TY  - JOUR
A1  - Hubrig, Swetlana
A1  - Scholz, Kathleen
A1  - Hamann, Wolf-Rainer
A1  - Schoeller, M.
A1  - Ignace, R.
A1  - Ilyin, Ilya
A1  - Gayley, K. G.
A1  - Oskinova, Lida
T1  - Searching for a magnetic field in Wolf-Rayet stars using FORS 2 spectropolarimetry
JF  - Monthly notices of the Royal Astronomical Society
N2  - 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.
KW  - techniques: polarimetric
KW  - stars: individual: WR 6
KW  - stars: magnetic field
KW  - stars: variables: general
KW  - stars: Wolf-Rayet
Y1  - 2016
U6  - https://doi.org/10.1093/mnras/stw558
SN  - 0035-8711
SN  - 1365-2966
VL  - 458
SP  - 3381
EP  - 3393
PB  - Oxford Univ. Press
CY  - Oxford
ER  - 
TY  - JOUR
A1  - Huenemoerder, D.
A1  - Gayley, K.
A1  - Hamann, Wolf-Rainer
A1  - Ignace, R.
A1  - Nichols, J.
A1  - Oskinova, Lida
A1  - Pollock, A. M. T.
A1  - Schulz, N.
T1  - High Resolution X-Ray Spectra of WR 6
JF  - Wolf-Rayet Stars : Proceedings of an International Workshop held in Potsdam, Germany, 1.–5. June 2015
N2  - As WR 6 is a putatively single WN4 star, and is relatively bright (V = 6.9), it is an ideal case for studying the wind mechanisms in these extremely luminous stars. To obtain higher resolution spectra at higher energy (above 1 keV) than previously obtained with the XMM/Newton RGS, we have observed WR 6 with the Chandra High Energy Transmission Grating Spectrometer for 450 ks. We have resolved emission lines of S, Si, Mg, Ne, and Fe, which all show a “fin"-shaped prole, characteristic of a self-absorbed uniformly expanding shell. Steep blue edges gives robust maximal expansion velocities of about 2000 km/s, somewhat larger than the 1700km/s derived from UV lines. The He-like lines all indicate that X-ray emitting plasmas are far from the photosphere – even at the higher energies where opacity is lowest { as was also the case for the longer wavelength lines observed with XMM-Newton/RGS. Abundances determined from X-ray spectral modeling indicate enhancements consistent with nucleosynthesis. The star was also variable in X-rays and in simultaneous optical photometry obtained with Chandra aspect camera, but not coherently with the optically known period of 3.765 days.
Y1  - 2015
U6  - http://nbn-resolving.de/urn/resolver.pl?urn:nbn:de:kobv:517-opus4-88236
SP  - 301
EP  - 304
ER  - 
TY  - JOUR
A1  - Huenemoerder, David P.
A1  - Gayley, K. G.
A1  - Hamann, Wolf-Rainer
A1  - Ignace, R.
A1  - Nichols, J. S.
A1  - Oskinova, Lida
A1  - Pollock, A. M. T.
A1  - Schulz, Norbert S.
A1  - Shenar, Tomer
T1  - Probing Wolf-Rayet winds: Chandra/HETG X-ray spectra of WR 6
JF  - The astrophysical journal : an international review of spectroscopy and astronomical physics
N2  - 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.
KW  - stars: individual (WR 6)
KW  - stars: massive
KW  - stars: Wolf-Rayet
Y1  - 2015
U6  - https://doi.org/10.1088/0004-637X/815/1/29
SN  - 0004-637X
SN  - 1538-4357
VL  - 815
IS  - 1
PB  - IOP Publ. Ltd.
CY  - Bristol
ER  - 
TY  - JOUR
A1  - Ignace, R.
A1  - Hole, K. T.
A1  - Oskinova, Lida
A1  - Rotter, J. P.
T1  - An X-Ray Study of Two B plus B Binaries: AH Cep and CW Cep
JF  - The astrophysical journal : an international review of spectroscopy and astronomical physics
N2  - AH Cep and CW Cep are both early B-type binaries with short orbital periods of 1.8. days and 2.7. days, respectively. All four components are B0.5V types. The binaries are also double-lined spectroscopic and eclipsing. Consequently, solutions for orbital and stellar parameters make the pair of binaries ideal targets for a study of the colliding winds between two B. stars. Chandra ACIS-I observations were obtained to determine X-ray luminosities. AH. Cep was detected with an unabsorbed X-ray luminosity at a 90% confidence interval of (9-33) x 10(30) erg s(-1), or (0.5-1.7) x 10(-7) L-Bol , relative to the combined Bolometric luminosities of the two components. While formally consistent with expectations for embedded wind shocks, or binary wind collision, the near-twin system of CW Cep was a surprising nondetection. For CW Cep, an upper limit was determined with L-X/L-Bol < 10(-8), again for the combined components. One difference between these two systems is that AH Cep is part of a multiple system. The X-rays from AH. Cep may not arise from standard wind shocks nor wind collision, but perhaps instead from magnetism in any one of the four components of the system. The possibility could be tested by searching for cyclic X-ray variability in AH. Cep on the short orbital period of the inner B. stars.
