3,843 research outputs found
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
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
Submillimetre point sources from the Archeops experiment: Very Cold Clumps in the Galactic Plane
Archeops is a balloon-borne experiment, mainly designed to measure the Cosmic
Microwave Background (CMB) temperature anisotropies at high angular resolution
(~ 12 arcminutes). By-products of the mission are shallow sensitivity maps over
a large fraction of the sky (about 30 %) in the millimetre and submillimetre
range at 143, 217, 353 and 545 GHz. From these maps, we produce a catalog of
bright submillimetre point sources. We present in this paper the processing and
analysis of the Archeops point sources. Redundancy across detectors is the key
factor allowing to sort out glitches from genuine point sources in the 20
independent maps. We look at the properties of the most reliable point sources,
totalling 304. Fluxes range from 1 to 10,000 Jy (at the frequencies covering
143 to 545 GHz). All sources are either planets (2) or of galactic origin.
Longitude range is from 75 to 198 degrees. Some of the sources are associated
with well-known Lynds Nebulae and HII compact regions in the galactic plane. A
large fraction of the sources have an IRAS counterpart. Except for Jupiter,
Saturn, the Crab and Cas A, all sources show a dust-emission-like modified
blackbody emission spectrum. Temperatures cover a range from 7 to 27 K. For the
coldest sources (T<10 K), a steep nu^beta emissivity law is found with a
surprising beta ~ 3 to 4. An inverse relationship between T and beta is
observed. The number density of sources at 353 GHz with flux brighter than 100
Jy is of the order of 1 per degree of Galactic longitude. These sources will
provide a strong check for the calibration of the Planck HFI focal plane
geometry as a complement to planets. These very cold sources observed by
Archeops should be prime targets for mapping observations by the Akari and
Herschel space missions and ground--based observatories.Comment: Version matching the published article (English improved). Published
in Astron. Astrophys, 21 pages, 13 figures, 4 tables Full article (with
complete tables) can be retrieved at
http://www.archeops.org/Archeops_Publicatio
Nonlinear coherent transport of waves in disordered media
We present a diagrammatic theory for coherent backscattering from disordered
dilute media in the nonlinear regime. The approach is non-perturbative in the
strength of the nonlinearity. We show that the coherent backscattering
enhancement factor is strongly affected by the nonlinearity, and corroborate
these results by numerical simulations. Our theory can be applied to several
physical scenarios like scattering of light in nonlinear Kerr media, or
propagation of matter waves in disordered potentials.Comment: 4 pages, 3 figure
Temperature dependence in random matrix models with pairing condensates
We address a number of issues raised by a manuscript of Klein, Toublan, and
Verbaarschot (hep-ph/0405180) in which the authors introduce a random matrix
model for QCD with two colors, two flavors, and fermions in the fundamental
representation. Their inclusion of temperature terms differs from the approach
adopted in previous work on this problem (Phys. Rev. D 64, 074016 (2001).) We
demonstrate that the two approaches are related by a transformation that leaves
the thermodynamic potential invariant and which therefore has no effect on
physical observables.Comment: 8 pages, revtex4. v2: typos corrected in reference
Geometry fluctuations in a two-dimensional quantum antiferromagnet
The paper considers the effects of random fluctuations of the local spin
connectivities (fluctuations of the geometry) on ground state properties of a
two-dimensional quantum antiferromagnet. We analyse the behavior of spins
described by the Heisenberg model as a function of what we call phason flip
disorder, following a terminology used for aperiodic systems. The calculations
were carried out both within linear spin wave theory and using quantum Monte
Carlo simulations. An "order by disorder" phenomenon is observed in this model,
wherein antiferromagnetism is found to be enhanced by phason disorder. The
value of the staggered order parameter increases with the number of defects,
accompanied by an increase in the ground state energy of the system.Comment: 5 pages, 7 figures. Shortened and corrected version (as accepted for
publication in Physical Review B
R\'enyi Entropies from Random Quenches in Atomic Hubbard and Spin Models
We present a scheme for measuring R\'enyi entropies in generic atomic Hubbard
and spin models using single copies of a quantum state and for partitions in
arbitrary spatial dimension. Our approach is based on the generation of random
unitaries from random quenches, implemented using engineered time-dependent
disorder potentials, and standard projective measurements, as realized by
quantum gas microscopes. By analyzing the properties of the generated unitaries
and the role of statistical errors, with respect to the size of the partition,
we show that the protocol can be realized in exisiting AMO quantum simulators,
and used to measure for instance area law scaling of entanglement in
two-dimensional spin models or the entanglement growth in many-body localized
systems.Comment: 5+9 page
Synthetic observations of first hydrostatic cores in collapsing low-mass dense cores II. Simulated ALMA dust emission maps
First hydrostatic cores are predicted by theories of star formation, but
their existence has never been demonstrated convincingly by (sub)millimeter
observations. Furthermore, the multiplicity at the early phases of the star
formation process is poorly constrained. The purpose of this paper is twofold.
First, we seek to provide predictions of ALMA dust continuum emission maps from
early Class 0 objects. Second, we show to what extent ALMA will be able to
probe the fragmentation scale in these objects. Following our previous paper
(Commer\c{c}on et al. 2012, hereafter paper I), we post-process three
state-of-the-art radiation-magneto-hydrodynamic 3D adaptive mesh refinement
calculations to compute the emanating dust emission maps. We then produce
synthetic ALMA observations of the dust thermal continuum from first
hydrostatic cores. We present the first synthetic ALMA observations of dust
continuum emission from first hydrostatic cores. We analyze the results given
by the different bands and configurations and we discuss for which combinations
of the two the first hydrostatic cores would most likely be observed. We also
show that observing dust continuum emission with ALMA will help in identifying
the physical processes occurring within collapsing dense cores. If the magnetic
field is playing a role, the emission pattern will show evidence of a
pseudo-disk and even of a magnetically driven outflow, which pure
hydrodynamical calculations cannot reproduce. The capabilities of ALMA will
enable us to make significant progress towards understanding fragmentation at
the early Class 0 stage and discovering first hydrostatic cores.Comment: 12 pages, 7 figures, accepted for publication in Astronomy and
Astrophysic
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