  We report on a timing and spectral analysis of a 50-ks NuSTAR observation of
IGR J16320-4751 (= AX J1631.9-4752); a high-mass X-ray binary hosting a
slowly-rotating neutron star. In this observation from 2015, the spin period
was 1,308.8+/-0.4 s giving a period derivative dP/dt ~ 2E-8 s s-1 when compared
with the period measured in 2004. In addition, the pulsed fraction decreased as
a function of energy, as opposed to the constant trend that was seen
previously. This suggests a change in the accretion geometry of the system
during the intervening 11 years. The phase-averaged spectra were fit with the
typical model for accreting pulsars: a power law with an exponential cutoff.
This left positive residuals at 6.4 keV attributable to the known iron K-alpha
line, as well as negative residuals around 14 keV from a candidate cyclotron
line detected at a significance of 5-sigma. We found no significant differences
in the spectral parameters across the spin period, other than the expected
changes in flux and component normalizations. A flare lasting around 5 ks was
captured during the first half of the observation where the X-ray emission
hardened and the local column density decreased. Finally, the binary orbital
period was refined to 8.9912+/-0.0078 d thanks to Swift/BAT monitoring data
from 2005-2022.
