Evidence for a nearly orthogonal rotator in GX 301–2 with phase-resolved cyclotron resonant scattering features

Cyclotron resonant scattering features (CRSFs) are the absorption features in the X-ray spectra of strongly magnetized accretion neutron stars (NSs), which are probably the most reliable probe to the surface magnetic fields of NSs. The high-mass X-ray binary GX 301–2 exhibits a very wide, variable, and complicated CRSF in the average spectra, which should be two absorption lines based on Nuclear Spectroscopic Telescope Array (NuStar) and Hard X-ray Modulation Telescope (Insight-HXMT) observations. With the Insight-HXMT frequent observations, we performed the phase-resolved spectroscopy and confirmed two cyclotron absorption lines in the phase-resolved spectra, with their centroid energy ratio ∼1.6–1.7 in the supercritical luminosity case. A major hindrance in understanding those CRSFs is the very poorly constrained magnetic inclination angle, which is also a fundamental property of an NS and key to understanding the emission characteristics of a pulsar. Comparing the phase-resolved CRSF with simulated X-ray spectra, the magnetic inclination angle is found to be ≳70°, i.e. nearly orthogonal between the NS’s spin and magnetic axis. The implications of an orthogonal rotator and magnetic structure evolution in the accreting X-ray binary are also discussed.