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Skinner S.L.,University of Colorado at Boulder | Zhekov S.A.,Space and Solar Terrestrial Research Institute | Gudel M.,University of Vienna | Schmutz W.,Physikalisch Meteorologisches Observatorium Davos PMOD | Sokal K.R.,University of Virginia
Astronomical Journal

Previous studies have demonstrated that putatively single nitrogen-type Wolf-Rayet stars (WN stars) without known companions are X-ray sources. However, almost all WN star X-ray detections so far have been of earlier WN2-WN6 spectral subtypes. Later WN7-WN9 subtypes (also known as WNL stars) have proved more difficult to detect, an important exception being WR79a (WN9ha). We present here new X-ray detections of the WNL stars WR16 (WN8h) and WR78 (WN7h). These new results, when combined with previous detections, demonstrate that X-ray emission is present in WN stars across the full range of spectral types, including later WNL stars. The two WN8 stars observed to date (WR16 and WR40) show unusually low X-ray luminosities (L x) compared to other WN stars, and it is noteworthy that they also have the lowest terminal wind speeds (v ∞). Existing X-ray detections of about a dozen WN stars reveal a trend of increasing L x with wind luminosity L wind = (1/2)v 2 ∞, suggesting that wind kinetic energy may play a key role in establishing X-ray luminosity levels in WN stars. © 2012. The American Astronomical Society. All rights reserved. Source

Naze Y.,University of Liege | Zhekov S.A.,University of Colorado at Boulder | Zhekov S.A.,Space and Solar Terrestrial Research Institute | Walborn N.R.,US Space Telescope Science Institute
Astrophysical Journal

High-resolution data of the peculiar magnetic massive star HD148937 were obtained with Chandra-HETGS, and are presented here in combination with a re-analysis of the older XMM-RGS data. The lines of the high-Z elements (Mg, Si, S) were found to be unshifted and relatively narrow (FWHM of about 800kms -1), i.e., narrower than the O line recorded by RGS, which possibly indicates that the hot plasma is multi-thermal and has several origins. These data further indicate a main plasma temperature of about 0.6keV and a formation of the X-ray emission at about one stellar radius above the photosphere. From the spectral fits and the H-to-He line ratios, the presence of very hot plasma is, however, confirmed, though with a smaller relative strength than for the prototype magnetic oblique rotator θ1OriC. Both stars thus share many similarities, but HD148937 appears less extreme than θ1OriC despite also having a large magnetic confinement parameter. © 2012. The American Astronomical Society. All rights reserved. Source

Atanassov A.M.,Space and Solar Terrestrial Research Institute
Advances in Space Research

The Spectral Airglow Temperature Imager (SATI) instrument registers airglow coming from an annular mesopause segment. The temperatures of various points of this segment are retrieved by sectors of the registered images containing spectral information. A stage of SATI spectrogram processing connected with deriving spectral information from images and the determination of the rotational temperature of oxygen molecules emitting at the altitude of the mesopause is considered. A "moving sector" approach for sector spectra calculation is proposed. Typical for this approach is the possibility for varying the number of calculated sector spectra from one image and the sector angle. The sector angle may be selected depending on the quality features of the images. Two versions, one based on averaging and a second - based on finding the median of the values of all pixels at equal distance from the image center are developed. The determination of a mean temperature based on a whole image mean spectrum is considered as a potential source of systematic and gross errors. A determination of the mean temperature by averaging sector temperatures is proposed. Applying criteria for evaluating the usability of sector temperatures based on azimuthal distribution of sector errors, emission intensities and backgrounds is possible. The moving sector approach is useful with the possibility for a detailed visual evaluation of the azimuthal distribution of retrieved sector variables - rotational temperature, emission rate, background and error. © 2012 COSPAR. Published by Elsevier Ltd. All rights reserved. Source

Zhekov S.A.,Space and Solar Terrestrial Research Institute
Monthly Notices of the Royal Astronomical Society

We have analysed the X-ray emission from a sample of close Wolf-Rayet+O (WR+O) binaries using data from the public Chandra and XMM-Newton archives. Global spectral fits show that two-temperature plasma is needed to match the X-ray emission from these objects, because the hot component (kT > 2 keV) is an important ingredient of the spectral models. In close WR+O binaries, X-rays likely originate in colliding stellar wind (CSW) shocks, driven by the massive winds of the binary components. The CSW shocks in these objects are expected to be radiative because of the high density of the plasma in the interaction region. In contrast, our analysis shows that, in our sample of close WR+O binaries, the CSW shocks are adiabatic. This is possible only if the mass-loss rates of the stellar components in the binary are at least one order of magnitude smaller than the values currently accepted. The most likely explanation for the X-ray properties of close WR+O binaries is that their winds are two-component flows. The more massive component (dense clumps) has a role in the optical/ultraviolet emission from these objects, while the smooth rarefied component is a key factor in the X-ray emission from these objects. © 2012 The Author Monthly Notices of the Royal Astronomical Society © 2012 RAS. Source

Park S.,University of Texas at Arlington | Zhekov S.A.,Space and Solar Terrestrial Research Institute | Burrows D.N.,Pennsylvania State University | Racusin J.L.,NASA | And 2 more authors.
Astrophysical Journal Letters

We have been monitoring the supernova remnant (SNR) 1987A with Chandra observations since 1999. Here we report on the latest change in the soft X-ray light curve of SNR 1987A. For the last 1.5yr (since day 8000), the soft X-ray flux has significantly flattened, staying (within uncertainties) at f X 5.7 × 10-12ergcm-2s-1 (corresponding to L X 3.6 × 1036ergs-1) in the 0.5-2keV band. This remarkable change in the recent soft X-ray light curve suggests that the forward shock is now interacting with a decreasing density structure, after interacting with an increasing density gradient over 10yr prior to day 8000. Possibilities may include the case that the shock is now propagating beyond a density peak of the inner ring. We briefly discuss some possible implications on the nature of the progenitor and the future prospects of our Chandra monitoring observations. © 2011. The American Astronomical Society. All rights reserved. Source

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