12,211 research outputs found
Method to Look for Imprints of Ultrahigh Energy Nuclei Sources
We propose a new method to search for heavy nuclei sources, on top of
background, in the Ultra-High Energy Cosmic Ray data. We apply this method to
the 69 events recently published by the Pierre Auger Collaboration and find a
tail of events for which it reconstructs the source at a few degrees from the
Virgo galaxy cluster. The reconstructed source is located at ~ 8.5 degrees from
M87. The probability to have such a cluster of events in some random background
and reconstruct the source position in any direction of the sky is about 7 x
10^(-3). The probability to reconstruct the source at less than 10 degrees from
M87 in a data set already containing such a cluster of events is about 4 x
10^(-3). This may be a hint at the Virgo cluster as a bright ultra-high energy
nuclei source. We investigate the ability of current and future experiments to
validate or rule out this possibility, and discuss several alternative
solutions which could explain the existing anisotropy in the Auger data.Comment: 12 pages (2 columns), 10 figures. Published in Physical Review
Preliminary Abundance Analysis of Galactic Bulge Main Sequence, Subgiant, and Giant Branch Stars Observed During Microlensing with Keck/HIRES
We present an abundance analysis of six main sequence turnoff, subgiant, and
giant branch stars toward the Galactic bulge that were observed with Keck/HIRES
during microlensing events. This is an early look at the first detailed
chemical analysis of main sequence stars in the Galactic bulge. Lensing events
allow the effective aperture of Keck to be increased beyond its current
dimensions; although, some events still stretched its spectroscopic
capabilities. Future large telescopes with high resolution and high throughput
spectrometers will allow the study of abundances in distant stellar populations
and in less evolved stars with greater ease.Comment: 8 pages including 2 figures. To appear in SPIE proceedings on
Astronomical Telescopes and Instrumentation. Uses spie.cl
Connected array antennas : analysis and design
Many of today's satellite communication and radar systems necessitate phased array antennas that are capable of wideband/multi-band operation and good polarization purity over a wide scan volume. However, the antenna solutions typically used for wideband wide-scan applications trade-off matching performance against polarization purity. For this reason, in recent years, a new approach has arisen for the design of broadband arrays, aiming at reducing cross polarization. This antenna solution consists of arrays of long dipoles or slots periodically fed, and are referred to as connected arrays of slot or dipoles. Connected array antennas represent one of the most promising concepts in the field of very wideband arrays, for being able to achieve both broad band and low cross polarization. The wideband performance is due to the fact that the connections between neighboring elements allow currents to remain nearly constant with frequency. Another attractive feature of connected arrays is their capability to achieve good polarization purity, in virtue of the planarity of the radiators. Besides the advantageous physical properties, connected arrays are based on simple geometries that lead to the derivation of analytical solutions for the antenna parameters. Closed-form expressions based on a spectral Green's function representation are derived for the input impedance, the current distribution over the array and the radiation patterns. Important advantages result from this representation with respect to numerical solutions: above all, the reduction of computational costs and the gain in physical insight on the wave phenomena. A convenient circuit representation of the array unit cell is derived. The circuit describes rigorously and analytically the transition between free-space radiation and guiding transmission line. Contrarily to standard Thévenin circuit for receiving antennas, this representation can be used to evaluate the power scattered by the antenna. The results have been applied to the analysis of the scattering and absorption of a real connected-dipole prototype array backed by a frequency selective ground plane. Good agreement was achieved between measurements and results from the equivalent network. A novel measurement technique based on passive RCS measurements in the main planes was used to characterize the active matching of the radiating part of the antenna in transmission. Finiteness effects can be particularly severe in connected arrays, due to electrical connection and the high mutual coupling between the elements. As a consequence, the overall behavior of a finite wideband array can be sensibly different with respect to infinite array analysis. Thus, it is crucial to include edge effects already in the preliminary assessment of the array performance. An efficient numerical procedure is derived for the characterization of the edge effects. The method requires only one unknown per elementary cell, independently from the cell geometrical parameters. This is possible thanks to the use of an appropriate connected array Green's function in the integral equation. This procedure is of general applicability and can be used for arrays with and without backing reflectors and for arbitrary scan angle. An alternative analytical representation is also derived to provide physical insight on the nature of the edge-waves. The analytical approximation of the spatial current distribution on the finite array is derived, for the specific case of a connected array of dipoles operating in free space, and scanning only in the E-plane. The key step is to represent the total current as sum of the infinite array contribution plus edge-born waves. The final analytical expression is given in terms of Fresnel functions and allows qualitative considerations on the nature of the electric current distribution, in terms of spreading and attenuation. The analytical expressions represent a powerful tool that can be used both for modeling and design. A connected array of dipoles with 40% bandwidth, when scanning in elevation to 45o, has been designed. Practical designs require the implementation of ad hoc feed structures that avoid common-mode currents to propagate on the feed lines. This problem has been addressed and feed structures that perform common-mode rejection have been designed. Measurements form a prototype demonstrator were presented for validation and showed good performance
