840 research outputs found
Onset of collective and cohesive motion
We study the onset of collective motion, with and without cohesion, of groups
of noisy self-propelled particles interacting locally. We find that this phase
transition, in two space dimensions, is always discontinuous, including for the
minimal model of Vicsek et al. [Phys. Rev. Lett. {\bf 75},1226 (1995)] for
which a non-trivial critical point was previously advocated. We also show that
cohesion is always lost near onset, as a result of the interplay of density,
velocity, and shape fluctuations.Comment: accepted for publication in Phys. Rev. Let
Near-infrared follow-up to the May 2008 activation of SGR 1627-41
On 28 May 2008, the Swift satellite detected the first reactivation of SGR
1627-41 since its discovery in 1998.
Following this event we began an observing campaign in near infrared
wavelengths to search for a possible counterpart inside the error circle of
this SGR, which is expected to show flaring activity simultaneous to the high
energy flares or at least some variability as compared to the quiescent state.
For the follow-up we used the 0.6m REM robotic telescope at La Silla
Observatory, which allowed a fast response within 24 hours and, through
director discretionary time, the 8.2m Very Large Telescope at Paranal
Observatory. There, we observed with NACO to produce high angular resolution
imaging with the aid of adaptive optics.
These observations represent the fastest near infrared observations after an
activation of this SGR and the deepest and highest spatial resolution
observations of the Chandra error circle.
5 sources are detected in the immediate vicinity of the most precise X-ray
localisation of this source. For 4 of them we do not detect variability,
although the X-ray counterpart experimented a significant decay during our
observation period. The 5th source is only detected in one epoch, where we have
the best image quality, so no variability constrains can be imposed and remains
as the only plausible counterpart. We can impose a limit of Ks > 21.6
magnitudes to any other counterpart candidate one week after the onset of the
activity. Our adaptive optics imaging, with a resolution of 0.2" provides a
reference frame for subsequent studies of future periods of activity.Comment: Accepted for publication in A&
Sacral neuromodulation as a treatment for neuropathic clitoral pain after abdominal hysterectomy
Sacral neuromodulation (SNM) may be beneficial in the treatment of patients with chronic pelvic pain, although it is not an FDA-approved indication. We present a case of a 51-year-old patient that presented with symptoms of lower urinary tract dysfunction and clitoral pain after an abdominal hysterectomy. Electrophysiological evaluation suggested a pudendal nerve lesion. After failure of conservative treatment, she was offered SNM as a treatment for her voiding symptoms. During test stimulation, she experienced only moderate improvement in voiding symptoms, but a striking improvement in pain symptoms. She underwent a two-stage implantation of a neurostimulator with a successful outcome after 6 months’ follow-up. The results of this report suggest that SNM may be effective in patients with neuropathic pelvic pain
Risk analysis of marine activities in the Belgian part of the North Sea (RAMA): final report
RAMA is a 2-year project (04/2004 - 04/2006) executed by two Belgian partners, Ecolas NV (Environmental Consultancy Agency) and the Maritime Institute (University of Ghent), and financed by the SPSD II research program, specific actions, of the Belgian Science Policy (BELPSO). RAMA aims to assess the environmental risks of spills by commercial shipping activities on the Belgian Part of the North Sea. Shipping patterns, transports of dangerous goods, probability of risks and the potential impact of spill incidents (oil & hazardous and noxious substances) will be assessed. The risk analysis within this project studies both the chances of a spill accident happening and the environmental impacts in case of an accident. The valorisation of the RAMA project will result in a thorough analysis of the current status of the shipping at the North Sea in relation to the issue of safety. The scope of the project will however go beyond the mere result of a fundamental risk analysis of the commercial shipping at the North Sea. It is also aiming at the formulation of recommendations to improve the safety level for the environment and at an optimization of response in the framework of the Belgian "North Sea Disaster Plan"
Nonlinear guided waves and spatial solitons in a periodic layered medium
We overview the properties of nonlinear guided waves and (bright and dark)
spatial optical solitons in a periodic medium created by a sequence of linear
and nonlinear layers. First, we consider a single layer with a cubic nonlinear
response (a nonlinear waveguide) embedded into a periodic layered linear
medium, and describe nonlinear localized modes (guided waves and Bragg-like
localized gap modes) and their stability. Then, we study modulational
instability as well as the existence and stability of discrete spatial solitons
in a periodic array of identical nonlinear layers, a one-dimensional nonlinear
photonic crystal. Both similarities and differences with the models described
by the discrete nonlinear Schrodinger equation (derived in the tight-binding
approximation) and coupled-mode theory (valid for the shallow periodic
modulations) are emphasized.Comment: 10 pages, 14 figure
Equivalence of Kinetic Theories of Bose-Einstein Condensation
We discuss the equivalence of two non-equilibrium kinetic theories that
describe the evolution of a dilute, Bose-Einstein condensed atomic gas in a
harmonic trap. The second-order kinetic equations of Walser et al. [PRA 63,
013607 (2001)] reduce to the Gross-Pitaevskii equation and the quantum
Boltzmann equation in the low and high temperature limits, respectively. These
kinetic equations can thus describe the system in equilibrium (finite
temperature) as well as in non-equilibrium (real time). We have found this
theory to be equivalent to the non-equilibrium Green's function approach
originally proposed by Kadanoff and Baym and more recently applied to
inhomogeneous trapped systems by M. Imamovi\'c-Tomasovi\'c and A. Griffin
[arXiv:cond-mat/9911402].Comment: REVTeX3, 6 pages, 2 eps figures, published version, minor change
Study of the luminous blue variable star candidate G26.47+0.02 and its environment
The luminous blue variable (LBV) stars are peculiar very massive stars. The
study of these stellar objects and their surroundings is important for
understanding the evolution of massive stars and its effects on the
interstellar medium. We study the LBV star candidate G26.47+0.02. Using several
large-scale surveys in different frequencies we performed a multiwavelength
study of G26.47+0.02 and its surroundings. We found a molecular shell (seen in
the 13CO J=1-0 line) that partially surrounds the mid-infrared nebula of
G26.47+0.02, which suggests an interaction between the strong stellar winds and
the molecular gas. From the HI absorption and the molecular gas study we
conclude that G26.47+0.02 is located at a distance of ~4.8 kpc. The radio
continuum analysis shows a both thermal and non-thermal emission toward this
LBV candidate, pointing to wind-wind collision shocks from a binary system.
This hypothesis is supported by a search of near-IR sources and the Chandra
X-ray analysis. Additional multiwavelength and long-term observations are
needed to detect some possible variable behavior, and if that is found, to
confirm the binary nature of the system.Comment: accepted in A&A 01/05/201
- …