Magnetic angle evolution in accreting neutron stars

Abstract

The rotation of a magnetised accreting neutron star (NS) in a binary system is described by its spin period and two angles: spin inclination $\alpha$ with respect to the orbital momentum and magnetic angle $\chi$ between the spin and the magnetic moment. Magnetospheric accretion spins the NS up and adjusts its rotation axis, decreasing $\alpha$ to nearly perfect alignment. Its effect upon the magnetic angle is more subtle and relatively unstudied. In this work, we model the magnetic angle evolution of a rigid spherical accreting NS. We find that the torque spinning the NS up may affect the magnetic angle while both $\alpha$ and $\chi$ significantly deviate from zero, and the spin-up torque varies with the phase of the spin period. As the rotation axis of the NS is being aligned with the spin-up torque, the magnetic axis becomes misaligned with the rotation axis. Under favourable conditions, magnetic angle may increase by $\Delta \chi \sim 15^{\circ }-20^{\circ }$. This orthogonalisation may be an important factor in the evolution of millisecond pulsars, as it partially compensates the $\chi$ decrease potentially caused by pulsar torques. If the direction of the spin-up torque changes randomly with time, as in wind-fed high-mass X-ray binaries, both the rotation axis of the NS and its magnetic axis become involved in a non-linear random-walk evolution. The ultimate attractor of this process is a bimodal distribution in $\chi$ peaking at $\chi =0^{\circ }$ and $\chi =90^{\circ }$.

12 pages, 7 figures. Resubmitted to MNRAS after minor revision. Any feedback is very welcome!

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• Quasi-universality of the magnetic deformation of neutron stars in general relativity and beyond
• A pulsar-helium star compact binary system formed by common envelope evolution
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• A short review of the pulsar magnetic inclination angles (II)
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• Formation of millisecond pulsar-helium star binaries
• Estimates of the Surface Magnetic Field Strength of Radio Pulsars
• Origin of young accreting neutron stars in high-mass X-ray binaries in supernova remnants
• The spin-orbit alignment hypothesis in millisecond pulsars
• Evolution of Crab Pulsar: Magnetic Inclination Angle and Spin