3,495 research outputs found
NASA refan program status
The objective of the refan program is to demonstrate the technical feasibility of substantially reducing the noise levels of existing JT8D powered aircraft. The program consists of the design, manufacturing and testing of the refan engines and modified nacelles and airplanes. Experimental testing was completed for the refan engine both at sea level and at altitude conditions. Ground testing for the B727 side- and center-engine installations and flight testing of the DC-9 with refan engines and acoustic nacelles were performed. Preliminary results presented show that substantial noise reductions were achieved
Closed String Thermal Torus From Thermo Field Dynamics
In this Letter a topological interpretation for the string thermal vacuum in
the Thermo Field Dynamics (TFD) approach is given. As a consequence, the
relationship between the Imaginary Time and TFD formalisms is achieved when
both are used to study closed strings at finite temperature. The TFD approach
starts by duplicating the system's degrees of freedom, defining an auxiliary
(tilde) string. In order to lead the system to finite temperature a Bogoliubov
transformation is implemented. We show that the effect of this transformation
is to glue together the string and the tilde string to obtain a torus. The
thermal vacuum appears as the boundary state for this identification. Also,
from the thermal state condition, a Kubo-Martin-Schwinger condition for the
torus topology is derived.Comment: 14 pages, revtex4, changes in the text and references. Version to be
published in Physics Letters
Current Algebra of Super WZNW Models
We derive the current algebra of supersymmetric principal chiral models with
a Wess-Zumino term. At the critical point one obtains two commuting super
Kac-Moody algebra as expected, but in general there are intertwining fields
connecting both right and left sectors, analogously to the bosonic case.
Moreover, in the present supersymmetric extension we have a quadratic algebra,
rather than an affine Lie algebra, due to the mixing between bosonic and
fermionic fields since the purely fermionic sector displays a Lie algebra as
well.Comment: 13 page
Plane waves in noncommutative fluids
We study the dynamics of the noncommutative fuid in the Snyder space
perturbatively at the first order in powers of the noncommutative parameter.
The linearized noncommutative fluid dynamics is described by a system of
coupled linear partial differential equations in which the variables are the
fluid density and the fluid potentials. We show that these equations admit a
set of solutions that are monocromatic plane waves for the fluid density and
two of the potentials and a linear function for the third potential. The
energy-momentum tensor of the plane waves is calculated.Comment: 11 pages. Version published as a Lette
Scalar field evolution in Gauss-Bonnet black holes
It is presented a thorough analysis of scalar perturbations in the background
of Gauss-Bonnet, Gauss-Bonnet-de Sitter and Gauss-Bonnet-anti-de Sitter black
hole spacetimes. The perturbations are considered both in frequency and time
domain. The dependence of the scalar field evolution on the values of the
cosmological constant and the Gauss-Bonnet coupling is
investigated. For Gauss-Bonnet and Gauss-Bonnet-de Sitter black holes, at
asymptotically late times either power-law or exponential tails dominate, while
for Gauss-Bonnet-anti-de Sitter black hole, the quasinormal modes govern the
scalar field decay at all times. The power-law tails at asymptotically late
times for odd-dimensional Gauss-Bonnet black holes does not depend on ,
even though the black hole metric contains as a new parameter. The
corrections to quasinormal spectrum due to Gauss-Bonnet coupling is not small
and should not be neglected. For the limit of near extremal value of the
(positive) cosmological constant and pure de Sitter and anti-de Sitter modes in
Gauss-Bonnet gravity we have found analytical expressions.Comment: 10 pages, to be published in Phys. Rev.
Darth Fader: Using wavelets to obtain accurate redshifts of spectra at very low signal-to-noise
We present the DARTH FADER algorithm, a new wavelet-based method for
estimating redshifts of galaxy spectra in spectral surveys that is particularly
adept in the very low SNR regime. We use a standard cross-correlation method to
estimate the redshifts of galaxies, using a template set built using a PCA
analysis on a set of simulated, noise-free spectra. Darth Fader employs wavelet
filtering to both estimate the continuum & to extract prominent line features
in each galaxy spectrum. A simple selection criterion based on the number of
features present in the spectrum is then used to clean the catalogue: galaxies
with fewer than six total features are removed as we are unlikely to obtain a
reliable redshift estimate. Applying our wavelet-based cleaning algorithm to a
simulated testing set, we successfully build a clean catalogue including
extremely low signal-to-noise data (SNR=2.0), for which we are able to obtain a
5.1% catastrophic failure rate in the redshift estimates (compared with 34.5%
prior to cleaning). We also show that for a catalogue with uniformly mixed SNRs
between 1.0 & 20.0, with realistic pixel-dependent noise, it is possible to
obtain redshifts with a catastrophic failure rate of 3.3% after cleaning (as
compared to 22.7% before cleaning). Whilst we do not test this algorithm
exhaustively on real data, we present a proof of concept of the applicability
of this method to real data, showing that the wavelet filtering techniques
perform well when applied to some typical spectra from the SDSS archive. The
Darth Fader algorithm provides a robust method for extracting spectral features
from very noisy spectra. The resulting clean catalogue gives an extremely low
rate of catastrophic failures, even when the spectra have a very low SNR. For
very large sky surveys, this technique may offer a significant boost in the
number of faint galaxies with accurately determined redshifts.Comment: 22 pages, 15 figures. Accepted for publication in Astronomy &
Astrophysic
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