54,685 research outputs found
Tunable waveguide lattices with non-uniform parity-symmetric tunneling
We investigate the single-particle time evolution and two-particle quantum
correlations in a one-dimensional -site lattice with a site-dependent
nearest neighbor tunneling function . Since
the bandwidth and the energy levels spacings for such a lattice both depend
upon , we show that the observable properties of a wavepacket, such as
its spread and the relative phases of its constitutents, vary dramatically as
is varied from positive to negative values. We also find that the
quantum correlations are exquisitely sensitive to the form of the tunneling
function. Our results suggest that arrays of waveguides with position-dependent
evanascent couplings will show rich dynamics with no counterpart in
present-day, traditional systems.Comment: 5 pages, 4 figure
An Update on the 0Z Project
We give an update on our 0Z Survey to find more extremely metal poor (EMP)
stars with [Fe/H] < -3 dex through mining the database of the Hamburg/ESO
Survey. We present the most extreme such stars we have found from ~1550
moderate resolution follow up spectra. One of these, HE1424-0241, has highly
anomalous abundance ratios not seen in any previously known halo giant, with
very deficient Si, moderately deficient Ca and Ti, highly enhanced Mn and Co,
and low C, all with respect to Fe. We suggest a SNII where the nucleosynthetic
yield for explosive alpha-burning nuclei was very low compared to that for the
hydrostatic alpha-burning element Mg, which is normal in this star relative to
Fe. A second, less extreme, outlier star with high [Sc/Fe] has also been found.
We examine the extremely metal-poor tail of the HES metallicity distribution
function (MDF). We suggest on the basis of comparison of our high resolution
detailed abundance analyses with [Fe/H](HES) for stars in our sample that the
MDF inferred from follow up spectra of the HES sample of candidate EMP stars is
heavily contaminated for [Fe/H](HES) < -3 dex; many of the supposed EMP stars
below that metallicity are of substantially higher Fe-metallicity, including
most of the very C-rich stars, or are spurious objects.Comment: to appear in conference proceedings "First Stars III", ed. B. O'Shea,
A. Heger & T.Abel, 4 pages, 2 figure
Consistency of pacing and metabolic responses during 2000-m rowing ergometry
PURPOSE: This study investigated the pacing strategy adopted and the consistency of performance and related physiological parameters across three 2000-m rowing-ergometer tests.
METHODS: Fourteen male well-trained rowers took part in the study. Each participant performed three 2000-m rowing-ergometer tests interspersed by 3-7 d. Throughout the trials, respiratory exchange and heart rate were recorded and power output and stroke rate were analyzed over each 500 m of the test. At the completion of the trial, assessments of blood lactate and rating of perceived exertion were measured.
RESULTS: Ergometer performance was unchanged across the 3 trials; however, pacing strategy changed from trial 1, which featured a higher starting power output and more progressive decrease in power, to trials 2 and 3, which were characterized by a more conservative start and an end spurt with increased power output during the final 500 m. Mean typical error (TE; %) across the three 2000-m trials was 2.4%, and variability was low to moderate for all assessed physiological variables (TE range = 1.4-5.1%) with the exception of peak lactate (TE = 11.5%).
CONCLUSIONS: Performance and physiological responses during 2000-m rowing ergometry were found to be consistent over 3 trials. The variations observed in pacing strategy between trial 1 and trials 2 and 3 suggest that a habituation trial is required before an intervention study and that participants move from a positive to a reverse-J-shaped strategy, which may partly explain conflicting reports in the pacing strategy exhibited during 2000-m rowing-ergometer trials
Gamma-ray Burst Afterglow with Continuous Energy Injection: Signature of a Highly-Magnetized Millisecond Pulsar
We investigate the consequences of a continuously injecting central engine on
the gamma-ray burst afterglow emission, focusing more specifically on a
highly-magnetized millisecond pulsar engine. For initial pulsar parameters
within a certain region of the parameter space, the afterglow lightcurves are
predicted to show a distinctive achromatic bump feature, the onset and duration
of which range from minutes to months, depending on the pulsar and the fireball
parameters. The detection of or upper limits on such features would provide
constraints on the burst progenitor and on magnetar-like central engine models.
