X‐Ray Line Profiles from Parameterized Emission within an Accelerating Stellar Wind

Owocki, Stan; Cohen, David H.

doi:10.1086/322413

Cited by 114 publications

(178 citation statements)

References 27 publications

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“…Our data reveal a slight blue-shift of the line centroids in all strong lines. The blue-shift is expected for lines formed in an accelerating stellar wind since radiation from the approaching side of the wind should be less attenuated by continuum opacity than that from the receding part (Owocki & Cohen 2001).…”

Section: The Rgs Spectrum Of 9 Sgrmentioning

confidence: 97%

A multi-wavelength investigation of the non-thermal radio emitting O-star 9 Sgr

Rauw

Blomme

Waldron

et al. 2002

A&A

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Abstract. We report the results of a multi-wavelength investigation of the O4 V star 9 Sgr (= HD 164794). Our data include observations in the X-ray domain with XMM-Newton, in the radio domain with the VLA as well as optical spectroscopy. 9 Sgr is one of a few presumably single OB stars that display non-thermal radio emission. This phenomenon is attributed to synchrotron emission by relativistic electrons accelerated in strong hydrodynamic shocks in the stellar wind. Given the enormous supply of photospheric UV photons in the wind of 9 Sgr, inverse Compton scattering by these relativistic electrons is a priori expected to generate a non-thermal power law tail in the X-ray spectrum. Our EPIC and RGS spectra of 9 Sgr reveal a more complex situation than expected from this simple theoretical picture. While the bulk of the thermal X-ray emission from 9 Sgr arises most probably in a plasma at temperature ∼3 × 10 6 K distributed throughout the wind, the nature of the hard emission in the X-ray spectrum is less clear. Assuming a non-thermal origin, our best fitting model yields a photon index of ≥2.9 for the power law component which would imply a low compression ratio of ≤1.79 for the shocks responsible for the electron acceleration. However, the hard emission can also be explained by a thermal plasma at a temperature ≥2 × 10 7 K. Our VLA data indicate that the radio emission of 9 Sgr was clearly non-thermal at the time of the XMM-Newton observation. Again, we derive a low compression ratio (1.7) for the shocks that accelerate the electrons responsible for the synchrotron radio emission. Finally, our optical spectra reveal long-term radial velocity variations suggesting that 9 Sgr could be a long-period spectroscopic binary.

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Section: The Rgs Spectrum Of 9 Sgrmentioning

confidence: 97%

A multi-wavelength investigation of the non-thermal radio emitting O-star 9 Sgr

Rauw

Blomme

Waldron

et al. 2002

A&A

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“…For example, resolved X-ray lines in O and B stars often appear symmetric (e.g., Schulz et al 2000;Miller et al 2002;Cohen et al 2003), whereas basic approaches to modeling distributed wind-shocks predict asymmetric profile shapes (e.g., MacFarlane et al 1991;Ignace 2001;Owocki & Cohen 2001;Ignace & Gayley 2002;Feldmeier et al 2003). Even though most of the emission is dominated by soft X-rays ( < ∼ 1 keV) as expected from wind However, relatively few hot stars have been observed with the latest generation instruments, and the interpretations are complicated by several factors.…”

Section: Introductionmentioning

confidence: 99%

XMM-Newton observations of the nitrogen-rich Wolf-Rayet star WR 1

Ignace

Oskinova²,

Brown

2003

A&A

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Abstract. We present XMM-Newton results for the X-ray spectrum from the N-rich Wolf-Rayet (WR) star WR 1. The EPIC instrument was used to obtain a medium-resolution spectrum. The following features characterize this spectrum: (a) significant emission "bumps" appear that are coincident with the wavelengths of typical strong lines, such as Mg, Si, and S; (b) little emission is detected above 4 keV, in contrast to recent reports of a hard component in the stars WR 6 and WR 110 which are of similar subtype; and (c) evidence for sulfur K-edge absorption at about 2.6 keV, which could only arise from absorption of X-rays by the ambient stellar wind. The lack of hard emission in our dataset is suggestive that WR 1 may truly be a single star, thus representing the first detailed X-ray spectrum that isolates the WR wind alone (in contrast to colliding wind zones). Although the properties of the S-edge are not well-constrained by our data, it does appear to be real, and its detection indicates that at least some of the hot gas in WR 1 must reside interior to the radius of optical depth unity for the total absorptive opacity at the energy of the edge.

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“…Traditionally, the X-ray emission of single OB and Wolf-Rayet (W-R) stars without companions has been attributed to shocks distributed throughout their winds that form as a result of line-driven flow instabilities (Lucy & White 1980;Lucy 1982;Baum et al 1992;Gayley & Owocki 1995;Feldmeier et al 1997;Owocki, Castor, & Rybicki 1988). Such emission is predicted to be relatively soft (kT < 1 keV), and X-ray emission lines formed in an optically thick outflowing wind are expected to be blueshifted and asymmetric because of higher attenuation of the redward portion of the line by receding material on the far side of the star (MacFarlane et al 1991;Owocki & Cohen 2001).…”

Section: Introductionmentioning

confidence: 99%

XMM‐Newtonand Very Large Array Observations of the Variable Wolf‐Rayet Star EZ Canis Majoris: Evidence for a Close Companion?

Skinner

Zhekov

Güdel

et al. 2002

ApJ

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We present new X-ray and radio observations of the Wolf-Rayet star EZ CMa (HD 50896) obtained with XMM-Newton and the Very Large Array (VLA). This WN4 star exhibits optical and UV variability at a period of 3.765 days whose cause is unknown. Binarity may be responsible, but the existence of a companion has not been proven. The radio spectral energy distribution of EZ CMa determined from VLA observations at five frequencies is in excellent agreement with predictions for free-free wind emission, and the ionized mass-loss rate allowing for distance uncertainties is _ M M ¼ 3:8 ðAE2:6Þ Â 10 À5 M yr À1 . The CCD X-ray spectra show prominent Si xiii and S xv emission lines and can be acceptably modeled as an absorbed multitemperature optically thin plasma, confirming earlier ASCA results. Nonsolar abundances are inferred with Fe notably deficient. The X-ray emission is dominated by cooler plasma at a temperature kT cool % 0:6 keV, but a harder component is also detected, and the derived temperature is kT hot % 3:0 4:2 keV if the emission is thermal. This is too high to be explained by radiative wind shock models, and the X-ray luminosity of the hard component is 3 orders of magnitude lower than expected for accretion onto a neutron star companion. We show that the hard emission could be produced by the Wolf-Rayet wind shocking onto a normal (nondegenerate) stellar companion at close separation. Finally, using comparable data sets we demonstrate that the X-ray and radio properties of EZ CMa are strikingly similar to those of the WN5-6 star WR 110. This similarity points to common X-ray and radio emission processes in WN stars and discredits the idea that EZ CMa is anomalous within its class.

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X‐Ray Line Profiles from Parameterized Emission within an Accelerating Stellar Wind

Cited by 114 publications

References 27 publications

A multi-wavelength investigation of the non-thermal radio emitting O-star 9 Sgr

A multi-wavelength investigation of the non-thermal radio emitting O-star 9 Sgr

XMM-Newton observations of the nitrogen-rich Wolf-Rayet star WR 1

XMM‐Newtonand Very Large Array Observations of the Variable Wolf‐Rayet Star EZ Canis Majoris: Evidence for a Close Companion?

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