17,264 research outputs found
Inductive and Electrostatic Acceleration in Relativistic Jet-Plasma Interactions
We report on the observation of rapid particle acceleration in numerical
simulations of relativistic jet-plasma interactions and discuss the underlying
mechanisms. The dynamics of a charge-neutral, narrow, electron-positron jet
propagating through an unmagnetized electron-ion plasma was investigated using
a three-dimensional, electromagnetic, particle-in-cell computer code. The
interaction excited magnetic filamentation as well as electrostatic plasma
instabilities. In some cases, the longitudinal electric fields generated
inductively and electrostatically reached the cold plasma wave-breaking limit,
and the longitudinal momentum of about half the positrons increased by 50% with
a maximum gain exceeding a factor of 2 during the simulation period. Particle
acceleration via these mechanisms occurred when the criteria for Weibel
instability were satisfied.Comment: Revised for Phys. Rev. Lett. Please see publised version for best
graphic
On the propagation of a two-dimensional viscous density current under surface waves
This study aims to develop an asymptotic theory for the slow spreading of a thin layer of viscous immiscible dense liquid on the bottom of a waterway under the combined effects of surface waves and density current. By virtue of the sharply different length and time scales (wave periodic excitation being effective at fast scales, while gravity and streaming currents at slow scales), a multiple-scale perturbation analysis is conducted. Evolution equations are deduced for the local and global profile distributions of the dense liquid layer as functions of the slow-time variables. When reflected waves are present, the balance between gravity and streaming will result, on a time scale one order of magnitude longer than the wave period, in an undulating water/liquid interface whose displacement amplitude is much smaller than the thickness of the dense liquid layer. On the global scale, the streaming current can predominate and drive the dense liquid to propagate with a distinct pattern in the direction of the surface waves. © 2002 American Institute of Physics.published_or_final_versio
Ferromagnetic Convection in a Heterogeneous Darcy Porous Medium Using a Local Thermal Non-equilibrium (LTNE) Model
The combined effects of vertical heterogeneity of permeability and local thermal non-equilibrium (LTNE) on the onset of ferromagnetic convection in a ferrofluid saturated Darcy porous medium in the presence of a uniform vertical magnetic field are investigated. A two-field model for temperature representing the solid and fluid phases separately is used. The eigenvalue problem is solved numerically using the Galerkin method for different forms of permeability heterogeneity function Γ(z) and their effect on the stability characteristics of the system has been analyzed in detail. It is observed that the general quadratic variation of Γ(z) with depth has more destabilizing effect on the system when compared to the homogeneous porous medium case. Besides, the influence of LTNE and magnetic parameters on the criterion for the onset of ferromagnetic convection is also assessed
Radio Polarization Observations of the Snail: A Crushed Pulsar Wind Nebula in G327.1-1.1 with a Highly Ordered Magnetic Field
Pulsar wind nebulae (PWNe) are suggested to be acceleration sites of cosmic
rays in the Galaxy. While the magnetic field plays an important role in the
acceleration process, previous observations of magnetic field configurations of
PWNe are rare, particularly for evolved systems. We present a radio
polarization study of the "Snail" PWN inside the supernova remnant G327.1-1.1
using the Australia Telescope Compact Array. This PWN is believed to have been
recently crushed by the supernova (SN) reverse shock. The radio morphology is
composed of a main circular body with a finger-like protrusion. We detected a
strong linear polarization signal from the emission, which reflects a highly
ordered magnetic field in the PWN and is in contrast to the turbulent
environment with a tangled magnetic field generally expected from
hydrodynamical simulations. This could suggest that the characteristic
turbulence scale is larger than the radio beam size. We built a toy model to
explore this possibility, and found that a simulated PWN with a turbulence
scale of about one-eighth to one-sixth of the nebula radius and a pulsar wind
filling factor of 50--75% provides the best match to observations. This implies
substantial mixing between the SN ejecta and pulsar wind material in this
system.Comment: 13 pages, 10 figures, Accepted for publication in Ap
Deep Chandra Observation of the Pulsar Wind Nebula Powered by the Pulsar J1846-0258 in the Supernova Remnant Kes 75
We present the results of detailed spatial and spectral analysis of the
pulsar wind nebula (PWN) in supernova remnant Kes 75 (G29.7-0.3) using a deep
exposure with Chandra X-ray observatory. The PWN shows a complex morphology
with clear axisymmetric structure. We identified a one-sided jet and two bright
clumps aligned with the overall nebular elongation, and an arc-like feature
perpendicular to the jet direction. Further spatial modeling with a torus and
jet model indicates a position angle 207\arcdeg\pm8 \arcdeg for the PWN
symmetry axis. We interpret the arc as an equatorial torus or wisp and the
clumps could be shock interaction between the jets and the surrounding medium.
