Torque bistability in the interaction between a neutron star magnetosphere and a thin accretion disc

Abstract

We present a time-dependent model of the interaction between a neutron star magnetosphere and a thin (Shakura-Sunyaev) accretion disc, where the extent of the magnetosphere is determined by balancing outward diffusion and inward advection of the stellar magnetic field at the inner edge of the disc. The nature of the equilibria available to the system is governed by the magnetic Prandtl number Pm and the ratio ξof the corotation radius to the Alfven radius. For ξ> Pm^0.3, the system can occupy one of two stable states, where the torques are of opposite signs. If the star is spinning up initially, in the absence of extraneous perturbations, ξdecreases until the spin-up equilibrium vanishes, the star subsequently spins down, and the torque asymptotes to zero. Vortex-in-cell simulations of the Kelvin-Helmholtz instability suggest that the transport speed across the mixing layer between the disc and magnetosphere is less than the shear speed when the layer is thin, unlike in previous models.

11 pages, 10 figures

References in corpus (10)

• Observations of Accreting Pulsars
• Nuclear-Powered Millisecond Pulsars and the Maximum Spin Frequency of Neutron Stars
• Spin-Up/Spin-Down of Magnetized Stars with Accretion Disks and Outflows
• Discovery of a High-Latitude Accreting Millisecond Pulsar in an Ultracompact Binary
• Accretion Onto Fast X-Ray Pulsars
• On the Dramatic Spin-up/Spin-down Torque Reversals in Accreting Pulsars
• Quasi-Periodic Oscillations from Rayleigh-Taylor and Kelvin-Helmholtz Instability at a Disk-Magnetosphere Interface
• Warped Disks as a Possible Origin of Torque Reversals in Accretion-Powered Pulsars
• Magnetic torques between accretion discs and stars
• Spectral Transition and Torque Reversal in X-ray Pulsar 4U 1626-67

Cited by in corpus (5)

• Formation of millisecond pulsars with CO white dwarf companions - II. Accretion, spin-up, true ages and comparison to MSPs with He white dwarf companions
• Theory of Disk Accretion onto Magnetic Stars
• Accretion torque on magnetized neutron stars
• Tracking hidden magnetospheric fluctuations in accretion-powered pulsars with a Kalman filter
• A diagnosis on torque reversals in 4U 1626-67