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

TL;DR

This paper reports the discovery of a cyclotron line in the X-ray pulsar IGR J16393-4643 using NuSTAR, enabling the first measurement of its neutron star's magnetic field and analyzing its spin evolution.

Contribution

The study presents the first detection of a cyclotron resonant scattering feature in IGR J16393-4643, providing a direct measurement of its magnetic field strength.

Findings

Magnetic field of neutron star measured at ~2.5 x 10^12 G.

Neutron star exhibits a long-term spin-up trend.

A break in the power density spectrum at the pulse frequency.

Abstract

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.