The lack of any observable counter jet implies a flow velocity larger than
0.4c. Comparing to an archival observation 6 years earlier, some small-scale
features in the PWN demonstrate strong variability: the flux of the inner jet
doubles and the peak of the northern clump broadens and shifts 2" outward. In
addition, the pulsar flux increases by 6 times, showing substantial spectral
softening from =1.1 to 1.9 and an emerging thermal component which was
not observed in the first epoch. The changes in the pulsar spectrum are likely
related to the magnetar-like bursts of the pulsar that occurred 7 days before
the Chandra observation, as recently reported from RXTE observations.Comment: Accepted by ApJ, 8 figures, some of them have been scaled down in
resolutio
Observing two dark accelerators around the Galactic Centre with Fermi Large Area Telescope
We report the results from a detailed ray investigation in the field
of two "dark accelerators", HESS J1745-303 and HESS J1741-302, with years
of data obtained by the Fermi Large Area Telescope. For HESS J1745-303, we
found that its MeV-GeV emission is mainly originated from the "Region A" of the
TeV feature. Its ray spectrum can be modeled with a single power-law
with a photon index of from few hundreds MeV to TeV. Moreover,
an elongated feature, which extends from "Region A" toward northwest for
, is discovered for the first time. The orientation of this
feature is similar to that of a large scale atomic/molecular gas distribution.
For HESS J1741-302, our analysis does not yield any MeV-GeV counterpart for
this unidentified TeV source. On the other hand, we have detected a new point
source, Fermi J1740.1-3013, serendipitously. Its spectrum is apparently curved
which resembles that of a ray pulsar. This makes it possibly
associated with PSR B1737-20 or PSR J1739-3023.Comment: 11 pages, 7 figures, 2 tables, accepted for publication in MNRA
Quantum Dot in 2D Topological Insulator: The Two-channel Kondo Fixed Point
In this work, a quantum dot couples to two helical edge states of a 2D
topological insulator through weak tunnelings is studied. We show that if the
electron interactions on the edge states are repulsive, with Luttinger liquid
parameter , the system flows to a stable two-channel fixed point at
low temperatures. This is in contrast to the case of a quantum dot couples to
two Luttinger liquid leads. In the latter case, a strong electron-electron
repulsion is needed, with , to reach the two-channel fixed point. This
two-channel fixed point is described by a boundary Sine-Gordon Hamiltonian with
a dependent boundary term. The impurity entropy at zero temperature is
shown to be . The impurity specific heat is when , and when . We
also show that the linear conductance across the two helical edges has
non-trivial temperature dependence as a result of the renormalization group
flow.Comment: 4+\epsilon page
Exploring Food Detection using CNNs
One of the most common critical factors directly related to the cause of a
chronic disease is unhealthy diet consumption. In this sense, building an
automatic system for food analysis could allow a better understanding of the
nutritional information with respect to the food eaten and thus it could help
in taking corrective actions in order to consume a better diet. The Computer
Vision community has focused its efforts on several areas involved in the
visual food analysis such as: food detection, food recognition, food
localization, portion estimation, among others. For food detection, the best
results evidenced in the state of the art were obtained using Convolutional
Neural Network. However, the results of all these different approaches were
gotten on different datasets and therefore are not directly comparable. This
article proposes an overview of the last advances on food detection and an
optimal model based on GoogLeNet Convolutional Neural Network method, principal
component analysis, and a support vector machine that outperforms the state of
the art on two public food/non-food datasets
Photonic Clusters
We show through rigorous calculations that dielectric microspheres can be
organized by an incident electromagnetic plane wave into stable cluster
configurations, which we call photonic molecules. The long-range optical
binding force arises from multiple scattering between the spheres. A photonic
molecule can exhibit a multiplicity of distinct geometries, including
quasicrystal-like configurations, with exotic dynamics. Linear stability
analysis and dynamical simulations show that the equilibrium configurations can
correspond with either stable or a type of quasi-stable states exhibiting
periodic particle motion in the presence of frictional dissipation.Comment: 4 pages, 3 figure
Electron-Beam Driven Relaxation Oscillations in Ferroelectric Nanodisks
Using a combination of computational simulations, atomic-scale resolution
imaging and phenomenological modelling, we examine the underlying mechanism for
nanodomain restructuring in lead zirconate titanate (PZT) nanodisks driven by
electron beams. The observed subhertz nanodomain dynamics are identified with
relaxation oscillations where the charging/discharging cycle time is determined
by saturation of charge traps and nanodomain wall creep. These results are
unusual in that they indicate very slow athermal dynamics in nanoscale systems.Comment: 5 pages, 2 figure
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