6,544 research outputs found
Electron cooling and Debye-Waller effect in photoexcited bismuth
By means of first principles calculations, we computed the effective
electron-phonon coupling constant governing the electron cooling in
photoexcited bismuth. strongly increases as a function of electron
temperature, which can be traced back to the semi-metallic nature of bismuth.
We also used a thermodynamical model to compute the time evolution of both
electron and lattice temperatures following laser excitation. Thereby, we
simulated the time evolution of (1 -1 0), (-2 1 1) and (2 -2 0) Bragg peak
intensities measured by Sciaini et al [Nature 458, 56 (2009)] in femtosecond
electron diffraction experiments. The effect of the electron temperature on the
Debye-Waller factors through the softening of all optical modes across the
whole Brillouin zone turns out to be crucial to reproduce the time evolution of
these Bragg peak intensities
Replica Conditional Sequential Monte Carlo
We propose a Markov chain Monte Carlo (MCMC) scheme to perform state
inference in non-linear non-Gaussian state-space models. Current
state-of-the-art methods to address this problem rely on particle MCMC
techniques and its variants, such as the iterated conditional Sequential Monte
Carlo (cSMC) scheme, which uses a Sequential Monte Carlo (SMC) type proposal
within MCMC. A deficiency of standard SMC proposals is that they only use
observations up to time to propose states at time when an entire
observation sequence is available. More sophisticated SMC based on lookahead
techniques could be used but they can be difficult to put in practice. We
propose here replica cSMC where we build SMC proposals for one replica using
information from the entire observation sequence by conditioning on the states
of the other replicas. This approach is easily parallelizable and we
demonstrate its excellent empirical performance when compared to the standard
iterated cSMC scheme at fixed computational complexity.Comment: To appear in Proceedings of ICML '1
Observational Test of Coronal Magnetic Field Models I. Comparison with Potential Field Model
Recent advances have made it possible to obtain two-dimensional line-of-sight
magnetic field maps of the solar corona from spectropolarimetric observations
of the Fe XIII 1075 nm forbidden coronal emission line. Together with the
linear polarization measurements that map the azimuthal direction of the
coronal magnetic field, these coronal vector magnetograms now allow for direct
observational testing of theoretical coronal magnetic field models. This paper
presents a study testing the validity of potential-field coronal magnetic field
models. We constructed a theoretical coronal magnetic field model of active
region AR 10582 observed by the SOLARC coronagraph in 2004 by a global
potential field extrapolation of the synoptic map of Carrington Rotation 2014.
Synthesized linear and circular polarization maps from thin layers of the
coronal magnetic field model above the active region along the line of sight
are compared with the observed maps. We found that reasonable agreement occurs
from layers located just above the sunspot of AR 10582, near the plane of the
sky. This result provides the first observational evidence that potential field
extrapolation can yield a reasonable approximation of the magnetic field
configuration of the solar corona for simple and stable active regions.Comment: 25 pages, 11 figures. ApJ in pres
Stochastic Cahn-Hilliard equation with double singular nonlinearities and two reflections
We consider a stochastic partial differential equation with two logarithmic
nonlinearities, with two reflections at 1 and -1 and with a constraint of
conservation of the space average. The equation, driven by the derivative in
space of a space-time white noise, contains a bi-Laplacian in the drift. The
lack of the maximum principle for the bi-Laplacian generates difficulties for
the classical penalization method, which uses a crucial monotonicity property.
Being inspired by the works of Debussche, Gouden\`ege and Zambotti, we obtain
existence and uniqueness of solution for initial conditions in the interval
. Finally, we prove that the unique invariant measure is ergodic, and
we give a result of exponential mixing
Detection of highly ionized O and Ne absorption lines in the X-ray spectrum of 4U1820-303 in the globular cluster, NGC 6624
We searched for absorption lines of highly ionized O and Ne in the energy
spectra of two Low-mass X-ray binaries, 4U1820-303 in the globular cluster
NGC6624 and Cyg X-2, observed with the Chandra LETG, and detected O VII, O VIII
and Ne IX absorption lines for 4U1820-303. The equivalent width of the O VII K
alpha line was 1.19 +0.47/-0.30 eV (90 % errors) and the significance was 6.5
sigma. Absorption lines were not detected for Cyg X-2 with a 90 % upper limit
on the equivalent width of 1.06 eV for O VII K alpha. The absorption lines
observed in 4U1820-303 are likely due to hot interstellar medium, because O
will be fully photo-ionized if the absorbing column is located close to the
binary system. The velocity dispersion is restricted to b = 200 - 420 km/s from
consistency between O VII K alpha and K beta lines, Ne/O abundance ratio, and H
column density. The average temperature and the O VII density are respectively
estimated to be log(T[K]) = 6.2 - 6.3 and n(OVII) = (0.7 - 2.3) x 10^{-6}
cm^{-3}. The difference of O VII column densities for the two sources may be
connected to the enhancement of the soft X-ray background (SXB) towards the
Galactic bulge region. Using the polytrope model of hot gas to account for the
SXB we corrected for the density gradient and estimated the midplane O VII
density at the solar neighborhood. The scale height of hot gas is then
estimated using the AGN absorption lines. It is suggested that a significant
portion of both the AGN absorption lines and the high-latitude SXB emission
lines can be explained by the hot gas in our Galaxy.Comment: Accepted for publication in ApJ. 7 pages, 9 eps figure
Pertinence d'une méthode de prédétermination des crues basée sur la simulation stochastique de pluies horaires.
