38 research outputs found
Winds in Collision: high-energy particles in massive binary systems
High-resolution radio observations have revealed that non-thermal radio
emission in WR stars arises where the stellar wind of the WR star collides with
that of a binary companion. These colliding-wind binary (CWB) systems offer an
important laboratory for investigating the underlying physics of particle
acceleration. Hydrodynamic models of the binary stellar winds and the
wind-collision region (WCR) that account for the evolution of the electron
energy spectrum, largely due to inverse Compton cooling, are now available.
Radiometry and imaging obtained with the VLA, MERLIN, EVN and VLBA provide
essential constraints to these models. Models of the radio emission from WR146
and WR147 are shown, though these very wide systems do not have defined orbits
and hence lack a number of important model parameters. Multi-epoch VLBI imaging
of the archetype WR+O star binary WR140 through a part of its 7.9-year orbit
has been used to define the orbit inclination, distance and the luminosity of
the companion star to enable the best constraints for any radio emitting CWB
system. Models of the spatial distribution of relativistic electrons and ions,
and the magnetic energy density are used to model the radio emission, and also
to predict the high energy emission at X-ray and gamma-ray energies. It is
clear that high-energy facilities e.g. GLAST and VERITAS, will be important for
constraining particle acceleration parameters such as the spectral index of the
energy spectrum and the acceleration efficiency of both ions and electrons, and
in turn, identify unique models for the radio spectra. This will be especially
important in future attempts to model the spectra of WR140 throughout its
complete orbit. A WCR origin for the synchrotron emission in O-stars, the
progenitors of WR stars, is illustrated by observations of Cyg OB2 No. 9.Comment: Invited review at the 8th EVN Symposium, Torun September 26-29, 2006.
11 pages, 12 figure
Theoretical X-ray Line Profiles from Colliding Wind Binaries
We present theoretical X-ray line profiles from a range of model colliding
wind systems. In particular, we investigate the effects of varying the stellar
mass-loss rates, the wind speeds, and the viewing orientation. We find that a
wide range of theoretical line profile shapes is possible, varying with orbital
inclination and phase. At or near conjunction, the lines have approximately
Gaussian profiles, with small widths (HWHM ~ 0.1 v_\infty) and definite blue-
or redshifts (depending on whether the star with the weaker wind is in front or
behind). When the system is viewed at quadrature, the lines are generally much
broader (HWHM ~ v_\infty), flat-topped and unshifted. Local absorption can have
a major effect on the observed profiles - in systems with mass-loss rates of a
few times 10^{-6} Msol/yr the lower energy lines (E <~ 1 kev) are particularly
affected. This generally results in blueward-skewed profiles, especially when
the system is viewed through the dense wind of the primary. The orbital
variation of the line widths and shifts is reduced in a low inclination binary.
The extreme case is a binary with i = 0 degrees, for which we would expect no
line profile variation.Comment: 15 pages, 15 figures. To appear in MNRA
Non-thermal processes in colliding-wind massive binaries: the contribution of Simbol-X to a multiwavelength investigation
Several colliding-wind massive binaries are known to be non-thermal emitters
in the radio domain. This constitutes strong evidence for the fact that an
efficient particle acceleration process is at work in these objects. The
acceleration mechanism is most probably the Diffusive Shock Acceleration (DSA)
process in the presence of strong hydrodynamic shocks due to the
colliding-winds. In order to investigate the physics of this particle
acceleration, we initiated a multiwavelength campaign covering a large part of
the electromagnetic spectrum. In this context, the detailed study of the hard
X-ray emission from these sources in the SIMBOL-X bandpass constitutes a
crucial element in order to probe this still poorly known topic of
astrophysics. It should be noted that colliding-wind massive binaries should be
considered as very valuable targets for the investigation of particle
acceleration in a similar way as supernova remnants, but in a different region
of the parameter space.Comment: 4 pages, 2 figures, to appear in Proc. of the Second Internqtionql
Simbol-X Symposium, held in Paris (France
Cold gas in the Intra Cluster Medium: implications for flow dynamics and powering optical nebulae
We show that the mechanical energy injection rate generated as the
intra-cluster medium (ICM) flows around cold clouds may be sufficient to power
the optical and near infra-red emission of nebulae observed in the central
regions of a sample of seven galaxy clusters. The energy injection rate is
extremely sensitive to the velocity difference between the ICM and cold clouds,
which may help to explain why optical and infra-red luminosity is often larger
than expected in systems containing AGNs. We also find that mass recycling is
likely to be important for the dynamics of the ICM. This effect will be
strongest in the central regions of clusters where there is more than enough
cold gas for its evaporation to contribute significantly to the density of the
hot phase.Comment: 8 pages, 2 figures, accepted for publication in MNRA
X-ray Spectral Variation of Eta Carinae through the 2003 X-ray Minimum
We report the results of an X-ray observing campaign on the massive, evolved
star Eta Carinae, concentrating on the 2003 X-ray minimum as seen by the
XMM-Newton observatory. These are the first spatially-resolved X-ray monitoring
observations of the stellar X-ray spectrum during the minimum. The hard X-ray
emission, believed to be associated with the collision of Eta Carinae's wind
with the wind from a massive companion star, varied strongly in flux on
timescales of days, but not significantly on timescales of hours. The lowest
X-ray flux in the 2-10 keV band seen by XMM-Newton was only 0.7% of the maximum
seen by RXTE just before the X-ray minimum. The slope of the X-ray continuum
above 5 keV did not vary in any observation, which suggests that the electron
temperature of the hottest plasma associated with the stellar source did not
vary significantly at any phase. Through the minimum, the absorption to the
stellar source increased by a factor of 5-10 to NH ~3-4E23 cm-2. The thermal Fe
XXV emission line showed significant excesses on both the red and blue sides of
the line outside the minimum and exhibited an extreme red excess during the
minimum. The Fe fluorescence line at 6.4 keV increased in equivalent width from
100 eV outside the minimum to 200 eV during the minimum. From these observed
features, we discuss two possible causes of the X-ray minimum; the eclipse of
the X-ray plasma and an intrinsic fading of the X-ray emissivity. The drop in
the colliding wind X-ray emission also revealed the presence of an additional
X-ray component which exhibited no variation on timescales of weeks to years.
