On the ultra-long spin period of 4U 1954+31

TL;DR

This paper proposes a new model explaining the ultra-long spin period of the neutron star in 4U 1954+31 by considering unsteady wind accretion, which reduces the need for an extremely strong magnetic field.

Contribution

It introduces a model accounting for unsteady wind accretion and angular momentum variation, explaining long neutron star spin periods without ultra-strong magnetic fields.

Findings

Neutron stars can attain long spin periods through variable wind accretion.

The model explains the observed spin period distribution of long-period neutron stars.

Long-term torque cancellation occurs due to alternating angular momentum of accreted matter.

Abstract

4U 1954+31 is a high-mass X-ray binary (HMXB) that contains a neutron star and an M supergiant companion. The neutron star has a spin period of ~5.4 hr. The traditional wind-accreting model requires an ultra-strong magnetic field for the neutron star to explain its extremely long spin period, which seems problematic for the neutron star with an age of a few tens of million years. In this work, we take into account the unsteady feature of wind accretion, which results in alternation of the direction of the wind matter's angular momentum. Accordingly, the torque exerted by the accreted wind matter varies between positive and negative from time to time, and largely cancels out over long time. In such a scenario, neutron stars can naturally attain long spin periods without the requirement of a very strong magnetic field. This may also provide a reasonable explanation for the spin period distribution of long-period neutron stars in HMXBs.