Multiple cyclotron line-forming regions in GX 301-2

Understand

We present two observations of the high-mass X-ray binary GX 301-2 with NuSTAR, taken at different orbital phases and different luminosities.

• We find that the continuum is well described by typical phenomenological models, like a very strongly absorbed NPEX model.
• However, for a statistically acceptable description of the hard X-ray spectrum we require two cyclotron resonant scattering features (CRSF), one at ~35 keV and the other at ~50 keV.
• Even though both features strongly overlap, the good resolution and sensitivity of NuSTAR allows us to disentangle them at >=99.9% significance.