Magnetic angle evolution in accreting neutron stars
Abstract
The rotation of a magnetized accreting neutron star (NS) in a binary system is described by its spin period and two angles: spin inclination α with respect to the orbital momentum and magnetic angle χ between the spin and the magnetic moment. Magnetospheric accretion spins the NS up and adjusts its rotation axis, decreasing α 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 α and χ 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 Δχ ~ 15°-20°. This orthogonalization may be an important factor in the evolution of millisecond pulsars, as it partially compensates the χ 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 χ peaking at χ = 0° and χ = 90°.
- Publication:
-
Monthly Notices of the Royal Astronomical Society
- Pub Date:
- August 2021
- DOI:
- 10.1093/mnras/stab1378
- arXiv:
- arXiv:2105.00754
- Bibcode:
- 2021MNRAS.505.1775B
- Keywords:
-
- accretion;
- accretion discs;
- stars: neutron;
- X-rays: binaries;
- Astrophysics - High Energy Astrophysical Phenomena
- E-Print:
- 12 pages, 7 figures. Resubmitted to MNRAS after minor revision. Any feedback is very welcome!