@inproceedings{IgnaceGayley2007,
  author    = {Ignace, R. and Gayley, K. G.},
  title     = {Circumstellar Magnetic Field Diagnostics from Line Polarization},
  url       = {http://nbn-resolving.de/urn:nbn:de:kobv:517-opus-18066},
  year      = {2007},
  language  = {en}
}
@article{OskinovaGayleyHamannetal.2012,
  author    = {Oskinova, Lida and Gayley, K. G. and Hamann, Wolf-Rainer and Huenemoerder, D. P. and Ignace, R. and Pollock, A. M. T.},
  title     = {HIGH-RESOLUTION X-RAY SPECTROSCOPY REVEALS THE SPECIAL NATURE OF WOLF-RAYET STAR WINDS},
  series = {ASTROPHYSICAL JOURNAL LETTERS},
  volume    = {747},
  journal   = {ASTROPHYSICAL JOURNAL LETTERS},
  number    = {2},
  publisher = {IOP PUBLISHING LTD},
  address   = {BRISTOL},
  issn      = {2041-8205},
  doi       = {10.1088/2041-8205/747/2/L25},
  pages     = {6},
  year      = {2012},
  abstract  = {We present the first high-resolutionX-ray spectrum of a putatively singleWolf-Rayet (WR) 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 (LDI) could create shocks. The X-ray emitting plasma reaches temperatures up to 50 MK and is embedded within the unshocked, "cool" stellar wind as revealed by characteristic spectral signatures. We detect a fluorescent Fe line at approximate to 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 LDI 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.},
  language  = {en}
}
@misc{OskinovaGayleyHamannetal.2012,
  author    = {Oskinova, Lida and Gayley, K. G. and Hamann, Wolf-Rainer and H{\"u}nem{\"o}rder, D. P. and Ignace, R. and Pollock, A. M. T.},
  title     = {High-Resolution X-Ray Spectroscopy reveals the special nature of Wolf-Rayet star winds (pg 747, 2012)},
  series = {The astrophysical journal : an international review of spectroscopy and astronomical physics ; Part 2, Letters},
  volume    = {752},
  journal   = {The astrophysical journal : an international review of spectroscopy and astronomical physics ; Part 2, Letters},
  number    = {2},
  publisher = {IOP Publ. Ltd.},
  address   = {Bristol},
  issn      = {2041-8205},
  doi       = {10.1088/2041-8205/752/2/L35},
  pages     = {1},
  year      = {2012},
  language  = {en}
}
@article{HuenemoerderGayleyHamannetal.2015,
  author    = {Huenemoerder, David P. and Gayley, K. G. and Hamann, Wolf-Rainer and Ignace, R. and Nichols, J. S. and Oskinova, Lida and Pollock, A. M. T. and Schulz, Norbert S. and Shenar, Tomer},
  title     = {Probing Wolf-Rayet winds: Chandra/HETG X-ray spectra of WR 6},
  series = {The astrophysical journal : an international review of spectroscopy and astronomical physics},
  volume    = {815},
  journal   = {The astrophysical journal : an international review of spectroscopy and astronomical physics},
  number    = {1},
  publisher = {IOP Publ. Ltd.},
  address   = {Bristol},
  issn      = {0004-637X},
  doi       = {10.1088/0004-637X/815/1/29},
  pages     = {16},
  year      = {2015},
  abstract  = {With a deep Chandra/HETGS exposure of WR 6, we have resolved emission lines whose profiles show that the X-rays originate from a uniformly expanding spherical wind of high X-ray-continuum optical depth. The presence of strong helium-like forbidden lines places the source of X-ray emission at tens to hundreds of stellar radii from the photosphere. Variability was present in X-rays and simultaneous optical photometry, but neither were correlated with the known period of the system or with each other. An enhanced abundance of sodium revealed nuclear-processed material, a quantity related to the evolutionary state of the star. The characterization of the extent and nature of the hot plasma in WR 6 will help to pave the way to a more fundamental theoretical understanding of the winds and evolution of massive stars.},
  language  = {en}
}
@article{HubrigScholzHamannetal.2016,
  author    = {Hubrig, Swetlana and Scholz, Kathleen and Hamann, Wolf-Rainer and Schoeller, M. and Ignace, R. and Ilyin, Ilya and Gayley, K. G. and Oskinova, Lida},
  title     = {Searching for a magnetic field in Wolf-Rayet stars using FORS 2 spectropolarimetry},
  series = {Monthly notices of the Royal Astronomical Society},
  volume    = {458},
  journal   = {Monthly notices of the Royal Astronomical Society},
  publisher = {Oxford Univ. Press},
  address   = {Oxford},
  issn      = {0035-8711},
  doi       = {10.1093/mnras/stw558},
  pages     = {3381 -- 3393},
  year      = {2016},
  abstract  = {To investigate if magnetic fields are present in Wolf-Rayet stars, we selected a few stars in the Galaxy and one in the Large Magellanic Cloud (LMC). We acquired low-resolution spectropolarimetric observations with the European Southern Observatory FORS 2 (FOcal Reducer low dispersion Spectrograph) instrument during two different observing runs. During the first run in visitor mode, we observed the LMC Wolf-Rayet star BAT99 7 and the stars WR 6, WR 7, WR 18, and WR 23 in our Galaxy. The second run in service mode was focused on monitoring the star WR 6. Linear polarization was recorded immediately after the observations of circular polarization. During our visitor observing run, the magnetic field for the cyclically variable star WR 6 was measured at a significance level of 3.3 sigma (< B-z > = 258 +/- 78 G). Among the other targets, the highest value for the longitudinal magnetic field, < B-z > = 327 +/- 141 G, was measured in the LMC star BAT99 7. Spectropolarimetric monitoring of the star WR 6 revealed a sinusoidal nature of the < B-z > variations with the known rotation period of 3.77 d, significantly adding to the confidence in the detection. The presence of the rotation-modulated magnetic variability is also indicated in our frequency periodogram. The reported field magnitude suffers from significant systematic uncertainties at the factor of 2 level, in addition to the quoted statistical uncertainties, owing to the theoretical approach used to characterize it. Linear polarization measurements showed no line effect in the stars, apart from WR 6. BAT99 7, WR 7, and WR 23 do not show variability of the linear polarization over two nights.},
  language  = {en}
}