High-resolution X-ray spectroscopy reveals the special nature of Wolf-Rayet star winds

The Astrophysical Journal Letters (Impact Factor: 5.34). 02/2012; 747(2). DOI: 10.1088/2041-8205/747/2/L25
Source: arXiv


We present the first high-resolution X-ray spectrum of a putatively single
Wolf-Rayet star. 400 ks observations of WR 6 by the XMM-Newton-telescope
resulted in a superb quality high-resolution X-ray spectrum. Spectral analysis
reveals that the X-rays originate far out in the stellar wind, more than 30
stellar radii from the photosphere, and thus outside the wind acceleration zone
where the line-driving instability could create shocks. The X-ray emitting
plasma reaches temperatures up to 50\,MK, and is embedded within the
un-shocked, "cool" stellar wind as revealed by characteristic spectral
signatures. We detect a fluorescent Fe line at approx 6.4 keV. The presence of
fluorescence is consistent with a two-component medium, where the cool wind is
permeated with the hot X-ray emitting plasma. The wind must have a very porous
structure to allow the observed amount of X-rays to escape. We find that
neither the line-driving instability nor any alternative binary scenario can
explain the data. We suggest a scenario where X-rays are produced when the fast
wind rams into slow "sticky clumps" that resist acceleration. Our new data show
that the X-rays in single WR-star are generated by some special mechanism
different from the one operating in the O-star winds.

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