KW  - stars: early-type
KW  - stars: individual (AH Cep, CW Cep)
KW  - stars: massive
KW  - stars: winds
KW  - outflows X-rays: binaries
Y1  - 2017
U6  - https://doi.org/10.3847/1538-4357/aa93ea
SN  - 0004-637X
SN  - 1538-4357
VL  - 850
PB  - IOP Publ. Ltd.
CY  - Bristol
ER  - 
TY  - JOUR
A1  - Ignace, R.
A1  - Toalá, Jesús Alberto
A1  - Oskinova, Lida
T1  - Inversion of Intensity Profiles for Bubble Emissivity
JF  - Wolf-Rayet Stars : Proceedings of an International Workshop held in Potsdam, Germany, 1.–5. June 2015
N2  - Under the assumption of spherical symmetry, the run of intensity with impact parameter for a spatially resolved and optically thin bubble can be inverted for an "effective emissivity" as a function of radius. The effective emissivity takes into account instrumental sensitivity and even interstellar absorption. This work was supported by a grant from NASA (G03-14008X).
Y1  - 2015
U6  - http://nbn-resolving.de/urn/resolver.pl?urn:nbn:de:kobv:517-opus4-88432
SP  - 358
ER  - 
TY  - JOUR
A1  - Leto, Paolo
A1  - Trigilio, C.
A1  - Oskinova, Lida
A1  - Ignace, R.
A1  - Buemi, C. S.
A1  - Umana, G.
A1  - Cavallaro, Francesco
A1  - Ingallinera, A.
A1  - Bufano, F.
A1  - Phillips, N. M.
A1  - Agliozzo, Claudia
A1  - Cerrigone, L.
A1  - Todt, Helge Tobias
A1  - Riggi, S.
A1  - Leone, Francesco
T1  - The polarization mode of the auroral radio emission from the early-type star HD 142301
JF  - Monthly notices of the Royal Astronomical Society
N2  - We report the detection of the auroral radio emission from the early-type magnetic star HD142301. New VLA observations of HD142301 detected highly polarized amplified emission occurring at fixed stellar orientations. The coherent emission mechanism responsible for the stellar auroral radio emission amplifies the radiation within a narrow beam, making the star where this phenomenon occurs similar to a radio lighthouse. The elementary emission process responsible for the auroral radiation mainly amplifies one of the two magneto-ionic modes of the electromagnetic wave. This explains why the auroral pulses are highly circularly polarized. The auroral radio emission of HD142301 is characterized by a reversal of the sense of polarization as the star rotates. The effective magnetic field curve of HD142301 is also available making it possible to correlate the transition from the left to the right-hand circular polarization sense ( and vice versa) of the auroral pulses with the known orientation of the stellar magnetic field. The results presented in this letter have implications for the estimation of the dominant magneto-ionic mode amplified within the HD142301 magnetosphere.
KW  - masers
KW  - polarization
KW  - stars: early-type
KW  - stars: individual: HD142301
KW  - stars: magnetic field
KW  - radio continuum: stars
Y1  - 2018
U6  - https://doi.org/10.1093/mnrasl/sly179
SN  - 0035-8711
SN  - 1365-2966
VL  - 482
IS  - 1
SP  - L4
EP  - L8
PB  - Oxford Univ. Press
CY  - Oxford
ER  - 
TY  - JOUR
A1  - Leto, Paolo
A1  - Trigilio, C.
A1  - Oskinova, Lida
A1  - Ignace, R.
A1  - Buemi, C. S.
A1  - Umana, G.
A1  - Ingallinera, A.
A1  - Todt, Helge Tobias
A1  - Leone, F.
T1  - The detection of variable radio emission from the fast rotating magnetic hot B-star HR 7355 and evidence for its X-ray aurorae
JF  - Monthly notices of the Royal Astronomical Society
N2  - In this paper, we investigate the multiwavelength properties of the magnetic early B-type star HR 7355. We present its radio light curves at several frequencies, taken with the Jansky Very Large Array, and X-ray spectra, taken with the XMM-Newton X-ray telescope. Modelling of the radio light curves for the Stokes I and V provides a quantitative analysis of the HR 7355 magnetosphere. A comparison between HR 7355 and a similar analysis for the Ap star CU Vir allows us to study how the different physical parameters of the two stars affect the structure of the respective magnetospheres where the non-thermal electrons originate. Our model includes a cold thermal plasma component that accumulates at high magnetic latitudes that influences the radio regime, but does not give rise to X-ray emission. Instead, the thermal X-ray emission arises from shocks generated by wind stream collisions close to the magnetic equatorial plane. The analysis of the X-ray spectrum of HR 7355 also suggests the presence of a non-thermal radiation. Comparison between the spectral index of the power-law X-ray energy distribution with the non-thermal electron energy distribution indicates that the non-thermal X-ray component could be the auroral signature of the non-thermal electrons that impact the stellar surface, the same non-thermal electrons that are responsible for the observed radio emission. On the basis of our analysis, we suggest a novel model that simultaneously explains the X-ray and the radio features of HR 7355 and is likely relevant for magnetospheres of other magnetic early-type stars.
