial changes in the mass-flow rate. Our results indicate that the magnitudes of the torques on either side of the torque reversal have a ratio similar for different systems independently of their spin periods, magnetic dipole moments and accretion rates during the transitions. Estimated torque reversal properties in our model are in agreement with the observed torque reversals of 4U 1626--67.