4,659 research outputs found
Earth magnetic field effects on the cosmic electron flux as background for Cherenkov Telescopes at low energies
Cosmic ray electrons and positrons constitute an important component of the
background for imaging atmospheric Cherenkov Telescope Systems with very low
energy thresholds. As the primary energy of electrons and positrons decreases,
their contribution to the background trigger rate dominates over protons, at
least in terms of differential rates against actual energies. After event
reconstruction, this contribution might become comparable to the proton
background at energies of the order of few GeV. It is well known that the flux
of low energy charged particles is suppressed by the Earth's magnetic field.
This effect strongly depends on the geographical location, the direction of
incidence of the charged particle and its mass. Therefore, the geomagnetic
field can contribute to diminish the rate of the electrons and positrons
detected by a given array of Cherenkov Telescopes.
In this work we study the propagation of low energy primary electrons in the
Earth's magnetic field by using the backtracking technique. We use a more
realistic geomagnetic field model than the one used in previous calculations.
We consider some sites relevant for new generations of imaging atmospheric
Cherenkov Telescopes. We also study in detail the case of 5@5, a proposed low
energy Cherenkov Telescope array.Comment: To appear in Astroparticle Physic
Discovering and Locating High-Energy Extra-Galactic Sources by Bayesian Mixture Modelling
Discovering and locating gamma-ray sources in the whole sky map is a declared target of the Fermi Gamma-ray Space Telescope collaboration. In this paper, we carry out an unsupervised analysis of the collection of high-energy photons accumulated by the Large Area Telescope, the principal instrument on board the Fermi spacecraft, over a period of around 7.5 years using a Bayesian mixture model. A fixed, though unknown, number of parametric components identify the extra-galactic emitting sources we are searching for, while a further component represents parametrically the dffuse gamma ray background due to both, extragalactic and galactic high energy photon emission. We determine the number of sources, their coordinates on the map and their intensities. The model parameters are estimated using a reversible jump MCMC algorithm which implements four different types of moves. These allow us to explore the dimension of the parameter space. The possible transitions remove from or add a source to the model, while leaving the background component unchanged. We furthermore present an heuristic procedure, based on the posterior distribution of the mixture weights, to qualify the nature of each detected source
BILDUNG VON HĂHEREN ALKYLBENZOLEN BEI DER REFORMING VON BENZINFRAKTIONEN AN EINEM BIMETALLISCHEN KATALYSATOR
On Existence and Properties of Approximate Pure Nash Equilibria in Bandwidth Allocation Games
In \emph{bandwidth allocation games} (BAGs), the strategy of a player
consists of various demands on different resources. The player's utility is at
most the sum of these demands, provided they are fully satisfied. Every
resource has a limited capacity and if it is exceeded by the total demand, it
has to be split between the players. Since these games generally do not have
pure Nash equilibria, we consider approximate pure Nash equilibria, in which no
player can improve her utility by more than some fixed factor through
unilateral strategy changes. There is a threshold (where
is a parameter that limits the demand of each player on a specific
resource) such that -approximate pure Nash equilibria always exist for
, but not for . We give both
upper and lower bounds on this threshold and show that the
corresponding decision problem is -hard. We also show that the
-approximate price of anarchy for BAGs is . For a restricted
version of the game, where demands of players only differ slightly from each
other (e.g. symmetric games), we show that approximate Nash equilibria can be
reached (and thus also be computed) in polynomial time using the best-response
dynamic. Finally, we show that a broader class of utility-maximization games
(which includes BAGs) converges quickly towards states whose social welfare is
close to the optimum
DIE ROLLE DER INSTRUMENTALEN ANALYTIK IN DER TECHNOLOGISCHEN FORSCHUNG ZUR HERSTELLUNG VON Câ - Cââ AROMATEN DURCH KATALYTISCHES BENZIN-REFORMING
Heterogeneous Catalytic Approaches in C-H Activation Reactions
This review summarizes the development of user-friendly, recyclable and easily separable heterogeneous catalysts for CâH activation during the last decade until December 2015
D-Foam Phenomenology: Dark Energy, the Velocity of Light and a Possible D-Void
In a D-brane model of space-time foam, there are contributions to the dark
energy that depend on the D-brane velocities and on the density of D-particle
defects. The latter may also reduce the speeds of photons linearly with their
energies, establishing a phenomenological connection with astrophysical probes
of the universality of the velocity of light. Specifically, the cosmological
dark energy density measured at the present epoch may be linked to the apparent
retardation of energetic photons propagating from nearby AGNs. However, this
nascent field of `D-foam phenomenology' may be complicated by a dependence of
the D-particle density on the cosmological epoch. A reduced density of
D-particles at redshifts z ~ 1 - a `D-void' - would increase the dark energy
while suppressing the vacuum refractive index, and thereby might reconcile the
AGN measurements with the relatively small retardation seen for the energetic
photons propagating from GRB 090510, as measured by the Fermi satellite.Comment: 10 pages, 3 figure
Simulating aerosol microphysics with the ECHAM/MADE GCM ? Part I: Model description and comparison with observations
International audienceThe aerosol dynamics module MADE has been coupled to the general circulation model ECHAM4 to simulate the chemical composition, number concentration, and size distribution of the global submicrometer aerosol. The present publication describes the new model system ECHAM4/MADE and presents model results in comparison with observations. The new model is able to simulate the full life cycle of particulate matter and various gaseous precursors including emissions of primary particles and trace gases, advection, convection, diffusion, coagulation, condensation, nucleation of sulfuric acid vapor, aerosol chemistry, cloud processing, and size-dependent dry and wet deposition. Aerosol components considered are sulfate (SO4), ammonium (NH4), nitrate (NO3), black carbon (BC), particulate organic matter (POM), sea salt, mineral dust, and aerosol liquid water. The model is numerically efficient enough to allow long term simulations, which is an essential requirement for application in general circulation models. In order to evaluate the results obtained with this new model system, calculated mass concentrations, particle number concentrations, and size distributions are compared to observations. The intercomparison shows, that ECHAM4/MADE is able to reproduce the major features of the geographical patterns, seasonal cycle, and vertical distributions of the basic aerosol parameters. In particular, the model performs well under polluted continental conditions in the northern hemispheric lower and middle troposphere. However, in comparatively clean remote areas, e.g. in the upper troposphere or in the southern hemispheric marine boundary layer, the current model version tends to underestimate particle number concentrations
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