NuSTAR discovery of a cyclotron line in the accreting X-ray pulsar IGR J16393-4643

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 50ks. We present the results of this observation which enabled the discovery of acyclotron 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 starfor the first time: B = (2.5+/-0.1) x10¹² G. The known pulsation period is now observed at 904.0+/-0.1 s. Since 2006, the neutron star has undergone along-term spin-up trend at a rate of P' = -2x10^-8 s/s^-1 (-0.6 s per year, or a frequency derivative of nu' = 3x10^-14 Hz/s^-1). In the power density spectrum, abreak appears at the pulse frequency which separates the zero slope at lowfrequency from the steeper slope at high frequency. This addition of angularmomentum to the neutron star could be due to the accretion of a quasi-sphericalwind, or it could be caused by the transient appearance of a prograde accretiondisk that is nearly in corotation with the neutron star whose magnetosphericradius is around 2x10⁸ cm.