The variable cyclotron line of GX 301-2

We present a 200 ksec observation of the High Mass X-ray Binary GX 301-2 taken in 2000 November with the Rossi X-ray Timing Explorer during the pre-periastron flare and the actual periastron passage of the neutron star. To model the spectrum we use a power law with the Fermi Dirac cutoff and a cyclotron line at higher energies plus either a reflection component or a heavily absorbed partial covering component. Although completely different, both models describe the data equally well. Phase resolved spectra show that the energy and the depth of the cyclotron resonant scattering feature vary strongly with pulse phase: It is deepest in the fall of the main pulse, the rise of the secondary pulse, and the pulse minimum in-between with tau(C) similar to0.3. In the other phase bins the line is much less deep with tau(C) similar to0.1. The energy of the line correlates strongly with its depth and varies by 25% from 30.1 keV in the fall of the secondary pulse to 37.9 keV in the fall of the main pulse.