3,612 research outputs found
In situ GISAXS study of the growth of Pd on MgO(001)
The morphology of growing Pd nano-particles on MgO(001) surfaces have been
investigated in situ, during growth, by grazing incidence small angle x-ray
scattering, for different substrate temperatures. The 2D patterns obtained are
quantitatively analyzed, and the average morphological parameters (shape, size)
deduced. Above 650 K, the aggregates adopt their equilibrium shape of truncated
octahedron, and the interfacial energy is deduced.Comment: 10 pages, 1 Table, 2 Figure
Constraints on cosmic-ray efficiency in the supernova remnant RCW 86 using multi-wavelength observations
Several young supernova remnants (SNRs) have recently been detected in the
high-energy and very-high-energy gamma-ray domains. As exemplified by RX
J1713.7-3946, the nature of this emission has been hotly debated, and direct
evidence for the efficient acceleration of cosmic-ray protons at the SNR shocks
still remains elusive. We analyzed more than 40 months of data acquired by the
Large Area Telescope (LAT) on-board the Fermi Gamma-Ray Space Telescope in the
HE domain, and gathered all of the relevant multi-wavelength (from radio to VHE
gamma-rays) information about the broadband nonthermal emission from RCW 86.
For this purpose, we re-analyzed the archival X-ray data from the ASCA/Gas
Imaging Spectrometer (GIS), the XMM-Newton/EPIC-MOS, and the RXTE/Proportional
Counter Array (PCA). Beyond the expected Galactic diffuse background, no
significant gamma-ray emission in the direction of RCW 86 is detected in any of
the 0.1-1, 1-10 and 10-100 GeV Fermi-LAT maps. In the hadronic scenario, the
derived HE upper limits together with the HESS measurements in the VHE domain
can only be accommodated by a spectral index Gamma <= 1.8, i.e. a value
in-between the standard (test-particle) index and the asymptotic limit of
theoretical particle spectra in the case of strongly modified shocks. The
interpretation of the gamma-ray emission by inverse Compton scattering of high
energy electrons reproduces the multi-wavelength data using a reasonable value
for the average magnetic field of 15-25 muG. For these two scenarios, we
assessed the level of acceleration efficiency. We discuss these results in the
light of existing estimates of the magnetic field strength, the effective
density and the acceleration efficiency in RCW 86.Comment: Accepted for publication in A&A; 10 pages and 4 figure
Large and small-scale structures and the dust energy balance problem in spiral galaxies
The interstellar dust content in galaxies can be traced in extinction at
optical wavelengths, or in emission in the far-infrared. Several studies have
found that radiative transfer models that successfully explain the optical
extinction in edge-on spiral galaxies generally underestimate the observed
FIR/submm fluxes by a factor of about three. In order to investigate this
so-called dust energy balance problem, we use two Milky Way-like galaxies
produced by high-resolution hydrodynamical simulations. We create mock optical
edge-on views of these simulated galaxies (using the radiative transfer code
SKIRT), and we then fit the parameters of a basic spiral galaxy model to these
images (using the fitting code FitSKIRT). The basic model includes smooth
axisymmetric distributions along a S\'ersic bulge and exponential disc for the
stars, and a second exponential disc for the dust. We find that the dust mass
recovered by the fitted models is about three times smaller than the known dust
mass of the hydrodynamical input models. This factor is in agreement with
previous energy balance studies of real edge-on spiral galaxies. On the other
hand, fitting the same basic model to less complex input models (e.g. a smooth
exponential disc with a spiral perturbation or with random clumps), does
recover the dust mass of the input model almost perfectly. Thus it seems that
the complex asymmetries and the inhomogeneous structure of real and
hydrodynamically simulated galaxies are a lot more efficient at hiding dust
than the rather contrived geometries in typical quasi-analytical models. This
effect may help explain the discrepancy between the dust emission predicted by
radiative transfer models and the observed emission in energy balance studies
for edge-on spiral galaxies.Comment: 9 pages, 5 figures, accepted for publication in A&
Dynamic roughening and fluctuations of dipolar chains
Nonmagnetic particles in a carrier ferrofluid acquire an effective dipolar
moment when placed in an external magnetic field. This fact leads them to form
chains that will roughen due to Brownian motion when the magnetic field is
decreased. We study this process through experiments, theory and simulations,
three methods that agree on the scaling behavior over 5 orders of magnitude.
