664 research outputs found
On the Normalization of the QSO's Lyman alpha Forest Power Spectrum
The calculation of the transmission power spectrum of QSO's Lyman alpha
absorption requires two parameters for the normalization: the continuum Fc and
mean transmission, i.e. average of e^{-tau}. Traditionally, the continuum is
obtained by a polynomial fitting truncating it at a lower order, and the mean
transmission is calculated over the entire wavelength range considered. The
flux F is then normalized by the average of Fc e^{-tau}. However, the
fluctuations in the transmitted flux are significantly correlated with the
local background flux on scales for which the field is intermittent. In this
paper, we develop a self-normalization algorithm of the transmission power
spectrum based on a multiresolution analysis. This self-normalized power
spectrum estimator needs neither a continuum fitting, nor pre-determining the
mean transmission. With simulated samples, we show that the self-normalization
algorithm can perfectly recover the transmission power spectrum from the flux
regardless of how the continuum varies with wavelength. We also show that the
self-normalized power spectrum is also properly normalized by the mean
transmission. Moreover, this power spectrum estimator is sensitive to the
non-linear behavior of the field. That is, the self-normalized power spectrum
estimator can distinguish between fields with or without the
fluctuation-background correlation. This cannot be accomplished by the power
spectrum with the normalization by an overall mean transmission. Therefore, the
self-normalized power spectrum would be useful for the discrimination among
models without the uncertainties caused by free (or fitting) parameters.Comment: 24 pages, 8 figures, to appear in ApJ tentatively in the Nov 1 2001
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Distinguishing f(R) theories from general relativity by gravitational lensing effect
The post-Newtonian formulation of a general class of f(R) theories is set up
to 3rd order approximation. It turns out that the information of a specific
form of f(R) gravity is encoded in the Yukawa potential, which is contained in
the perturbative expansion of the metric components. Although the Yukawa
potential is canceled in the 2nd order expression of the effective refraction
index of light, detailed analysis shows that the difference of the lensing
effect between the f(R) gravity and general relativity does appear at the 3rd
order when is larger than the distance to the
gravitational source. However, the difference between these two kinds of
theories will disappear in the axially symmetric spacetime region. Therefore
only in very rare case the f(R) theories are distinguishable from general
relativity by gravitational lensing effect at the 3rd order post-Newtonian
approximation.Comment: 14 page
Ly Leaks in the Absorption Spectra of High Redshift QSOs
Spectra of high redshift QSOs show deep Gunn-Peterson absorptions on the blue
sides of the \Lya emissions lines. They can be decomposed into components
called \Lya leaks, defined to be emissive regions in complementary to otherwise
zero-fluxed absorption gaps. Just like \Lya absorption forests at low
redshifts, \Lya leaks are both easy to find in observations and containing rich
sets of statistical properties that can be used to study the early evolution of
the IGM. Among all properties of a leak profile, we investigate its equivalent
width in this paper, since it is weakly affected by instrumental resolution and
noise. Using 10 Keck QSO spectra at , we have measured the number
density distribution function , defined to be the number of leaks per
equivalent width and per redshift , in the redshift range .