A Compact Fireball Model of Gamma Ray Bursts
It is proposed that the gamma ray burst photons near the peak of the spectrum
at several hundred KeV are produced on very compact scales, where photon
production is limited by blackbody effects and/or the requirement of energetic
quanta () for efficient further production. The fast variation of
order milliseconds in the time profile is then a natural expectation, given the
other observed GRB parameters. Analytic calculations are presented to show that
the escape of non-thermal, energetic gamma rays can emerge within a second of
the thermal photons from a gammasphere of below cm. The minimum
asymptotic bulk Lorentz factor in this model is found to be of order several
hundred if the photosphere is of order cm and greater for
larger or smaller photospheric radii. It is suggested that prompt UHE gamma
rays might provide a new constraint on the asymptotic Lorentz factor of the
outflow.Comment: To appear in ApJ, revisions requested by the refere
Sudden Stops: Determinants and Output Effects in the First Era of Globalization, 1880-1913
Using a sample of 20 emerging countries from 1880 to 1913, we study the determinants and output effects of sudden stops in capital inflows during an era of intensified globalization. We find that higher levels of original sin (hard currency debt to total debt) and large current account deficits associated with reliance on foreign capital greatly increased the likelihood of experiencing a sudden stop. Trade openness and stronger commitment to the gold standard had the opposite effect. These results are robust for many sudden stop definitions used in the literature. Finally, we use a treatment effects model to show that after controlling for endogeneity sudden stops have a strong negative association with growth in per capita output. We also show that banking, currency and debt crises that were preceded by a sudden stop have much greater negative relation with growth than in the absence of a sudden stop.
Preliminary design issues for inertial rings in Ambient Assisted Living applications
A wearable 9dof inertial system able to measure hand posture and movement is presented. The design issues for the deployment of measurement instrumentation based on no-invasive ring-shaped inertial units and of a wireless sensor network by them composed are described. Compromises between the physical and functional proprieties of a wearable device and the requirements for the hardware development are discussed with attention to an handsome design concept aesthetically effective. Techniques of power saving based on an optimized firmware programming are mentioned to realize a performing battery powered system featured by an exhaustive operation time. The printed circuit board (PCB) design rules, the choice of the components and materials, the fusion of inertial data with optical sensors outcomes are also discussed. Previous experience in the field of wearable systems are mentioned in the presentation of the results that emphasize the functional and application potential of a 9dof inertial system integrated in a ring-shaped device. � 2015 IEEE
Common-mode resonances in ultra wide band connected arrays of dipoles : measurements from the demonstrator and exit strategy
A 6-9 GHz prototype array of dual-polarized connected dipoles has been manufactured. The array is composed by two orthogonal sets of 8×8 elements for each polarization (128), arranged in an egg-crate configuration. Preliminary measurements highlighted the presence of unpredicted common-mode resonances excited in the vertical feeding lines. An analysis of the common-mode excitation is carried out and exit strategies for a design of a resonance-free connected array are presented. © 2009 IEEE
PCB Slot Based Transformers to Avoid Common-Mode Resonances in Connected Arrays of Dipoles
Environmental/occupational exposure to radon and non-pulmonary neoplasm risk: A review of epidemiologic evidence
Although Radon (Rn) is a known agent for lung cancer, the link between Rn exposure and other non-pulmonary neoplasms remains unclear. The aim of this review is to investigate the role of Rn in the development of tumors other than lung cancer in both occupational and environmental exposure. Particularly, our attention has been focused on leukemia and tumors related to brain and central nervous system (CNS), skin, stomach, kidney, and breast. The epidemiologic literature has been systematically reviewed focusing on workers, general population, and pediatric population. A weak increase in leukemia risk due to Rn exposure was found, but bias and confounding factors cannot be ruled out. The results of studies conducted on stomach cancer are mixed, although with some prevalence for a positive association with Rn exposure. In the case of brain and CNS cancer and skin cancer, results are inconclusive, while no association was found for breast and kidney cancers. Overall, the available evidence does not support a conclusion that a causal association has been established between Rn exposure and the risk of other non-pulmonary neoplasms mainly due to the limited number and heterogeneity of existing studies. To confirm this result, a statistical analysis should be necessary, even if it is now not applicable for the few studies available
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