1,153 research outputs found
Overview of MHz air shower radio experiments and results
In this paper, I present a review of the main results obtained in the last 10
years in the field of radio-detection of cosmic-ray air showers in the MHz
range. All results from all experiments cannot be reported here so that I will
focus on the results more than on the experiments themselves. Modern
experiments started in 2003 with CODALEMA and LOPES. In 2006, small-size
autonomous prototypes setup were installed at the Pierre Auger Observatory
site, to help the design of the Auger Engineering Radio Array (AERA). We will
discuss the principal aspects of the radio data analysis and the determination
of the primary cosmic ray characteristics: the arrival direction, the lateral
distribution of the electric field, the correlation with the primary energy,
the emission mechanisms and the sensitivity to the composition of the cosmic
rays.Comment: Proceedings of the ARENA2012 conference (Erlangen, Germany), to be
published in AIP Conference Proceeding
Relativistic Fermi acceleration with shock compressed turbulence
This paper presents numerical simulations of test particle Fermi acceleration
at relativistic shocks of Lorentz factor Gamma_sh = 2-60, using a realistic
downstream magnetic structure obtained from the shock jump conditions. The
upstream magnetic field is described as pure Kolmogorov turbulence; the
corresponding downstream magnetic field lies predominantly in the plane
tangential to the shock surface and the coherence length is smaller along the
shock normal than in the tangential plane. Acceleration is nonetheless
efficient and leads to powerlaw spectra with index s = 2.6-2.7 at large shock
Lorentz factor Gamma_sh >> 1, markedly steeper than for isotropic scattering
downstream. The acceleration timescale t_acc in the upstream rest frame becomes
a fraction of Larmor time t_L in the ultra-relativistic limit, t_acc ~ 10
t_L/Gamma_sh. Astrophysical applications are discussed, in particular the
acceleration in gamma-ray bursts internal and external shocks.Comment: 11 pages; 10 figures; submitted to MNRA
Radio emission from the air shower sudden death
We present a new mechanism for air shower radio emission due to the sudden
absorption of secondary particles when the shower front hits the ground. The
electrons present in excess during the air shower development imply a net
residual negative charge in the shower front. We show that for showers hitting
the ground before the complete extinction of their electromagnetic component,
the sudden vanishing of the net residual negative charge generates an electric
field contribution in the kHz---MHz range. We characterize this radio
contribution as a function of primary energy, arrival direction and antenna
position, using the simulation code SELFAS2. We discuss the interest of this
new predicted signal on detection and analysis of ultra-high energy cosmic-rays
and we argue that the region in the shower of maximum emission of the electric
field should not coincide with the region of maximum development.Comment: 8 pages, 15 figures, Submitted to Astroparticle Physics on October 6,
201
On the efficiency of Fermi acceleration at relativistic shocks
It is shown that Fermi acceleration at an ultra-relativistic shock wave
cannot operate on a particle for more than 1 1/2 Fermi cycle (i.e., u -> d -> u
-> d) if the particle Larmor radius is much smaller than the coherence length
of the magnetic field on both sides of the shock, as is usually assumed. This
conclusion is shown to be in excellent agreement with recent numerical
simulations. We thus argue that efficient Fermi acceleration at
ultra-relativistic shock waves requires significant non-linear processing of
the far upstream magnetic field with strong amplification of the small scale
magnetic power. The streaming or transverse Weibel instabilities are likely to
play a key role in this respect.Comment: 4 pages, 2 figures; to appear in ApJ Letter
Graph-based skin lesion segmentation of multispectral dermoscopic images
International audienceAccurate skin lesion segmentation is critical for automated early skin cancer detection and diagnosis. We present a novel method to detect skin lesion borders in multispectral der-moscopy images. First, hairs are detected on infrared images and removed by inpainting visible spectrum images. Second, skin lesion is pre-segmented using a clustering of a superpixel partition. Finally, the pre-segmentation is globally regular-ized at the superpixel level and locally regularized in a narrow band at the pixel level
Angular Power Spectrum Estimation of Cosmic Ray Anisotropies with Full or Partial Sky Coverage
We study the angular power spectrum estimate in order to search for large
scale anisotropies in the arrival directions distribution of the highest-energy
cosmic rays. We show that this estimate can be performed even in the case of
partial sky coverage and validated over the full sky under the assumption that
the observed fluctuations are statistically spatial stationary. If this
hypothesis - which can be tested directly on the data - is not satisfied, it
would prove, of course, that the cosmic ray sky is non isotropic but also that
the power spectrum is not an appropriate tool to represent its anisotropies,
whatever the sky coverage available. We apply the method to simulations of the
Pierre Auger Observatory, reconstructing an input power spectrum with the
Southern site only and with both Northern and Southern ones. Finally, we show
the improvement that a full-sky observatory brings to test an isotropic
distribution, and we discuss the sensitivity of the Pierre Auger Observatory to
large scale anisotropies.Comment: 16 pages, 6 figures, version accepted for publication by JCA
New data model for graph-cut segmentation: application to automatic melanoma delineation
International audienceWe propose a new data model for graph-cut image segmentation, defined according to probabilities learned by a classification process. Unlike traditional graph-cut methods, the data model takes into account not only color but also texture and shape information. For melanoma images, we also introduce skin chromophore features and automatically derive "seed" pixels used to train the classifier from a coarse initial segmentation. On natural images, our method successfully segments objects having similar color but different texture. Its application to melanoma delineation compares favorably to manual delineation and related graph-cut segmentation methods
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