1,147 research outputs found
Numerical Simulations of Gamma-Ray Burst Explosions
Gamma-ray bursts are a complex, non-linear system that evolves very rapidly
through stages of vastly different conditions. They evolve from scales of few
hundred kilometers where they are very dense and hot to cold and tenuous on
scales of parsecs. As such, our understanding of such a phenomenon can truly
increase by combining theoretical and numerical studies adopting different
numerical techniques to face different problems and deal with diverse
conditions. In this review, we will describe the tremendous advancement in our
comprehension of the bursts phenomenology through numerical modeling. Though we
will discuss studies mainly based on jet dynamics across the progenitor star
and the interstellar medium, we will also touch upon other problems such as the
jet launching, its acceleration, and the radiation mechanisms. Finally, we will
describe how combining numerical results with observations from Swift and other
instruments resulted in true understanding of the bursts phenomenon and the
challenges still lying ahead.Comment: 14 Pages, Journal of High Energy Astrophysics for the dedicated
issue: "Swift: Ten Years of Discovery", in pres
Effects of a competitive period on the anthropometric profile of soccer referees
El objetivo de este estudio fue analizar el efecto de un periodo competitivo de 10 semanas sobre las caracterĂsticas antropomĂ©tricas, la composiciĂłn corporal y el somatotipo de ĂĄrbitros de fĂștbol. 14 ĂĄrbitros de fĂștbol (28,8 ± 5,1 años) de distintas categorĂas nacionales de fĂștbol de España participaron en este estudio. Se observĂł un descenso significativo tras este periodo competitivo en el sumatorio de ocho pliegues (Î = -6,07%, p < 0,05, d = 0,38, bajo). AdemĂĄs, se observĂł un descenso significativo en el porcentaje de masa adiposa (Î= -2,29%, p < 0,05, d = 0,19, trivial) y en el componente endomorfo (Î= -6,82%, p < 0,05, d = 0,32, bajo) en el postest. Un periodo competitivo de 10 semanas parece ser
suficiente como para provocar modificaciones en el somatotipo y un descenso del sumatorio de pliegues y de la masa adiposa en ĂĄrbitrosThe aim of this study was to analyze the effect of a 10-week competitive period on body composition, anthropometric characteristics and somatotype in soccer referees. Fourteen officials (28.8 ± 5.1 yr) from different national soccer categories of Spain took part in the study. A decrease in the sum of eight skinfold thicknesses (Î = -6.07%, p < 0.05, d = 0.38, low) was observed after the competitive period. Moreover, decreases in adipose mass (Î= -2.29%, p < 0.05, d = 0.19, trivial) and endomorphic component were also observed. A 10-week competitive period has demonstrated to decrease both skinfold thicknesses and adipose mass, changing the somatotype of the referee
Group delay in THz spectroscopy with ultra-wideband log-spiral antennae
We report on the group delay observed in continuous-wave terahertz
spectroscopy based on photomixing with phase-sensitive homodyne detection. We
discuss the different contributions of the experimental setup to the phase
difference \Delta\phi(\nu) between transmitter arm and receiver arm. A simple
model based on three contributions yields a quantitative description of the
overall behavior of \Delta\phi(\nu). Firstly, the optical path-length
difference gives rise to a term linear in frequency. Secondly, the
ultra-wideband log-spiral antennae effectively radiate and receive in a
frequency-dependent active region, which in the most simple model is an annular
area with a circumference equal to the wavelength. The corresponding term
changes by roughly 6 pi between 100 GHz and 1 THz. The third contribution stems
from the photomixer impedance. In contrast, the derivative (d\Delta\phi / d\nu)
is dominated by the contribution of periodic modulations of \Delta\phi(\nu)
caused by standing waves, e.g., in the photomixers' Si lenses. Furthermore, we
discuss the Fourier-transformed spectra, which are equivalent to the waveform
in a time-domain experiment. In the time domain, the group delay introduced by
the log-spiral antennae gives rise to strongly chirped signals, in which low
frequencies are delayed. Correcting for the contributions of antennae and
photomixers yields sharp peaks or "pulses" and thus facilitates a
