7,161 research outputs found
Nearby supernova remnants and the cosmic-ray spectral hardening at high energies
Recent measurements of cosmic-ray spectra of several individual nuclear
species by the CREAM, TRACER, and ATIC experiments indicate a change in the
spectral index of the power laws at TeV energies. Possible explanations among
others include non linear diffusive shock acceleration of cosmic-rays,
different cosmic-ray propagation properties at higher and lower energies in the
Galaxy and the presence of nearby sources. In this paper, we show that if
supernova remnants are the main sources of cosmic rays in our Galaxy, the
effect of the nearby remnants can be responsible for the observed spectral
changes. Using a rigidity dependent escape of cosmic-rays from the supernova
remnants, we explain the apparent observed property that the hardening of the
helium spectrum occurs at relatively lower energies as compared to the protons
and also that the spectral hardening does not persist beyond TeV
energies.Comment: 6 pages, MNRAS accepted, minor text correction
GeV-TeV cosmic-ray spectral anomaly as due to re-acceleration by weak shocks in the Galaxy
Recent cosmic-ray measurements have found an anomaly in the cosmic-ray energy
spectrum at GeV-TeV energies. Although the origin of the anomaly is not clearly
understood, suggested explanations include effect of cosmic-ray source
spectrum, propagation effects, and the effect of nearby sources. In this paper,
we propose that the spectral anomaly might be an effect of re-acceleration of
cosmic rays by weak shocks in the Galaxy. After acceleration by strong
supernova remnant shock waves, cosmic rays undergo diffusive propagation
through the Galaxy. During the propagation, cosmic rays may again encounter
expanding supernova remnant shock waves, and get re-accelerated. As the
probability of encountering old supernova remnants is expected to be larger
than the younger ones due to their bigger sizes, re-acceleration is expected to
be mainly due to weaker shocks. Since weaker shocks generate a softer particle
spectrum, the resulting re-accelerated component will have a spectrum steeper
than the initial cosmic-ray source spectrum produced by strong shocks. For a
reasonable set of model parameters, it is shown that such re-accelerated
component can dominate the GeV energy region while the non-reaccelerated
component dominates at higher energies, explaining the observed GeV-TeV
spectral anomaly.Comment: 12 pages, A&A accepte
Limits to superweak amplification of beam shifts
The magnitudes of beam shifts (Goos-H\"anchen and Imbert-Fedorov, spatial and
angular) are greatly enhanced when a reflected light beam is postselected by an
analyzer, by analogy with superweak measurements in quantum theory.
Particularly strong enhancements can be expected close to angles at which no
light is transmitted for a fixed initial and final polarizations. We derive a
formula for the angular and spatial shifts at such angles (which includes the
Brewster angle), and we show that their maximum size is limited by higher-order
terms from the reflection coefficients occurring in the Artmann shift formula.Comment: 3 pages, 2 figures, Optics Letters styl
A New Class of Cellular Automata for Reaction-Diffusion Systems
We introduce a new class of cellular automata to model reaction-diffusion
systems in a quantitatively correct way. The construction of the CA from the
reaction-diffusion equation relies on a moving average procedure to implement
diffusion, and a probabilistic table-lookup for the reactive part. The
applicability of the new CA is demonstrated using the Ginzburg-Landau equation.Comment: 4 pages, RevTeX 3.0 , 3 Figures 214972 bytes tar, compressed,
uuencode
Self-affine Manifolds
This paper studies closed 3-manifolds which are the attractors of a system of
finitely many affine contractions that tile . Such attractors are
called self-affine tiles. Effective characterization and recognition theorems
for these 3-manifolds as well as theoretical generalizations of these results
to higher dimensions are established. The methods developed build a bridge
linking geometric topology with iterated function systems and their attractors.
A method to model self-affine tiles by simple iterative systems is developed
in order to study their topology. The model is functorial in the sense that
there is an easily computable map that induces isomorphisms between the natural
subdivisions of the attractor of the model and the self-affine tile. It has
many beneficial qualities including ease of computation allowing one to
determine topological properties of the attractor of the model such as
connectedness and whether it is a manifold. The induced map between the
attractor of the model and the self-affine tile is a quotient map and can be
checked in certain cases to be monotone or cell-like. Deep theorems from
geometric topology are applied to characterize and develop algorithms to
recognize when a self-affine tile is a topological or generalized manifold in
all dimensions. These new tools are used to check that several self-affine
tiles in the literature are 3-balls. An example of a wild 3-dimensional
self-affine tile is given whose boundary is a topological 2-sphere but which is
not itself a 3-ball. The paper describes how any 3-dimensional handlebody can
be given the structure of a self-affine 3-manifold. It is conjectured that
every self-affine tile which is a manifold is a handlebody.Comment: 40 pages, 13 figures, 2 table
Topological aberration of optical vortex beams and singularimetry of dielectric interfaces
The splitting of a high-order optical vortex into a constellation of unit
vortices, upon total reflection, is described and analyzed. The vortex
constellation generalizes, in a local sense, the familiar longitudinal
Goos-H\"anchen and transverse Imbert-Federov shifts of the centroid of a
reflected optical beam. The centroid shift is related to the centre of the
constellation, whose geometry otherwise depends on higher-order terms in an
expansion of the reflection matrix. We present an approximation of the field
around the constellation of increasing order as an Appell sequence of complex
polynomials whose roots are the vortices, and explain the results by an analogy
with the theory of optical aberration.Comment: 5 pages, 3 figures, REVTeX 4.
Cosmogenic gamma-rays and neutrinos constrain UHECR source models
We use CRPropa 3 to show how the expected cosmogenic neutrino and gamma-ray
spectra depend on the maximum energy of ultra-high energy cosmic rays (UHECRs)
at their sources, on the spectral index at injection and on the chemical
composition of UHECRs. The isotropic diffuse gamma-ray background measured by
Fermi/LAT is already close to touching upon a model with co-moving source
evolution and with the chemical composition, spectral index and maximum
acceleration energy optimized to provide the best fit to the UHECR spectrum and
composition measured by the Pierre Auger Collaboration. Additionally, the
detectable fraction of protons present at the highest energies in UHECRs, for
experiments with sensitivities to the single-flavor neutrino flux at
EeV in the range of - GeV cm s
sr, is shown as a function of the evolution of UHECR sources.
Experiments that reach this sensitivity will be able to significantly constrain
the proton fraction for realistic source evolution models.Comment: Proc. 35th ICRC, Busan, South Korea, PoS(ICRC2017)56
The Freezing of Random RNA
We study secondary structures of random RNA molecules by means of a
renormalized field theory based on an expansion in the sequence disorder. We
show that there is a continuous phase transition from a molten phase at higher
temperatures to a low-temperature glass phase. The primary freezing occurs
above the critical temperature, with local islands of stable folds forming
within the molten phase. The size of these islands defines the correlation
length of the transition. Our results include critical exponents at the
transition and in the glass phase.Comment: 4 pages, 8 figures. v2: presentation improve
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