KW  - stars: chemically peculiar
KW  - stars: early-type
KW  - stars: individual: HR 7355
KW  - stars: magnetic field
KW  - radio continuum: stars
KW  - X-rays: stars
Y1  - 2017
U6  - https://doi.org/10.1093/mnras/stx267
SN  - 0035-8711
SN  - 1365-2966
VL  - 467
SP  - 2820
EP  - 2833
PB  - Oxford Univ. Press
CY  - Oxford
ER  - 
TY  - JOUR
A1  - Leto, Paolo
A1  - Trigilio, C.
A1  - Oskinova, Lida
A1  - Ignace, R.
A1  - Buemi, C. S.
A1  - Umana, G.
A1  - Ingallinera, A.
A1  - Leone, Francesco
A1  - Phillips, N. M.
A1  - Agliozzo, Claudia
A1  - Todt, Helge Tobias
A1  - Cerrigone, L.
T1  - A combined multiwavelength VLA/ALMA/Chandra study unveils the complex magnetosphere of the B-type star HR5907
JF  - Monthly notices of the Royal Astronomical Society
N2  - We present new radio/millimeter measurements of the hot magnetic star HR5907 obtained with the VLA and ALMA interferometers. We find that HR5907 is the most radio luminous early type star in the cm-mm band among those presently known. Its multi-wavelength radio light curves are strongly variable with an amplitude that increases with radio frequency. The radio emission can be explained by the populations of the non-thermal electrons accelerated in the current sheets on the outer border of the magnetosphere of this fast-rotating magnetic star. We classify HR5907 as another member of the growing class of strongly magnetic fast-rotating hot stars where the gyro-synchrotron emission mechanism efficiently operates in their magnetospheres. The new radio observations of HR5907 are combined with archival X-ray data to study the physical condition of its magnetosphere. The X-ray spectra of HR5907 show tentative evidence for the presence of non-thermal spectral component. We suggest that non-thermal X-rays originate a stellar X-ray aurora due to streams of non-thermal electrons impacting on the stellar surface. Taking advantage of the relation between the spectral indices of the X-ray power-law spectrum and the non-thermal electron energy distributions, we perform 3-D modelling of the radio emission for HR5907. The wavelength-dependent radio light curves probe magnetospheric layers at different heights above the stellar surface. A detailed comparison between simulated and observed radio light curves leads us to conclude that the stellar magnetic field of HR 5907 is likely non-dipolar, providing further indirect evidence of the complex magnetic field topology of HR5907.
KW  - stars: chemically peculiar
KW  - stars: early-type
KW  - stars: individual: HR 5907
KW  - stars: magnetic field
KW  - radio continuum: stars
KW  - X-rays: stars
Y1  - 2018
U6  - https://doi.org/10.1093/mnras/sty244
SN  - 0035-8711
SN  - 1365-2966
VL  - 476
IS  - 1
SP  - 562
EP  - 579
PB  - Oxford Univ. Press
CY  - Oxford
ER  - 
TY  - JOUR
A1  - Oskinova, Lida
A1  - Gayley, K. G.
A1  - Hamann, Wolf-Rainer
A1  - Huenemoerder, D. P.
A1  - Ignace, R.
A1  - Pollock, A. M. T.
T1  - HIGH-RESOLUTION X-RAY SPECTROSCOPY REVEALS THE SPECIAL NATURE OF
 WOLF-RAYET STAR WINDS
JF  - ASTROPHYSICAL JOURNAL LETTERS
N2  - 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.
KW  - stars: individual (WR 6)
KW  - stars: winds, outflows
KW  - stars: Wolf-Rayet
KW  - X-rays: stars
Y1  - 2012
U6  - https://doi.org/10.1088/2041-8205/747/2/L25
SN  - 2041-8205
VL  - 747
IS  - 2
PB  - IOP PUBLISHING LTD
CY  - BRISTOL
ER  - 
TY  - GEN
A1  - Oskinova, Lida
A1  - Gayley, K. G.
A1  - Hamann, Wolf-Rainer
A1  - Hünemörder, D. P.
A1  - Ignace, R.
A1  - Pollock, A. M. T.
T1  - High-Resolution X-Ray Spectroscopy reveals the special nature of Wolf-Rayet star winds (pg 747, 2012)
T2  - The astrophysical journal : an international review of spectroscopy and astronomical physics ; Part 2, Letters
Y1  - 2012
U6  - https://doi.org/10.1088/2041-8205/752/2/L35
SN  - 2041-8205
VL  - 752
IS  - 2
PB  - IOP Publ. Ltd.
CY  - Bristol
ER  -