An achromatic bump such as that in GRB 000301C afterglow may be caused by a
millisecond pulsar with P0=3.4 millisecond and Bp=2.7e14 Gauss.Comment: 5 pages, emulateapj style, to appear in ApJ Letters, updated with the
accepted version, a few corrections are mad
Radiation spectra and polarization in magnetar bursts
We present Monte Carlo simulations of radiative transfer in magnetar
atmospheres. We include the effects of vacuum polarization, electron and proton
scattering, and free-free absorption. Simulations are performed for the
atmosphere model with the magnetic field perpendicular and also tilted with
respect to the neutron star surface, and we show that the average spectrum does
not strongly depend on the orientation of the magnetic field. We investigate
the region of the parameter space where the vacuum absorption-like feature
appears in the spectrum and we analyze the shape of the proton cyclotron line.
Our results indicate that the existence of the vacuum polarization feature
should be a general attribute of soft gamma-ray repeaters burst spectra,
provided that the energy release takes place at the sufficiently dense region,
and the atmosphere scaleheight is large enough. We discuss the existence of
such a feature in recent observational data on these sources.Comment: submitted to Ap
Electrodynamics of Magnetars IV: Self-Consistent Model of the Inner Accelerator, with Implications for Pulsed Radio Emission
We consider the voltage structure in the open-field circuit and outer
magnetosphere of a magnetar. The standard polar-cap model for radio pulsars is
modified significantly when the polar magnetic field exceeds 1.8x10^{14} G.
Pairs are created by accelerated particles via resonant scattering of thermal
X-rays, followed by the nearly instantaneous conversion of the scattered photon
to a pair. A surface gap is then efficiently screened by e+- creation, which
regulates the voltage in the inner part of the circuit to ~10^9 V. We also
examine the electrostatic gap structure that can form when the magnetic field
is somewhat weaker, and deduce a voltage 10-30 times larger over a range of
surface temperatures. We examine carefully how the flow of charge back to the
star above the gap depends on the magnitude of the current that is extracted
from the surface of the star, on the curvature of the magnetic field lines, and
on resonant drag. The rates of different channels of pair creation are
determined self-consistently, including the non-resonant scattering of X-rays,
and collisions between gamma rays and X-rays. We find that the electrostatic
gap solution has too small a voltage to sustain the observed pulsed radio
output of magnetars unless i) the magnetic axis is nearly aligned with the
rotation axis and the light of sight; or ii) the gap is present on the closed
as well as the open magnetic field lines. Several properties of the radio
magnetars -- their rapid variability, broad pulses, and unusually hard radio
spectra -- are consistent with a third possibility, that the current in the
outer magnetosphere is strongly variable, and a very high rate of pair creation
is sustained by a turbulent cascade.Comment: 32 pages, submitted to the Astrophysical Journa
A simple and efficient numerical scheme to integrate non-local potentials
As nuclear wave functions have to obey the Pauli principle, potentials issued
from reaction theory or Hartree-Fock formalism using finite-range interactions
contain a non-local part. Written in coordinate space representation, the
Schrodinger equation becomes integro-differential, which is difficult to solve,
contrary to the case of local potentials, where it is an ordinary differential
equation. A simple and powerful method has been proposed several years ago,
with the trivially equivalent potential method, where non-local potential is
replaced by an equivalent local potential, which is state-dependent and has to
be determined iteratively. Its main disadvantage, however, is the appearance of
divergences in potentials if the wave functions have nodes, which is generally
the case. We will show that divergences can be removed by a slight modification
of the trivially equivalent potential method, leading to a very simple, stable
and precise numerical technique to deal with non-local potentials. Examples
will be provided with the calculation of the Hartree-Fock potential and
associated wave functions of 16O using the finite-range N3LO realistic
interaction.Comment: 8 pages, 2 figures, submitted to Eur. Phys. J.
Risk factors associated with Rift Valley fever epidemics in South Africa in 2008-11.
Rift Valley fever (RVF) is a zoonotic and vector-borne disease, mainly present in Africa, which represents a threat to human health, animal health and production. South Africa has experienced three major RVF epidemics (1950-51, 1973-75 and 2008-11). Due to data scarcity, no previous study has quantified risk factors associated with RVF epidemics in animals in South Africa. Using the 2008-11 epidemic datasets, a retrospective longitudinal study was conducted to identify and quantify spatial and temporal environmental factors associated with RVF incidence. Cox regressions with a Besag model to account for the spatial effects were fitted to the data. Coefficients were estimated by Bayesian inference using integrated nested Laplace approximation. An increase in vegetation density was the most important risk factor until 2010. In 2010, increased temperature was the major risk factor. In 2011, after the large 2010 epidemic wave, these associations were reversed, potentially confounded by immunity in animals, probably resulting from earlier infection and vaccination. Both vegetation density and temperature should be considered together in the development of risk management strategies. However, the crucial need for improved access to data on population at risk, animal movements and vaccine use is highlighted to improve model predictions
- …