The variable cyclotron line in GX 301-2

¹ Institut für Astronomie und Astrophysik – Astronomie, Sand 1, 72076 Tübingen, Germany e-mail: kreyken@astro.uni-tuebingen.de
² INTEGRAL Science Data Centre, 6 Ch. d'Écogia, 1290 Versoix, Switzerland
³ Department of Physics, University of Warwick, Coventry CV4 7AL, UK
⁴ Space Sciences Laboratory, University of California, Berkeley, Berkeley, CA, 94702-7450, USA
⁵ Max-Planck-Institut für extraterrestrische Physik, Giessenbachstr. 1, 85740 Garching, Germany
⁶ Center for Astrophysics and Space Sciences, University of California at San Diego, La Jolla, CA 92093-0424, USA

Received: 10 December 2003
Accepted: 11 August 2004

Abstract

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 RXTE continuously 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  G and  G.