X-ray Emission from Nitrogen-Type Wolf-Rayet Stars

TL;DR

This study presents new X-ray observations of nitrogen-type Wolf-Rayet stars, revealing their X-ray emission characteristics, spectral properties, and potential links to stellar wind properties, challenging existing models of shock-induced X-ray production.

Contribution

It provides the first comprehensive X-ray detection survey of WN stars across spectral subtypes, highlighting the presence of both cool and hot plasma components and suggesting wind properties influence X-ray emission.

Findings

WN stars are confirmed X-ray sources with diverse spectral features.

Hot plasma components exceed predictions of radiative wind shock models.

X-ray luminosity appears more related to wind properties than bolometric luminosity.

Abstract

We summarize new X-ray detections of four nitrogen-type Wolf-Rayet (WR) stars obtained in a limited survey aimed at establishing the X-ray properties of WN stars across their full range of spectral subtypes. None of the detected stars is so far known to be a close binary. We report Chandra detections of WR 2 (WN2), WR 18 (WN4), and WR 134 (WN6), and an XMM-Newton detection of WR79a (WN9ha). These observations clearly demonstrate that both WNE and WNL stars are X-ray sources. We also discuss Chandra archive detections of the WN6h stars WR 20b, WR 24, and WR 136 and ROSAT non-detections of WR 16 (WN8h) and WR 78 (WN7h). The X-ray spectra of all WN detections show prominent emission lines and an admixture of cool (kT < 1 keV) and hot (kT > 2 keV) plasma. The hotter plasma is not predicted by radiative wind shock models and other as yet unidentified mechanisms are at work. Most stars show X-ray absorption in excess of that expected from visual extinction (Av), likely due to their strong winds or cold circumstellar gas. Existing data suggest a falloff in X-ray luminosity toward later WN7-9 subtypes, which have higher Lbol but slower, denser winds than WN2-6 stars. This provides a clue that wind properties may be a more crucial factor in determining emergent X-ray emission levels than bolometric luminosity.