On the hysteresis effect in transitions between accretion and propeller regimes

TL;DR

This paper investigates the hysteresis effect in accretion and propeller regime transitions in neutron star systems, revealing how surface interactions and disc thickness influence transition luminosities and spectral states.

Contribution

It introduces a model explaining the hysteresis in accretion regimes based on the intersection of Alfvén and equilibrium surfaces at different latitudes.

Findings

Transition from propeller to accretion occurs at higher luminosity than vice versa.

Disc thickness affects the critical fastness parameter for regime change.

Hysteresis explains spectral transition behaviors in low-mass X-ray binaries.

Abstract

Some observations and numerical simulations of disc-magnetosphere interaction show that accretion can proceed in the propeller regime. When the Alfv\'en radius is beyond the corotation radius, matter climbs up to the high latitudes where the Alfv\'en surface is inside the equilibrium surface and can accrete. We calculate the fraction of the mass flux in the disc that can accrete onto the neutron star depending on the fastness parameter and the inclination angle between rotation and magnetic axis. We find that, for a narrow range of the fastness parameter, the Alfv\'en and the equilibrium surfaces intersect at two different critical latitudes. While the system is transiting from the propeller to the accretion regime (the initial rise of an outburst), the disc is already thick and the part of the disc between these two critical latitudes cannot accrete. In transitions from the accretion to the propeller regime (decay of an outburst), the disc is thin, hence, full accretion of matter proceeds until the Alfv\'en radius moves beyond the equilibrium radius at the disc-midplane. Therefore, the accretion regime commences at a smaller fastness parameter than it ceases. As a result, the transition from the propeller to the accretion regime occurs at a luminosity higher than the transition from the accretion to the propeller regime. We discuss the implications of our results for spectral transitions exhibited by low-mass X-ray binaries.