time-domain-like analysis of our continuous-wave data.Comment: 7 pages, 7 figure
Relationship between the cardiovascular capacity and repeated sprints ability in high-standard soccer referees
Los objetivos de este estudio fueron por un lado, describir el rendimiento en una prueba de rendimiento cardiovascular realizada en laboratorio y en otra de esprints repetidos (RSA) en ĂĄrbitros de fĂștbol de alto nivel, y por otro lado, analizar la asociaciĂłn entre los resultados obtenidos en ambas pruebas. Doce ĂĄrbitros de fĂștbol de alto nivel (Divisiones 1ÂȘ, 2ÂȘ y 2ÂȘ B, 28,8±5,1 años, 73,2±6,6 kg, 1,8±0,1 m 22,82±1,38 kg·m-2) participaron en el estudio. Los ĂĄrbitros obtuvieron un consumo mĂĄximo de oxĂgeno (VO2max) de 59,50±4,56ml·kg-1·min-1. El tiempo medio de los esprints repetidos (RSAMedia) en 15 y 30 m fue de 2,54±0,09 s y 4,43±0,14 s, respectivamente, tardando 15,26±0,53 s y 26,60±0,82 s en completar los seis esprints (RSATotal). Los Ăndices de fatiga en 15 y 30 m fueron de 2,25±1,05% y 3,33±1,59% (RSASdec) y de 3,45±2,92% y 6,31±4,63% (RSACambio). Se obtuvieron asociaciones positivas entre distintas variables cardiovasculares y de tiempo en RSA. Por el contrario, el consumo de oxĂgeno en las intensidades de umbral ventilarorio 1 (VT1) y umbral ventilatorio 2 (VT2) correlacionĂł negativamente con el Ăndice de fatiga RSASdec. Una mayor capacidad cardiovascular en ĂĄrbitros de fĂștbol, a pesar de poder producir un efecto negativo sobre el tiempo empleado en el RSA, puede minimizar la fatiga durante los esprints repetidos
Electric and magnetic dipolar response of Germanium spheres: Interference effects, scattering anisotropy and optical forces
We address the scattering cross sections, and their consequences, for
submicrometer Germanium spheres. It is shown that there is a wide window in the
near infrared where light scattering by these particles is fully described by
their induced electric and magnetic dipoles. In this way, we observe remarkable
anisotropic scattering angular distributions, as well as zero forward or
backward scattered intensities, which until recently was theoretically
demonstrated only for hypothetically postulated magnetodielectric spheres.
Also, interesting new effects of the optical forces exerted on these objects
are now obtained.Comment: 13 pages, 4 figure
Exception for the zero-forward-scattering theory
Studies on single scattering of electromagnetic waves by magnetic particles were reported in the 1980s by Kerker et al. [J. Opt. Soc. Am. 73, 765 (1983)] . They obtained that very small spherical particles with electric permittivity and magnetic permeability values such that Δ=(4âÎŒ)/(2ÎŒ+1) do not produce forward scattering. We show here that this condition contains an interesting exception at ( Δ=â2 , ÎŒ=â2 ) when electric and magnetic resonances are present and around which the scattered field distribution is computed and described showing a polarization-insensitive behavior at the point ( Δ=â2 , ÎŒ=â2 )
Physical Education Classes as a Precursor to the Mediterranean Diet and the Practice of Physical Activity
Physical activity and a healthy, balanced diet are remaining unresolved issues among young people. According to the World Health Organization, young people do not get enough exercise during the week, and physical education classes are the best way to promote healthy habits. This study aims to analyze how the role of the teacher influences the frustration of psychological needs, coping strategies, motivation, and the adoption of healthy eating habits through the Mediterranean diet and the regular practice of physical activity. The study involved 1031 boys and 910 girls between the ages of 13 and 18. To explain the relationships between the different variables included in this study, a model of structural equations has been developed. The results showed that autonomy support negatively predicted the frustration of four psychological needs. The failure to meet four psychological needs negatively predicted resilience. Likewise, resilience positively predicted autonomous motivation, and this positively predicted the Mediterranean diet and the practice of physical activity. Thus, the results obtained in the present study are in line with those of various studies wherein physical education classes were seen to help consolidate healthy living habits
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