  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.
