The variable cyclotron line in GX 301-2

We present pulse phase resolved spectra of the hypergiant high mass X-ray binary GX 301-2. We observed the source in 2001 October with RXTEcontinuously for a total on-source time of almost 200 ks. We model the continuum with both, a heavily absorbed partial covering model and a reflection model. In either case we find that the well known cyclotron resonant scattering feature (CRSF) at ~35 keV is – although present at all pulse phases – strongly variable over the pulse: the line position varies by 25% from 30 keV in the fall of the secondary pulse to 38 keV in the fall of the main pulse where it is deepest. The line variability implies that we are seeing regions of magnetic field strength varying between $3.4\times 10^{12}$G and $4.2\times 10^{12}$G.