These new observational statistics, in both the differential and cumulative
forms, fit well to hydro numerical simulations of uniform ionizing background
in the CDM cosmology. In this model, Ly leaks are mainly due
to low density voids. It supports the early studies that the IGM at
would still be in a highly ionized state with neutral hydrogen fraction . Measurements of at would be effective to probe the
reionization of the IGM.Comment: 3 figs, accepted by ApJ
The Impact of Organizational Aspects on Successful Adoption of Decision Support System in Developing Countries: The Context of Libyan Higher Education Ministry
Decision Support Systems are the information application programs that analyze data and present it in a way that makes it easier to make acumen decisions dependent on the prevailing circumstance. This paper provides research grounds for implementing and designing the decision support systems. The paper generally talks about the adoption of decision support systems in developing countries. Most of the firms in the developing countries reveal an advanced use of computerized services. Most developing countries employ the use of DSS in the organizations not because of being competitive but because of the business requirements which are always immediate (Power 124). The organizations that have successfully employed the use of DSS have been seen to be successful. The researcher begins with a brief introduction and background of the study topic, continue with a section of data analyzing, exploration, reduction, factor analysis and discussion, provide numerous recommendations based on the information obtained and conclude the paper. Keywords: Decision Support Systems, Adoption of Decision Support Systems, Developing Countries, Factors Influencing the Decision Support System
Bifurcations and chaos in a gear assembly with clearances for solar array drive assembly
Solar array drive assembly is an important part of the spacecraft. It is used to rotate the solar panels. The gear assembly in solar array drive assembly plays a key role in transferring power safely. Nonlinear behavior of gear assembly, like the chaotic motion, can highly affect the stability and operating life of solar array drive assembly. Clearances in gear assembly which were neglected for simplification in past years have increased the risk of failure and become a problem in accurate control. To investigate the clearances effect on nonlinear behavior, this paper establishes a new dynamic model of the gear assembly with bilateral clearances. The main difference comparing to general spur gears is its unique hysteresis stiffness may also influence the clearance effects. Transformation of the hysteresis loop is observed from theoretical equations using different parameters. Bifurcations and chaotic analysis of the system are carried out by numerical simulations in this study. The results show that the variation of clearances may induce the chaotic behavior into gear transmission even when the primary response is stable. When the system step into the chaotic region, it has a high risk of unstable vibration and fuzzy output. The influence of excitation frequency on the chaotic motion of the system is also provided. Chaos thresholds are calculated to avoid nonlinear behavior of the system in design and control. This study makes it possible to predict the unstable clearance interval in this system and avoid the system stepping into chaotic motion. Analyzing and predicting the chaotic behaviors can contribute to the further studies on design and control of the solar array drive assembly
Two-way collinear interaction of longitudinal waves in an elastic medium with quadratic nonlinearity
A numerical implementation of two-way collinear interaction of nonlinear ultrasonic longitudinal waves in an elastic medium with quadratic nonlinearity is conducted in this work. A semi-discrete central scheme is used here to solve the numerical problem. The pulse-inversion technique is applied to accentuate the generated resonant waves and remove the fundamental components. The produced resonant waves can be clearly observed in the frequency domain. Variation trends of the resonant waves together with second harmonics along the propagation path are analyzed and results show that apart from the obvious growing of the transverse component with difference frequency, the longitudinal component and the resonant wave of sum frequency have notable responses as well. The spatial distribution of resonant waves will provide necessary information for the related experiments
Abundance and Clustering of C IV Absorption Systems in the SCDM, LCDM and CHDM Models
We have developed a method for calculating the two-point correlation function
of nonlinearly evolved mass and collapsed halos in the Press-Schechter
formalism. The nonlinear gravitational interaction is treated as the sum of
various individual spherical top-hat clustering. Because no collapsed halo of
mass M can exist in initial regions (or top-hat spheres) of mass less than M,
the bias that massive halos have stronger correlation than the background mass
can be naturally introduced. We apply this method to derive constraints on
popular dark-matter models from the spatial number density and the correlation
function of C IV absorption systems in QSO spectra. Considering C IV systems
should behosted by collapsed halos, one can obtain an upper limit to the
threshold mass of the collapsed halos by requiring their number density to be
larger than that of observed C IV systems. On the other hand, in order to
explain the observed clustering of C IV systems, a lower limit to the threshold
mass will be set for the hosting halos. The cold dark matter model, the Lamda
CDM model and the cold-plus-hot dark matter model are tested using the C IV
observations.Comment: 23 pages, LaTeX, plus 11 figures in the PS format to be published in
Ap
Dispersion and Property Manipulation of Carbon Nanotubes by Self-Assemibles of Amphiphilic Molecules
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