659 research outputs found
Le français dans le monde
Titre de l'écran-titre (visionné le 16 nov. 2006)Publ. comme: no spécial, janvier 2002 de la revue Le français dans le mond
Applying Magnetized Accretion-Ejection Models to Microquasars: a preliminary step
We present in this proceeding some aspects of a model that should explain the
spectral state changes observed in microquasars. In this model, ejection is
assumed to take place only in the innermost disc region where a large scale
magnetic field is anchored. Then, in opposite to conventional ADAF models, the
accretion energy can be efficiently converted in ejection and not advected
inside the horizon. We propose that changes of the disc physical state (e.g.
transition from optically thick to optically thin states) can strongly modify
the magnetic accretion-ejection structure resulting in the spectral
variability. After a short description of our scenario, we give some details
concerning the dynamically self-consistent magnetized accretion-ejection model
used in our computation. We also present some preliminary results of spectral
energy distribution.Comment: Proceeding of the fith Microquasar Workshop, June 7 - 13, 2004,
Beijing, China. Accepted for publication in the Chinese Journal of Astronomy
and Astrophysic
Fractal dimension of transport coefficients in a deterministic dynamical system
In many low-dimensional dynamical systems transport coefficients are very
irregular, perhaps even fractal functions of control parameters. To analyse
this phenomenon we study a dynamical system defined by a piece-wise linear map
and investigate the dependence of transport coefficients on the slope of the
map. We present analytical arguments, supported by numerical calculations,
showing that both the Minkowski-Bouligand and Hausdorff fractal dimension of
the graphs of these functions is 1 with a logarithmic correction, and find that
the exponent controlling this correction is bounded from above by 1 or
2, depending on some detailed properties of the system. Using numerical
techniques we show local self-similarity of the graphs. The local
self-similarity scaling transformations turn out to depend (irregularly) on the
values of the system control parameters.Comment: 17 pages, 6 figures; ver.2: 18 pages, 7 figures (added section 5.2,
corrected typos, etc.
Universal fluctuations in subdiffusive transport
Subdiffusive transport in tilted washboard potentials is studied within the
fractional Fokker-Planck equation approach, using the associated continuous
time random walk (CTRW) framework. The scaled subvelocity is shown to obey a
universal law, assuming the form of a stationary Levy-stable distribution. The
latter is defined by the index of subdiffusion alpha and the mean subvelocity
only, but interestingly depends neither on the bias strength nor on the
specific form of the potential. These scaled, universal subvelocity
fluctuations emerge due to the weak ergodicity breaking and are vanishing in
the limit of normal diffusion. The results of the analytical heuristic theory
are corroborated by Monte Carlo simulations of the underlying CTRW
Level Sets of the Takagi Function: Local Level Sets
The Takagi function \tau : [0, 1] \to [0, 1] is a continuous
non-differentiable function constructed by Takagi in 1903. The level sets L(y)
= {x : \tau(x) = y} of the Takagi function \tau(x) are studied by introducing a
notion of local level set into which level sets are partitioned. Local level
sets are simple to analyze, reducing questions to understanding the relation of
level sets to local level sets, which is more complicated. It is known that for
a "generic" full Lebesgue measure set of ordinates y, the level sets are finite
sets. Here it is shown for a "generic" full Lebesgue measure set of abscissas
x, the level set L(\tau(x)) is uncountable. An interesting singular monotone
function is constructed, associated to local level sets, and is used to show
the expected number of local level sets at a random level y is exactly 3/2.Comment: 32 pages, 2 figures, 1 table. Latest version has updated equation
numbering. The final publication will soon be available at springerlink.co
Dynamical percolation on general trees
H\"aggstr\"om, Peres, and Steif (1997) have introduced a dynamical version of
percolation on a graph . When is a tree they derived a necessary and
sufficient condition for percolation to exist at some time . In the case
that is a spherically symmetric tree, H\"aggstr\"om, Peres, and Steif
(1997) derived a necessary and sufficient condition for percolation to exist at
some time in a given target set . The main result of the present paper
is a necessary and sufficient condition for the existence of percolation, at
some time , in the case that the underlying tree is not necessary
spherically symmetric. This answers a question of Yuval Peres (personal
communication). We present also a formula for the Hausdorff dimension of the
set of exceptional times of percolation.Comment: 24 pages; to appear in Probability Theory and Related Field
Analisis Hidrolika Bangunan Krib Permeabel pada Saluran Tanah (Uji Model Laboratorium)
One of the structures to protect river bank erosion is groyne. Groyne can serve and control water flow, reducing flow velocity and scour of river bank. The purposes of this study is to analyze the changes in the river bed elevation (morphology) and the depth of scour in the upstream groyne caused by the permeable groyne installed at the river meanders. The experiment was conducted at Fluid Mechanics and Hydraulics Laboratory, Sriwijaya University. The study tested the hydraulics models, a trapezoidal channel, meanders angle of 90˚, five permeable groynes at meanders, and the water flowing in the channels was clear water. The observations were carried out with a flow rate was 63,32 Lt / min, three variations of permeable groynes angle were 45˚, 90˚ and 135˚ to the upstream within 1 hour, 2,5 hours and 4 hours for each angle variations . The results of this study showed that the flow velocity of meanders was decreasing to the end of the meanders, and the changes of channel only occurred at the riverbed. Maximum riverbed changes (Bt / Bo) for permeable groyne angle of 45˚, 90˚ and 135 ˚ were 1,376 cm, 1,346 cm dan 1,452 cm. The maximum depth of scour (ds/y) for permeable groyne angle of 45˚, 90˚ and 135˚ were 1,05 cm, 0,95 cm dan 1,17 cm. Thus, permeable groyne with angle of 90 proved to be the best with the smallest riverbed changes (Bt /Bo) was 1,346 cm and the coefficient of determination (R2) was 0,9384, and also the smallest scour depth (ds/y) was 0,95 cm and the coefficient of determination (R2) was 0,8317 compared to other groyne permeable angles
Simultaneous Optimization of MP2RAGE T-1-weighted (UNI) and FLuid And White matter Suppression (FLAWS) brain images at 7T using Extended Phase Graph (EPG) Simulations
Wavelets techniques for pointwise anti-Holderian irregularity
In this paper, we introduce a notion of weak pointwise Holder regularity,
starting from the de nition of the pointwise anti-Holder irregularity. Using
this concept, a weak spectrum of singularities can be de ned as for the usual
pointwise Holder regularity. We build a class of wavelet series satisfying the
multifractal formalism and thus show the optimality of the upper bound. We also
show that the weak spectrum of singularities is disconnected from the casual
one (denoted here strong spectrum of singularities) by exhibiting a
multifractal function made of Davenport series whose weak spectrum di ers from
the strong one
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