  The high-mass X-ray binary and accreting X-ray pulsar IGR J16393-4643 was
observed by NuSTAR in the 3-79 keV energy band for a net exposure time of 50
ks. We present the results of this observation which enabled the discovery of a
cyclotron resonant scattering feature with a centroid energy of 29.3(+1.1/-1.3)
keV. This allowed us to measure the magnetic field strength of the neutron star
for the first time: B = (2.5+/-0.1)e12 G. The known pulsation period is now
observed at 904.0+/-0.1 s. Since 2006, the neutron star has undergone a
long-term spin-up trend at a rate of P' = -2e-8 s/s (-0.6 s per year, or a
frequency derivative of nu' = 3e-14 Hz/s ). In the power density spectrum, a
break appears at the pulse frequency which separates the zero slope at low
frequency from the steeper slope at high frequency. This addition of angular
momentum to the neutron star could be due to the accretion of a quasi-spherical
wind, or it could be caused by the transient appearance of a prograde accretion
disk that is nearly in corotation with the neutron star whose magnetospheric
radius is around 2e8 cm.
