Study of the many fluorescent lines and the absorption variability in GX 301−2 with XMM-Newton

¹ Dr. Karl Remeis-Sternwarte & ECAP, Universität Erlangen-Nürnberg, Sternwartstr. 7, 96049 Bamberg, Germany
e-mail: felix.fuerst@sternwarte.uni-erlangen.de
² Center for Astrophysics & Space Sciences, University of California, San Diego, 9500 Gilman Drive, La Jolla, CA 92093, USA
³ CRESST and NASA Goddard Space Flight Center, Astrophysics Science Division, Code 661, Greenbelt, MD 20771, USA
⁴ Center for Space Science and Technology, University of Maryland Baltimore County, 1000 Hilltop Circle, Baltimore, MD 21250, USA
⁵ CEA Saclay, DSM/IRFU/SAp – UMR AIM (7158) CNRS/CEA/Université Paris Diderot, Orme des Merisiers, Bat. 709, 91191 Gif-sur-Yvette, France
⁶ ISOC, European Space Astronomy Centre (ESAC), PO Box 78, 28691 Villanueva de la Cañada, Spain
⁷ ISDC data center for astrophysics, Université de Genève, chemin d’Écogia, 16 1290 Versoix, Switzerland

Received: 8 July 2011
Accepted: 12 September 2011

Abstract

We present an in-depth study of the high mass X-ray binary (HMXB) GX 301−2 during its pre-periastron flare using data from the XMM-Newton satellite. The energy spectrum shows a power law continuum absorbed by a large equivalent hydrogen column on the order of 10²⁴ cm^-2 and a prominent Fe   Kα fluorescent emission line. Besides the Fe   Kα line, evidence for Fe   Kβ, Ni   Kα, Ni   Kβ, S Kα, Ar Kα, Ca Kα, and Cr Kα fluorescent lines is found. The observed line strengths are consistent with fluorescence in a cold absorber. This is the first time that Cr Kα is seen in emission in the X-ray spectrum of a HMXB. In addition to the modulation by the strong pulse period of  ~685 s the source is highly variable and shows different states of activity. We perform time-resolved as well as pulse-to-pulse resolved spectroscopy to investigate differences between these states of activity. We find that fluorescent line fluxes are strongly variable and generally follow the overall flux. The N_H value is variable by a factor of 2, but not correlated to continuum normalization. We find an interval of low flux in the light curve in which the pulsations cease almost completely, without any indication of an increasing absorption column. We investigate this dip in detail and argue that it is most likely that during the dip the accretion ceased and the afterglow of the fluorescent iron accounted for the main portion of the X-ray flux. A similar dip was found earlier in RXTE data, and we compare our findings to these results.