14,888 research outputs found
A Modified Version of the Waxman Algorithm
The iterative algorithm recently proposed by Waxman for solving eigenvalue
problems, which relies on the method of moments, has been modified to improve
its convergence considerably without sacrificing its benefits or elegance. The
suggested modification is based on methods to calculate low-lying eigenpairs of
large bounded hermitian operators or matrices
A New Constraint on the Escape Fraction in Distant Galaxies Using Gamma-ray Burst Afterglow Spectroscopy
We describe a new method to measure the escape fraction fesc of ionizing
radiation from distant star-forming galaxies using the afterglow spectra of
long-duration gamma-ray bursts (GRBs). Optical spectra of GRB afterglows allow
us to evaluate the optical depth of the host ISM, according to the neutral
hydrogen column density N(HI) observed along the sightlines toward the
star-forming regions where the GRBs are found. Different from previous effort
in searching for faint, transmitted Lyman continuum photons, our method is not
subject to background subtraction uncertainties and does not require prior
knowledge of either the spectral shape of the host galaxy population or the IGM
Lya forest absorption along these GRB sightlines. Because most GRBs occur in
sub-L_* galaxies, our study also offers the first constraint on fesc for
distant low-mass galaxies that dominate the cosmic luminosity density. We have
compiled a sample of 27 GRBs at redshift z>2 for which the underlying N(HI) in
the host ISM are known. These GRBs together offer a statistical sampling of the
integrated optical depth to ionizing photons along random sightlines from
star-forming regions in the host galaxies, and allow us to estimate the mean
escape fraction averaged over different viewing angles. We find
=0.02\pm 0.02 and place a 95% c.l. upper limit <= 0.075 for these
hosts. We discuss possible biases of our approach and implications of the
result. Finally, we propose to extend this technique for measuring at
z~0.2 using spectra of core-collapse supernovae.Comment: Five journal pages, including one figure; ApJL in pres
Class Attendance and Students’ Evaluations of Teaching: Do No-Shows Bias Course Ratings and Rankings?
Background: Many university departments use students’ evaluations of teaching (SET) to compare and rank courses. However, absenteeism from class is often nonrandom and, therefore, SET for different courses might not be comparable. Objective: The present study aims to answer two questions. Are SET positively biased due to absenteeism? Do procedures, which adjust for absenteeism, change course rankings? Research Design: The author discusses the problem from a missing data perspective and present empirical results from regression models to determine which factors are simultaneously associated with students’ class attendance and course ratings. In order to determine the extent of these biases, the author then corrects average ratings for students’ absenteeism and inspect changes in course rankings resulting from this adjustment. Subjects: The author analyzes SET data on the individual level. One or more course ratings are available for each student. Measures: Individual course ratings and absenteeism served as the key outcomes. Results: Absenteeism decreases with rising teaching quality. Furthermore, both factors are systematically related to student and course attributes. Weighting students’ ratings by actual absenteeism leads to mostly small changes in ranks, which follow a power law. Only a few, average courses are disproportionally influenced by the adjustment. Weighting by predicted absenteeism leads to very small changes in ranks. Again, average courses are more strongly affected than courses of very high or low in quality. Conclusions: No-shows bias course ratings and rankings. SET are more appropriate to identify high- and low-quality courses than to determine the exact ranks of average courses
A New Variable Modified Chaplygin Gas Model Interacting with Scalar Field
In this letter we present a new form of the well known Chaplygin gas model by
introducing inhomogeneity in the EOS. This model explains crossing.
Also we have given a graphical representation of the model using
parameters. We have also considered an interaction of this model with the
scalar field by introducing a phenomenological coupling function and have shown
that the potential decays with time.Comment: 7 pages, 3 figure
Asymptotic self-similarity breaking at late times in cosmology
We study the late time evolution of a class of exact anisotropic cosmological
solutions of Einstein's equations, namely spatially homogeneous cosmologies of
Bianchi type VII with a perfect fluid source. We show that, in contrast to
models of Bianchi type VII which are asymptotically self-similar at late
times, Bianchi VII models undergo a complicated type of self-similarity
breaking. This symmetry breaking affects the late time isotropization that
occurs in these models in a significant way: if the equation of state parameter
satisfies the models isotropize as regards the shear
but not as regards the Weyl curvature. Indeed these models exhibit a new
dynamical feature that we refer to as Weyl curvature dominance: the Weyl
curvature dominates the dynamics at late times. By viewing the evolution from a
dynamical systems perspective we show that, despite the special nature of the
class of models under consideration, this behaviour has implications for more
general models.Comment: 29 page
Resistance of a domain wall in the quasiclassical approach
Starting from a simple microscopic model, we have derived a kinetic equation
for the matrix distribution function. We employed this equation to calculate
the conductance in a mesoscopic F'/F/F' structure with a domain wall (DW).