This component may be produced by the collision of high speed outflows at v
\~1000-2000 km s-1 from Eta Carinae with ambient gas within a few thousand AU
from the star.Comment: 35 pages, 14 figures, accepted for publication in Ap
Suzaku monitoring of hard X-ray emission from η carinae over a single binary orbital cycle
The Suzaku X-ray observatory monitored the supermassive binary system η Carinae 10 times during the whole 5.5 yr orbital cycle between 2005 and 2011. This series of observations presents the first long-term monitoring of this enigmatic system in the extremely hard X-ray band between 15 and 40 keV. During most of the orbit, the 15-25 keV emission varied similarly to the 2-10 keV emission, indicating an origin in the hard energy tail of the kT ∼ 4 keV wind-wind collision (WWC) plasma. However, the 15-25 keV emission declined only by a factor of three around periastron when the 2-10 keV emission dropped by two orders of magnitude due probably to an eclipse of the WWC plasma. The observed minimum in the 15-25 keV emission occurred after the 2-10 keV flux had already recovered by a factor of ∼3. This may mean that the WWC activity was strong, but hidden behind the thick primary stellar wind during the eclipse. The 25-40 keV flux was rather constant through the orbital cycle, at the level measured with INTEGRAL in 2004. This result may suggest a connection of this flux component to the γ-ray source detected in this field. The helium-like Fe Kα line complex at ∼6.7 keV became strongly distorted toward periastron as seen in the previous cycle. The 5-9 keV spectra can be reproduced well with a two-component spectral model, which includes plasma in collision equilibrium and a plasma in non-equilibrium ionization (NEI) with τ ∼ 1011 cm-3 s-1. The NEI plasma increases in importance toward periastron
Coordinated Monitoring of the Eccentric O-star Binary Iota Orionis: The X-ray Analysis
We analyse two ASCA observations of the highly eccentric O9III + B1III binary
Iota Orionis obtained at periastron and apastron. Based on the assumption of a
strong colliding winds shock between the stellar components, we expected to see
significant variation in the X-ray emission between these phases. The
observations proved otherwise: the X-ray luminosities and spectral
distributions were remarkably similar. The only noteworthy feature was the hint
of a proximity effect during periastron passage, supported also in the optical.
We discuss the accuracy of our results, and also analyse archival ROSAT
observations. We investigate why we do not see a clear colliding winds
signature. A simple model shows that the wind attenuation to the expected
position of the shock apex is negligible throughout the orbit, which poses the
puzzling question of why the expected 1/D variation (ie. a factor of 7.5) in
the intrinsic luminosity is not seen in the data. Two scenarios are proposed:
either the colliding winds emission is unexpectedly weak such that intrinsic
shocks in the winds dominate the emission, or, alternatively, that the emission
observed is colliding winds emission but in a more complex form than we would
naively expect. Complex hydrodynamical models are then analyzed. Despite
strongly phase-variable emission from the models, both were consistent with the
observations. We find that if the mass-loss rates of the stars are low then
intrinsic wind shocks could dominate the emission. However, when we assume
higher mass-loss rates of the stars, we find that the observed emission could
also be consistent with a purely colliding winds origin. To distinguish between
the different models X-ray observations with improved phase coverage will be
necessary.Comment: 18 pages, 14 figures, uses mn.st
An Introduction to the Chandra Carina Complex Project
The Great Nebula in Carina provides an exceptional view into the violent
massive star formation and feedback that typifies giant HII regions and
starburst galaxies. We have mapped the Carina star-forming complex in X-rays,
using archival Chandra data and a mosaic of 20 new 60ks pointings using the
Chandra X-ray Observatory's Advanced CCD Imaging Spectrometer, as a testbed for
understanding recent and ongoing star formation and to probe Carina's regions
of bright diffuse X-ray emission. This study has yielded a catalog of
properties of >14,000 X-ray point sources; >9800 of them have multiwavelength
counterparts. Using Chandra's unsurpassed X-ray spatial resolution, we have
separated these point sources from the extensive, spatially-complex diffuse
emission that pervades the region; X-ray properties of this diffuse emission
suggest that it traces feedback from Carina's massive stars. In this
introductory paper, we motivate the survey design, describe the Chandra
observations, and present some simple results, providing a foundation for the
15 papers that follow in this Special Issue and that present detailed catalogs,
methods, and science results.Comment: Accepted for the ApJS Special Issue on the Chandra Carina Complex
Project (CCCP), scheduled for publication in May 2011. All 16 CCCP Special
Issue papers are available at
http://cochise.astro.psu.edu/Carina_public/special_issue.html through 2011 at
least. 43 pages; 18 figure