The RMS width goes initially as , then as before it
saturates. We show how these results complement existing results on polymer
chains, and how the chain dynamics may be described by a recent non-Markovian
formulation of anomalous diffusion.Comment: 4 pages, 3 figures, submitted to Phys. Rev. Let
A Persistent High-Energy Flux from the Heart of the Milky Way : Integral's view of the Galactic Center
The Ibis/Isgri imager on Integral detected for the first time a hard X-ray
source, IGR J17456-2901, located within 1' of Sgr A* over the energy range
20-100 keV. Here we present the results of a detailed analysis of ~7 Ms of
Integral observations of the GC. With an effective exposure of 4.7 Ms we have
obtained more stringent positional constraints on this HE source and
constructed its spectrum in the range 20-400 keV. Furthermore, by combining the
Isgri spectrum with the total X-ray spectrum corresponding to the same physical
region around SgrA* from XMM data, and collected during part of the Integral
observations, we constructed and present the first accurate wide band HE
spectrum for the central arcmins of the Galaxy. Our complete analysis of the
emission properties of IGR shows that it is faint but persistent with no
variability above 3 sigma contrary to what was alluded to in our first paper.
This result, in conjunction with the spectral characteristics of the X-ray
emission from this region, suggests that the source is most likely not
point-like but, rather, that it is a compact, yet diffuse, non-thermal emission
region. The centroid of IGR is estimated to be R.A.=17h45m42.5,
decl.=-28deg59'28'', offset by 1' from the radio position of Sgr A* and with a
positional uncertainty of 1'. Its 20-400 keV luminosity at 8 kpc is L=5.4x10^35
erg/sec. Very recently, Hess detected of a source of ~TeV g-rays also located
within 1' of Sgr A*. We present arguments in favor of an interpretation
according to which the photons detected by Integral and Hess arise from the
same compact region of diffuse emission near the central BH and that the
supernova remnant Sgr A East could play an important role as a contributor of
very HE g-rays to the overall spectrum from this region.Comment: 14 pages, 11 figures, Accepted for publication in Ap
The Stabilized Poincare-Heisenberg algebra: a Clifford algebra viewpoint
The stabilized Poincare-Heisenberg algebra (SPHA) is the Lie algebra of
quantum relativistic kinematics generated by fifteen generators. It is obtained
from imposing stability conditions after attempting to combine the Lie algebras
of quantum mechanics and relativity which by themselves are stable, however not
when combined. In this paper we show how the sixteen dimensional Clifford
algebra CL(1,3) can be used to generate the SPHA. The Clifford algebra path to
the SPHA avoids the traditional stability considerations, relying instead on
the fact that CL(1,3) is a semi-simple algebra and therefore stable. It is
therefore conceptually easier and more straightforward to work with a Clifford
algebra. The Clifford algebra path suggests the next evolutionary step toward a
theory of physics at the interface of GR and QM might be to depart from working
in space-time and instead to work in space-time-momentum.Comment: 14 page
The Green Computing Observatory: a data curation approach for green IT
International audienceThe Green Computing Observatory (GCO) is a collaborative effort to provide the scientific community with a comprehensive set of traces of energy consumption of a production cluster. These traces include the detailed monitoring of the hardware and software, as well as global site information such as the overall consumption and overall cooling. The acquired data is transformed into an XML format built from a specifically designed ontology and published through the Grid Observatory website
Shape-Selection of Thermodynamically Stabilized Colloidal Pd and Pt Nanoparticles Controlled via Support Effects
Colloidal
chemistry, in combination with nanoparticle (NP)/support
epitaxial interactions is used here to synthesize shape-selected and
thermodynamically stable metallic NPs over a broad range of NP sizes.
The morphology of three-dimensional palladium and platinum NPs supported
on TiO<sub>2</sub>(110) was investigated using scanning tunneling
microscopy. Well-defined Pd and Pt NPs were synthesized via inverse
micelle encapsulation. The initially spherical NPs were found to become
faceted and form an epitaxial relationship with the support after
high-temperature annealing (e.g., 1100 °C). Shape selection was
achieved for almost all Pd NPs, namely, a truncated octahedron shape
with (111) top and interfacial facets. The Pt NPs were however found
to adopt a variety of shapes. The epitaxial relationship of the NPs
with the support was evidenced by the alignment of the clusterâs
edges with TiO<sub>2</sub>(110)-[001] atomic rows and was found to
be responsible for the shape control. The ability of synthesizing
thermally stable shape-selected metal NPs demonstrated here is expected
to be of relevance for applications in the field of catalysis, since
the activity and selectivity of NP catalysts has been shown to strongly
depend on the NP shape
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