165 research outputs found
MARCOS, a numerical tool for the simulation of multiple time-dependent non-linear diffusive shock acceleration
We present a new code aimed at the simulation of diffusive shock acceleration
(DSA), and discuss various test cases which demonstrate its ability to study
DSA in its full time-dependent and non-linear developments. We present the
numerical methods implemented, coupling the hydrodynamical evolution of a
parallel shock (in one space dimension) and the kinetic transport of the
cosmic-rays (CR) distribution function (in one momentum dimension), as first
done by Falle. Following Kang and Jones and collaborators, we show how the
adaptive mesh refinement technique (AMR) greatly helps accommodating the
extremely demanding numerical resolution requirements of realistic (Bohm-like)
CR diffusion coefficients. We also present the paral lelization of the code,
which allows us to run many successive shocks at the cost of a single shock,
and thus to present the first direct numerical simulations of linear and
non-linear multiple DSA, a mechanism of interest in various astrophysical
environments such as superbubbles, galaxy clusters and early cosmological
flows.Comment: accepted for publication in MNRAS by the Royal Astronomical Society
and Blackwell Publishin
3D Simulations of the Thermal X-ray Emission from Young Supernova Remnants Including Efficient Particle Acceleration
Supernova remnants (SNRs) are believed to be the major contributors to
Galactic cosmic rays. The detection of non-thermal emission from SNRs
demonstrates the presence of energetic particles, but direct signatures of
protons and other ions remain elusive. If these particles receive a sizeable
fraction of the explosion energy, the morphological and spectral evolution of
the SNR must be modified. To assess this, we run 3D hydrodynamic simulations of
a remnant coupled with a non-linear acceleration model. We obtain the
time-dependent evolution of the shocked structure, impacted by the
Rayleigh-Taylor hydrodynamic instabilities at the contact discontinuity and by
the back-reaction of particles at the forward shock. We then compute the
progressive temperature equilibration and non-equilibrium ionization state of
the plasma, and its thermal emission in each cell. This allows us to produce
the first realistic synthetic maps of the projected X-ray emission from the
SNR. Plasma conditions (temperature, ionization age) can vary widely over the
projected surface of the SNR, especially between the ejecta and the ambient
medium owing to their different composition. This demonstrates the need for
spatially-resolved spectroscopy. We find that the integrated emission is
reduced with particle back-reaction, with the effect being more significant for
the highest photon energies. Therefore different energy bands, corresponding to
different emitting elements, probe different levels of the impact of particle
acceleration. Our work provides a framework for the interpretation of SNR
observations with current X-ray missions (Chandra, XMM-Newton, Suzaku) and with
upcoming X-ray missions (such as Astro-H).Comment: Accepted for publication in ApJ. Figures quality has been reduced for
the arXi
An XMM-Newton study of the mixed-morphology supernova remnant W28 (G6.4-0.1)
We have performed an XMM-Newton imaging and spectroscopic study of supernova
remnant (SNR) W28, a prototype mixed-morphology or thermal composite SNR,
believed to be interacting with a molecular cloud. The observed hot X-ray
emitting plasma is characterized by low metal abundances, showing no evidence
of ejecta. The X-rays arising from the deformed northeast shell consist of a
thermal component with a temperature of keV plus a hard component of
either thermal (temperature keV) or non-thermal (photon index
-2.4) origin. The X-ray emission in the SNR interior is blobby and the
corresponding spectra are best described as the emission from a cold
( keV) plasma in non-equilibrium ionization with an ionization
timescale of cm s plus a hot ( keV)
gas in collisional ionization equilibrium. Applying the two-temperature model
to the smaller central regions, we find non-uniform interstellar absorption,
temperature and density distribution, which indicates that the remnant is
evolving in a non-uniform environment with denser material in the east and
north. The cloudlet evaporation mechanism can essentially explain the
properties of the X-ray emission in the center and thermal conduction may also
play a role for length scales comparable to the remnant radius. A recombining
plasma model with an electron temperature of keV is also feasible
for describing the hot central gas with the recombination age of the gas
estimated at yr.Comment: 16 pages, 7 figures, 5 tables, ApJ in pres
Optical properties of single ZnTe nanowires grown at low temperature
Optically active gold-catalyzed ZnTe nanowires have been grown by molecular
beam epitaxy, on a ZnTe(111) buffer layer, at low temperature 350\degree under
Te rich conditions, and at ultra-low density (from 1 to 5 nanowires per
micrometer^{2}. The crystalline structure is zinc blende as identified by
transmission electron microscopy. All nanowires are tapered and the majority of
them are oriented. Low temperature micro-photoluminescence and
cathodoluminescence experiments have been performed on single nanowires. We
observe a narrow emission line with a blue-shift of 2 or 3 meV with respect to
the exciton energy in bulk ZnTe. This shift is attributed to the strain induced
by a 5 nm-thick oxide layer covering the nanowires, and this assumption is
supported by a quantitative estimation of the strain in the nanowires
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