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

TL;DR

This study presents the first high-resolution X-ray spectrum of a single Wolf-Rayet star, revealing that its X-ray emission originates far out in the stellar wind, indicating a unique mechanism distinct from O-star winds.

Contribution

It provides new high-resolution spectral data showing X-ray emission from a single Wolf-Rayet star's wind, suggesting a novel production mechanism involving wind-clump interactions.

Findings

X-rays originate more than 30 stellar radii from the star

The plasma reaches temperatures up to 50 MK

The wind has a porous structure allowing X-ray escape

Abstract

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.