4,230 research outputs found
Archeops: an instrument for present and future cosmology
Archeops is a balloon-borne instrument dedicated to measure the cosmic
microwave background (CMB) temperature anisotropies. It has, in the millimetre
domain (from 143 to 545 GHz), a high angular resolution (about 10 arcminutes)
in order to constrain high l multipoles, as well as a large sky coverage
fraction (30%) in order to minimize the cosmic variance. It has linked, before
WMAP, Cobe large angular scales to the first acoustic peak region. From its
results, inflation motivated cosmologies are reinforced with a flat Universe
(Omega_tot=1 within 3%). The dark energy density and the baryonic density are
in very good agreement with other independent estimations based on supernovae
measurements and big bang nucleosynthesis. Important results on galactic dust
emission polarization and their implications for Planck are also addressed.Comment: 4 pages, 2 figures, to appear in Proceedings of the Multiwavelength
Cosmology Conference, June 2003, Mykonos Island, Greec
Quantum microscopic approach to low-energy heavy ion collisions
The Time-dependent Hartree-Fock (TDHF) theory is applied to the study of
heavy ion collisions at energies around the Coulomb barrier. The competition
between fusion and nucleon transfer mechanisms is investigated. For
intermediate mass systems such as 16O+208Pb, proton transfer favors fusion by
reducing the Coulomb repulsion. A comparison with sub-barrier transfer
experimental data shows that pairing correlations are playing an important role
in enhancing proton pair transfer. For heavier and more symmetric systems, a
fusion hindrance is observed due to the dominance of the quasi-fission process.
Typical quasi-fission time of few zeptoseconds are obtained. Actinide
collisions are also investigated both within the TDHF approach and with the
Ballian-V\'en\'eroni prescription for fluctuation and correlation of one-body
observables. The possible formation of new heavy neutron-rich nuclei in
actinide collisions is discussed.Comment: Invited Plenary Talk given at NN201
Modeling high-energy pulsar lightcurves from first principles
Current models of gamma-ray lightcurves in pulsars suffer from large
uncertainties on the precise location of particle acceleration and radiation.
Here, we present an attempt to alleviate these difficulties by solving for the
electromagnetic structure of the oblique magnetosphere, particle acceleration,
and the emission of radiation self-consistently, using 3D spherical
particle-in-cell simulations. We find that the low-energy radiation is
synchro-curvature radiation from the polar-cap regions within the light
cylinder. In contrast, the high-energy emission is synchrotron radiation that
originates exclusively from the Y-point and the equatorial current sheet where
relativistic magnetic reconnection accelerates particles. In most cases,
synthetic high-energy lightcurves contain two peaks that form when the current
sheet sweeps across the observer's line of sight. We find clear evidence of
caustics in the emission pattern from the current sheet. High-obliquity
solutions can present up to two additional secondary peaks from energetic
particles in the wind region accelerated by the reconnection-induced flow near
the current sheet. The high-energy radiative efficiency depends sensitively on
the viewing angle, and decreases with increasing pulsar inclination. The
high-energy emission is concentrated in the equatorial regions where most of
the pulsar spindown is released and dissipated. These results have important
implications for the interpretation of gamma-ray pulsar data.Comment: 14 pages, 11 figures, Accepted for publication in MNRA
Delay aversion
We address the following question: When can one person properly be said to be more delay averse than another? In reply, several (nested) comparison methods are developed. These methods yield a theory of delay aversion which parallels that of risk aversion. The applied strength of this theory is demonstrated in a variety of dynamic economic settings, including the classical optimal growth and tree cutting problems, repeated games, and bargaining. Both time-consistent and time-inconsistent scenarios are considered.Delay aversion, impatience, consumption smoothing, time consistency
Analysis of Nonlinear Noisy Integrate\&Fire Neuron Models: blow-up and steady states
Nonlinear Noisy Leaky Integrate and Fire (NNLIF) models for neurons networks
can be written as Fokker-Planck-Kolmogorov equations on the probability density
of neurons, the main parameters in the model being the connectivity of the
network and the noise. We analyse several aspects of the NNLIF model: the
number of steady states, a priori estimates, blow-up issues and convergence
toward equilibrium in the linear case. In particular, for excitatory networks,
blow-up always occurs for initial data concentrated close to the firing
potential. These results show how critical is the balance between noise and
excitatory/inhibitory interactions to the connectivity parameter
Post-Reconstruction Deconvolution of PET Images by Total Generalized Variation Regularization
Improving the quality of positron emission tomography (PET) images, affected
by low resolution and high level of noise, is a challenging task in nuclear
medicine and radiotherapy. This work proposes a restoration method, achieved
after tomographic reconstruction of the images and targeting clinical
situations where raw data are often not accessible. Based on inverse problem
methods, our contribution introduces the recently developed total generalized
variation (TGV) norm to regularize PET image deconvolution. Moreover, we
stabilize this procedure with additional image constraints such as positivity
and photometry invariance. A criterion for updating and adjusting automatically
the regularization parameter in case of Poisson noise is also presented.
Experiments are conducted on both synthetic data and real patient images.Comment: First published in the Proceedings of the 23rd European Signal
Processing Conference (EUSIPCO-2015) in 2015, published by EURASI
Ab-initio pulsar magnetosphere: the role of general relativity
It has recently been demonstrated that self-consistent particle-in-cell
simulations of low-obliquity pulsar magnetospheres in flat spacetime show weak
particle acceleration and no pair production near the poles. We investigate the
validity of this conclusion in a more realistic spacetime geometry via
general-relativistic particle-in-cell simulations of the aligned pulsar
magnetospheres with pair formation. We find that the addition of frame-dragging
effect makes local current density along the magnetic field larger than the
Goldreich-Julian value, which leads to unscreened parallel electric fields and
the ignition of a pair cascade. When pair production is active, we observe
field oscillations in the open field bundle which could be related to pulsar
radio emission. We conclude that general relativistic effects are essential for
the existence of pulsar mechanism in low obliquity rotators.Comment: 5 pages, 4 figure, submitted to ApJLetter
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