In the limit of a small exchange energy and an abrupt DW, the conductance
of the structure is equal to . Assuming that the scattering times
for electrons with up and down spins are close to each other we show that the
account for a finite width of the DW leads to an increase in this conductance.
We have also calculated the spatial distribution of the electric field in the F
wire. In the opposite limit of large (adiabatic variation of the
magnetization in the DW) the conductance coincides in the main approximation
with the conductance of a single domain structure . The account for rotation of
the magnetization in the DW leads to a negative correction to this conductance.
Our results differ from the results in papers published earlier.Comment: 11 pages; replaced with revised versio
Analysis of the application of the optical method to the measurements of the water vapor content in the atmosphere - Part 1: Basic concepts of the measurement technique
We retrieved the total content of the atmospheric water vapor (or Integrated
Water Vapor, IWV) from extensive sets of photometric data obtained since 1995
at Lindenberg Meteorological Observatory with star and sun photometers.
Different methods of determination of the empirical parameters that are
necessary for the retrieval are discussed. The instruments were independently
calibrated using laboratory measurements made at Pulkovo Observatory with the
VKM-100 multi-pass vacuum cell. The empirical parameters were also calculated
by the simulation of the atmospheric absorption by water vapor, using the
MODRAN-4 program package for different model atmospheres. The results are
compared to those presented in the literature, obtained with different
instruments and methods of the retrieval. The reliability of the empirical
parameters, used for the power approximation that links the water vapor content
with the observed absorption, is analyzed. Currently, the total (from
measurements, calibration, and calculations) errors yield the standard
uncertainty of about 10% in the total column water vapor. We discuss the
possibilities for improving the accuracy of calibration to ~1% as indispensable
condition in order to make it possible to use data obtained by optical
photometry as an independent reference for other methods (GPS, MW-radiometers,
lidar, etc).Comment: 28 pages, 8 figures, 3 tables. In submitting to Atmospheric
Measurement Technique
Global dynamics of the mixmaster model
The asymptotic behaviour of vacuum Bianchi models of class A near the initial
singularity is studied, in an effort to confirm the standard picture arising
from heuristic and numerical approaches by mathematical proofs. It is shown
that for solutions of types other than VIII and IX the singularity is velocity
dominated and that the Kretschmann scalar is unbounded there, except in the
explicitly known cases where the spacetime can be smoothly extended through a
Cauchy horizon. For types VIII and IX it is shown that there are at most two
possibilities for the evolution. When the first possibility is realized, and if
the spacetime is not one of the explicitly known solutions which can be
smoothly extended through a Cauchy horizon, then there are infinitely many
oscillations near the singularity and the Kretschmann scalar is unbounded
there. The second possibility remains mysterious and it is left open whether it
ever occurs. It is also shown that any finite sequence of distinct points
generated by iterating the Belinskii-Khalatnikov-Lifschitz mapping can be
realized approximately by a solution of the vacuum Einstein equations of
Bianchi type IX.Comment: 16 page
Flux-Induced Vortex in Mesoscopic Superconducting Loops
We predict the existence of a quantum vortex for an unusual situation. We
study the order parameter in doubly connected superconducting samples embedded
in a uniform magnetic field. For samples with perfect cylindrical symmetry, the
order parameter has been known for long and no vortices are present in the
linear regime. However, if the sample is not symmetric, there exist ranges of
the field for which the order parameter vanishes along a line, parallel to the
field. In many respects, the behavior of this line is qualitatively different
from that of the vortices encountered in type II superconductivity. For samples
with mirror symmetry, this flux-induced vortex appears at the thin side for
small fluxes and at the opposite side for large fluxes. We propose direct and
indirect experimental methods which could test our predictions.Comment: 6 pages, Latex, 4 figs., uses RevTex, extended to situations far from
cylindrical symmetr
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