56 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
A Chandra Grating Observation of the Dusty Wolf-Rayet Star WR48a
We present results of a Chandra High Energy Transmission Grating (HETG)
observation of the carbon-rich Wolf-Rayet (WR) star WR48a. These are the first
high-resolution spectra of this object in X-rays. Blue-shifted centroids of the
spectral lines of about -360 km/s and line widths of 1000 - 1500 km/s (FWHM)
were deduced from the analysis of the line profiles of strong emission lines.
The forbidden line of Si XIII is strong and not suppressed, indicating that the
rarefied 10-30 MK plasma forms far from strong sources of far-UV emission, most
likely in a wind collision zone. Global spectral modeling showed that the X-ray
spectrum of WR48a suffered higher absorption in the October 2012 Chandra
observation compared to a previous January 2008 XMM-Newton observation. The
emission measure of the hot plasma in WR48a decreased by a factor ~ 3 over the
same period of time. The most likely physical picture that emerges from the
analysis of the available X-ray data is that of colliding stellar winds in a
wide binary system with an elliptical orbit. We propose that the unseen
secondary star in the system is another WR star or perhaps a luminous blue
variable.Comment: 13 pages, 5 figures, 2 tables; Accepted for publication in The
Astrophysical Journa
Supernova Remnant 1987A: Opening the Future by Reaching the Past
We report an up-turn in the soft X-ray light curve of supernova remnant (SNR)
1987A in late 2003 (~6200 days after the explosion), as observed with the
Chandra X-ray Observatory. Since early 2004, the rapid increase of the 0.5-2
keV band X-ray light curve can no longer be described by the exponential
density distribution model with which we successfully fitted the data between
1990 and 2003. Around day ~6200, we also find that the fractional contribution
to the observed soft X-ray flux from the decelerated shock begins to exceed
that of the fast shock and that the X-ray brightening becomes "global" rather
than "spotty". We interpret these results as evidence that the blast wave has
reached the main body of the dense circumstellar material all around the inner
ring. This interpretation is supported by other recent observations, including
a deceleration of the radial expansion of the X-ray remnant, a significant
up-turn in the mid-IR intensities, and the prevalence of the optical hot spots
around the entire inner ring, all of which occur at around day 6000. In
contrast to the soft X-ray light curve, the hard band (3-10 keV) X-ray light
curve increases at a much lower rate which is rather similar to the radio light
curve. The hard X-ray emission may thus originate from the reverse shock where
the radio emission is likely produced. Alternatively, the low increase rate of
the hard X-rays may simply be a result of the continuous softening of the
overall X-ray spectrum.Comment: AASTex preprint style 12 pages including 1 table and 4 figures,
Accepted by ApJ
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