International audienceExtreme events are rarely observed, so their analysis is generally based on observations of more frequent values. The relevance of flood frequency analysis (FFA) method depends on its capability to estimate the frequency of extreme values with reasonable accuracy using extrapolation. A FFA method based on stochastic simulation of flood event is assessed based on its reliability and stability. For such an assessment, different training/testing decompositions are performed for a set of data from more than 1,000 gauging stations. We showed that the method enables relevant "predictive" estimates, e.g. by assigning correct return periods to the record values that are systematically absent in calibration data sets. The model is also highly stable vis-a-vis the sampling. This characteristic is linked to the use of regional statistical rainfall data and a simple rainfall-runoff model that requires calibrating only one parameterLes Ă©vĂ©nements extrĂȘmes sont par nature rarement observĂ©s, c'est pourquoi leur estimation est gĂ©nĂ©ralement basĂ©e sur l'observation de valeurs plus courantes. La pertinence d'une mĂ©thode de prĂ©dĂ©termination des Ă©vĂ©nements extrĂȘmes dĂ©pend donc de sa capacitĂ© Ă raisonnablement extrapoler les distributions de frĂ©quences vers les valeurs extrĂȘmes. Dans cette Ă©tude, une mĂ©thode de prĂ©dĂ©termination de crues basĂ©e sur la simulation de scĂ©narios de pluies horaires, est Ă©valuĂ©e sur sa capacitĂ© Ă produire des estimations justes et stables. Cette Ă©valuation s'appuie sur diffĂ©rents tests d'Ă©chantillonnage sur les pĂ©riodes de calage et de validation, appliquĂ©s sur un jeu de donnĂ©es consĂ©quent (plus de 1000 stations). Nous montrons que la mĂ©thode est capable de fournir une estimation pertinente sur les Ă©vĂ©nements extrĂȘmes bien que ceux-ci soit systĂ©matiquement ĂŽtĂ©s de la pĂ©riode de calage. La mĂ©thode montre aussi une grande stabilitĂ© face Ă l'Ă©chantillonnage. Cette caractĂ©ristique est liĂ©e Ă l'utilisation d'une information statistique rĂ©gionale sur la pluie et Ă la simplicitĂ© de la modĂ©lisation hydrologique paramĂ©trĂ©e par un seul paramĂštre
Temperature and Heavy Element Abundance Profiles of Cool Clusters of Galaxies from ASCA
We perform a spatially resolved X-ray spectroscopic study of a set of 18
relaxed clusters of galaxies with gas temperatures below 4 keV. Spectral
analysis was done using ASCA/SIS data coupled with the spatial information
contained in ROSAT/PSPC and Einstein/IPC observations. We derive the
temperature profiles using single-temperature fits and also correct for the
presence of cold gas at the cluster centers. For all of the clusters in the
sample, we derive Si and Fe abundance profiles. For a few of the clusters, we
also derive Ne and S abundance profiles. We present a comparison of the
elemental abundances derived at similar overdensities as well as element
mass-to-light ratios. We conclude that the preferential accretion of low
entropy, low abundance gas into the potentials of groups and cold clusters can
explain most of the observed trends in metallicity. In addition, we discuss the
importance of energy input from SNe II on cluster scaling relations and on the
relation between the observed scatter in the retainment of SN Ia products with
differences between the epoch of cluster formation.Comment: 14 pages, several changes are introduced, ApJ 2001, v 555 (July 1, in
press
The Enrichment History of Hot Gas in Poor Galaxy Groups
We have analyzed the ASCA SIS and GIS data for seventeen groups and
determined the average temperature and abundance of the hot x-ray emitting gas.
For groups with gas temperatures less than 1.5 keV we find that the abundance
is correlated with the gas temperature and luminosity. We have also determined
the abundance of the alpha-elements and iron independently for those groups
with sufficient counts. We find that for the cool groups (i.e. kT <1.5 keV) the
ratio of alpha-elements to iron is ~1, about half that seen in clusters.
Spectral fits with the S, Si and Fe abundances allowed to vary separately
suggest the S/Fe ratio is similar to that seen in clusters while the Si/Fe
ratio in groups is half the value determined for richer systems. The mass of
metals per unit blue luminosity drops rapidly in groups as the temperature
drops. There are two possible explanations for this decrease. One is that the
star formation in groups is very different from that in rich clusters. The
other explanation is that groups lose much of their enriched material via winds
during the early evolution of ellipticals. If the latter is true, we find that
poor groups will have contributed significantly (roughly 1/3 of the metals) to
the enrichment of the intergalactic medium.Comment: 19 Pages with 2 figures, Accepted for publication in the
Astrophysical Journa
Ram extrusion force for a frictional plastic material: model prediction and application to cement paste
the original publication is available at http://www.springerlink.com/content/pn3034u81144458g/International audienceWe developed a model to predict the ram extrusion force of frictional plastic materials such as cement-based pastes. The extrusion of cement-based materials has already been studied, but the interaction between shaping force and paste behaviour still have to be understood. Our model is based on the plastic frictional behaviour of cement-based materials and integrates the physical mechanisms that govern material extrusion flow and extrusion force increase. When the process starts, a pressure gradient is created in the extruder due to wall friction of the paste that is submitted to plug flow. It induces a consolidation of the material. As a result, a large increase of extrusion force appears. A Coulomb law is used to model cement-based materials, which is considered as consolidating granular media. Such modelling is compared with experimental results. Tests were carried out on extrudible cement pastes. Modelling and experimental results are in good agreement
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