5,510 research outputs found
Morphology and hardness ratio exploitation under limited statistics
Gamma-ray astronomy has produced for several years now sky maps for low
photon statistics, non-negligible background and comparatively poor angular
resolution. Quantifying the significance of spatial features remains difficult.
Besides, spectrum extraction requires regions with large statistics while maps
in energy bands allow only qualitative interpretation. The two main competing
mechanisms in the VHE domain are the Inverse-Compton emission from accelerated
electrons radiating through synchrotron in the X-ray domain and the
interactions between accelerated hadrons and the surrounding medium, leading to
the production and subsequent decay of Pi0 mesons. The spectrum of the VHE
emission from leptons is predicted to steepen with increasing distance from the
acceleration zone, owing to synchrotron losses (i.e. cooled population). It
would remain approximately constant for hadrons. Ideally, spectro-imaging
analysis would have the same spatial scale in the TeV and X-ray domains, to
distinguish the local emission mechanisms. More realistically, we investigate
here the possibility of improving upon the currently published HESS results by
using more sophisticated tools.Comment: 4 pages, 6 figures, Proceeding for a poster at the GAMMA08 Heidelberg
Symposiu
Magnetoresistance and collective Coulomb blockade in super-lattices of ferromagnetic CoFe nanoparticles
We report on transport properties of millimetric super-lattices of CoFe
nanoparticles surrounded by organic ligands. R(T)s follow R(T) =
R_0.exp(T/T_0)^0.5 with T_0 ranging from 13 to 256 K. At low temperature I(V)s
follow I=K[(V-V_T)/V_T]^ksi with ksi ranging 3.5 to 5.2. I(V) superpose on a
universal curve when shifted by a voltage proportional to the temperature.
Between 1.8 and 10 K a high-field magnetoresistance with large amplitude and a
strong voltage-dependence is observed. Its amplitude only depends on the
magnetic field/temperature ratio. Its origin is attributed to the presence of
paramagnetic states present at the surface or between the nanoparticles. Below
1.8 K, this high-field magnetoresistance abruptly disappears and inverse
tunnelling magnetoresistance is observed, the amplitude of which does not
exceed 1%. At this low temperature, some samples display in their I(V)
characteristics abrupt and hysteretic transitions between the Coulomb blockade
regime and the conductive regime. The increase of the current during these
transitions can be as high as a factor 30. The electrical noise increases when
the sample is near the transition. The application of a magnetic field
decreases the voltage at which these transitions occur so magnetic-field
induced transitions are also observed. Depending on the applied voltage, the
temperature and the amplitude of the magnetic field, the magnetic-field induced
transitions are either reversible or irreversible. These abrupt and hysteretic
transitions are also observed in resistance-temperature measurements. They
could be the soliton avalanches predicted by Sverdlov et al. [Phys. Rev. B 64,
041302 (R), 2001] or could also be interpreted as a true phase transition
between a Coulomb glass phase to a liquid phase of electrons
Encoding dynamics for multiscale community detection: Markov time sweeping for the Map equation
The detection of community structure in networks is intimately related to
finding a concise description of the network in terms of its modules. This
notion has been recently exploited by the Map equation formalism (M. Rosvall
and C.T. Bergstrom, PNAS, 105(4), pp.1118--1123, 2008) through an
information-theoretic description of the process of coding inter- and
intra-community transitions of a random walker in the network at stationarity.
However, a thorough study of the relationship between the full Markov dynamics
and the coding mechanism is still lacking. We show here that the original Map
coding scheme, which is both block-averaged and one-step, neglects the internal
structure of the communities and introduces an upper scale, the `field-of-view'
limit, in the communities it can detect. As a consequence, Map is well tuned to
detect clique-like communities but can lead to undesirable overpartitioning
when communities are far from clique-like. We show that a signature of this
behavior is a large compression gap: the Map description length is far from its
ideal limit. To address this issue, we propose a simple dynamic approach that
introduces time explicitly into the Map coding through the analysis of the
weighted adjacency matrix of the time-dependent multistep transition matrix of
the Markov process. The resulting Markov time sweeping induces a dynamical
zooming across scales that can reveal (potentially multiscale) community
structure above the field-of-view limit, with the relevant partitions indicated
by a small compression gap.Comment: 10 pages, 6 figure
EMBRACE@Nancay: An Ultra Wide Field of View Prototype for the SKA
A revolution in radio receiving technology is underway with the development
of densely packed phased arrays for radio astronomy. This technology can
provide an exceptionally large field of view, while at the same time sampling
the sky with high angular resolution. Such an instrument, with a field of view
of over 100 square degrees, is ideal for performing fast, all-sky, surveys,
such as the "intensity mapping" experiment to measure the signature of Baryonic
Acoustic Oscillations in the HI mass distribution at cosmological redshifts.
The SKA, built with this technology, will be able to do a billion galaxy
survey. I will present a very brief introduction to radio interferometry, as
well as an overview of the Square Kilometre Array project. This will be
followed by a description of the EMBRACE prototype and a discussion of results
and future plans.Comment: to appear in proceedings of the INFIERI Summer School INtelligent
Signal Processing for FrontIEr Research and Industry, Paris 201
Terahertz imaging of sub-wavelength particles with Zenneck surface waves
Impact of sub-wavelength-size dielectric particles on Zenneck surface waves on planar metallic antennas is investigated at terahertz (THz) frequencies with THz near-field probe microscopy. Perturbations of the surface waves show the particle presence, despite its sub-wavelength size. The experimental configuration, which utilizes excitation of surface waves at metallic edges, is suitable for THz imaging of dielectric sub-wavelength size objects. As a proof of concept, the effects of a small strontium titanate rectangular particle and a titanium dioxide sphere on the surface field of a bow-tie antenna are experimentally detected and verified using full-wave simulations
DWDM-PON/mm-Wave wireless converged Next Generation Access Topology using coherent heterodyne detection
A radio-over-fibre system using coherent optical heterodyne detection scheme is proposed, to achieve seamless integration of a photonic Remote Antenna Unit (RAU) into a Next Generation Dense Wavelength Division Multiplexed Passive Optical Network (NG DWDM-PON). The proposed scheme significantly simplifies the optical mm-wave generation and data recovery as it doesn't require any high-bandwidth modulator at the central office or high-frequency Local Oscillators (LOs) at either the central office or the customer unit; or optical phase-locking techniques to generate the mm-wave wireless signal. A proof-of-concept transmission utilizing 1 Gb/s On-Off Keying is experimentally demonstrated
Characterization of a dense aperture array for radio astronomy
EMBRACE@Nancay is a prototype instrument consisting of an array of 4608
densely packed antenna elements creating a fully sampled, unblocked aperture.
This technology is proposed for the Square Kilometre Array and has the
potential of providing an extremely large field of view making it the ideal
survey instrument. We describe the system,calibration procedures, and results
from the prototype.Comment: 17 pages, accepted for publication in A&
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