324 research outputs found
Colliding Stellar Wind Models with Nonequilibrium Ionization: X-rays from WR 147
The effects of nonequilibrium ionization are explicitly taken into account in
a numerical model which describes colliding stellar winds (CSW) in massive
binary sytems. This new model is used to analyze the most recent X-ray spectra
of the WR+OB binary system WR 147. The basic result is that it can adequately
reproduce the observed X-ray emission (spectral shape, observed flux) but some
adjustment in the stellar wind parameters is required. Namely, (i) the stellar
wind velocities must be higher by a factor of 1.4 - 1.6; (ii) the mass loss
must be reduced by a factor of ~ 2. The reduction factor for the mass loss is
well within the uncertainties for this parameter in massive stars, but given
the fact that the orbital parameters (e.g., inclination angle and eccentricity)
are not well constrained for WR 147, even smaller corrections to the mass loss
might be sufficient. Only CSW models with nonequilibrium ionization and equal
(or nearly equal) electron and ion postshock temperature are successful.
Therefore, the analysis of the X-ray spectra of WR 147 provides evidence that
the CSW shocks in this object must be collisionless.Comment: 10 pages, 4 figures, 1 Table; accepted for publication in MNRA
XMM-Newton Observations Reveal Very High X-ray Luminosity from the Carbon-rich Wolf-Rayet Star WR 48a
We present XMM-Newton observations of the dusty Wolf-Rayet star WR 48a. This
is the first detection of this object in X-rays. The XMM-Newton EPIC spectra
are heavily absorbed and the presence of numerous strong emission lines
indicates a thermal origin of the WR 48a X-ray emission, with dominant
temperature components at kT_cool approx. 1 keV and kT_hot approx. 3~keV, the
hotter component dominating the observed flux. No significant X-ray variability
was detected on time scales < 1 day. Although the distance to WR 48a is
uncertain, if it is physically associated with the open clusters Danks 1 and 2
at d ~ 4 kpc, then the resultant X-ray luminosity L_X ~ 10^(35) ergs/s makes it
the most X-ray luminous Wolf-Rayet star in the Galaxy detected so far, after
the black-hole candidate Cyg X-3. We assume the following scenarios as the most
likely explanation for the X-ray properties of WR 48a: (1) colliding stellar
winds in a wide WR+O binary system, or in a hierarchical triple system with
non-degenerate stellar components; (2) accretion shocks from the WR 48a wind
onto a close companion (possibly a neutron star). More specific information
about WR48a and its wind properties will be needed to distinguish between the
above possibilities.Comment: 10 pages, 3 figures, 1 table, accepted for publication in The
Astrophysical Journal Letter
Chandra HETG Observations of the Colliding Stellar Wind System WR 147
We present an extended analysis of deep Chandra HETG observations of the
WR+OB binary system WR 147 that was resolved into a double X-ray source (Zhekov
& Park, 2010, ApJ, 709, L119). Our analysis of the profiles of strong emission
lines shows that their centroids are blue-shifted in the spectrum of the
northern X-ray source. We find no suppressed forbidden line in the He-like
triplets which indicates that the X-ray emitting region is not located near
enough to the stars in the binary system to be significantly affected by their
UV radiation. The most likely physical picture that emerges from the entire set
of HETG data suggests that the northern X-ray source can be associated with the
colliding stellar wind region in the wide WR+OB binary system, while the X-rays
of its southern counterpart, the WN8 star, are result from stellar wind
shocking onto a close companion (a hypothesized third star in the system).Comment: 22 pages, 6 figures, 2 Tables; accepted for publication in The
Astrophysical Journa
X-ray Evolution of SNR 1987A: The Radial Expansion
We present the evolution of the radial expansion of SNR 1987A as measured
using Chandra X-ray observations taken over the last 10 years. To characterize
the complex structure of the remnant and isolate the expansion measurement, we
fit the images to several empirical models including: a simple circular torus,
a torus with bilateral lobes, and a torus with four tangentially extended
lobes. We discuss the results of this measure in the context of the overall
evolution of the supernova remnant, for which we believe we have measured the
end of the free expansion phase and its transition to the adiabatic phase (at
least along the equatorial ring). The timing of this event is in agreement with
early predictions of the remnant evolution.Comment: Accepted for publication in ApJ, 21 pages, 4 figures, 3 table
X-rays from Colliding Stellar Winds: the case of close WR+O binary systems
We have analysed the X-ray emission from a sample of close 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 as 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. CSW shocks in these objects are expected to be radiative due to the
high density of the plasma in the interaction region. Opposite to this, our
analysis shows that the CSW shocks in the sample of close WR+O binaries 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 could be that their winds are two-component flows. The
more massive component (dense clumps) play role for the optical/UV emission
from these objects, while the smooth rarefied component is a key factor for
their X-ray emission.Comment: MNRAS, accepted for publication (Feb 6, 2012); 13 pages, 6 figures, 3
table
Evolution of the Chandra CCD Spectra of SNR 1987A: Probing the Reflected-Shock Picture
We continue to explore the validity of the reflected shock structure (RSS)
picture in SNR 1987A that was proposed in our previous analyses of the X-ray
emission from this object. We used an improved version of our RSS model in a
global analysis of 14 CCD spectra from the monitoring program with Chandra. In
the framework of the RSS picture, we are able to match both the expansion
velocity curve deduced from the analysis of the X-ray images and light curve.
Using a simplified analysis, we also show that the X-rays and the non-thermal
radio emission may originate from the same shock structure (the blast wave). We
believe that using the RSS model in the analysis of grating data from the
Chandra monitoring program of SNR 1987A that cover a long enough time interval,
will allow us to build a more realistic physical picture and model of SNR
1987A.Comment: 14 pages, 1 Table, 8 figures, accepted for publication in MNRA
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