11,420 research outputs found
Reducing the number of time delays in coupled dynamical systems
When several dynamical systems interact, the transmission of the information
between them necessarily implies a time delay. When the time delay is not
negligible, the study of the dynamics of these interactions deserve a special
treatment. We will show here that under certain assumptions, it is possible to
set to zero a significant amount of time-delayed connections without altering
the global dynamics. We will focus on graphs of interactions with identical
time delays and bidirectional connections. With these premises, it is possible
to find a configuration where a number of time delays have been removed
with , where is the number of dynamical
systems on a connected graph
Distribution functions for a family of axially symmetric galaxy models
We present the derivation of distribution functions for the first four
members of a family of disks, previously obtained in (MNRAS, 371, 1873, 2006),
which represent a family of axially symmetric galaxy models with finite radius
and well behaved surface mass density. In order to do this we employ several
approaches that have been developed starting from the potential-density pair
and, essentially using the method introduced by Kalnajs (Ap. J., 205, 751,
1976) we obtain some distribution functions that depend on the Jacobi integral.
Now, as this method demands that the mass density can be properly expressed as
a function of the gravitational potential, we can do this only for the first
four discs of the family. We also find another kind of distribution functions
by starting with the even part of the previous distribution functions and using
the maximum entropy principle in order to find the odd part and so a new
distribution function, as it was pointed out by Dejonghe (Phys. Rep., 133, 217,
1986). The result is a wide variety of equilibrium states corresponding to
several self-consistent finite flat galaxy models.Comment: 12 pages, 7 figures, updated version, accepted for publication in
Rev. Acad. Colomb. Cienc. Ex. Fis. Na
Testing the performance and accuracy of the RELXILL model for the relativistic X-ray reflection from accretion disks
The reflection spectroscopic model RELXILL is commonly implemented in
studying relativistic X-ray reflection from accretion disks around black holes.
We present a systematic study of the model's capability to constrain the
dimensionless spin and ionization parameters from 6,000 NuSTAR
simulations of a bright X-ray source employing the lamppost geometry. We employ
high count spectra to show the limitations in the model without being confused
with limitations in signal-to-noise. We find that both parameters are
well-recovered at 90% confidence with improving constraints at higher
reflection fraction, high spin, and low source height. We test spectra across a
broad range - first at 1010 and then 10 total source counts
across the effective 3-79 keV band of NuSTAR, and discover a strong dependence
of the results on how fits are performed around the starting parameters, owing
to the complexity of the model itself. A blind fit chosen over an approach that
carries some estimates of the actual parameter values can lead to significantly
worse recovery of model parameters. We further stress on the importance to span
the space of nonlinear-behaving parameters like carefully and
thoroughly for the model to avoid misleading results. In light of selecting
fitting procedures, we recall the necessity to pay attention to the choice of
data binning and fit statistics used to test the goodness of fit by
demonstrating the effect on the photon index . We re-emphasize and
implore the need to account for the detector resolution while binning X-ray
data and using Poisson fit statistics instead while analyzing Poissonian data.Comment: 9 pages, 6 figures, accepted for publication in Ap
Enhancing the Radiative Rate in III-V Semiconductor Plasmonic Core-Shell Nanowire Resonators
We investigate the radiative properties of plasmonic core-shell nanowire resonators and, using boundary element method calculations,
demonstrate enhanced radiative decay rate by up to 3500 times in
nanoscale compound semi-conductor/metal cavities. Calculation of the
local density of optical states enables identification of new types of
modes in cavities with mode volumes on the order of
10^(-4)(λ/n)^3. These modes dramatically enhance the radiative
decay rate and significantly modify the polarization of far-field
emission
PT-symmetry broken by point-group symmetry
We discuss a PT-symmetric Hamiltonian with complex eigenvalues. It is based
on the dimensionless Schr\"{o}dinger equation for a particle in a square box
with the PT-symmetric potential . Perturbation theory clearly
shows that some of the eigenvalues are complex for sufficiently small values of
. Point-group symmetry proves useful to guess if some of the eigenvalues
may already be complex for all values of the coupling constant. We confirm
those conclusions by means of an accurate numerical calculation based on the
diagonalization method. On the other hand, the Schr\"odinger equation with the
potential exhibits real eigenvalues for sufficiently small
values of . Point group symmetry suggests that PT-symmetry may be broken
in the former case and unbroken in the latter one
X-ray Reflection Spectroscopy of the Black Hole GX 339-4: Exploring the Hard State with Unprecedented Sensitivity
We analyze {\it simultaneously} six composite {\it RXTE} spectra of GX 339--4
in the hard state comprising 77 million counts collected over 196 ks. The
source spectra are ordered by luminosity and spanthe range 1.6\% to 17\% of the
Eddington luminosity. Crucially, using our new tool {\tt pcacorr}, we
re-calibrate the data to a precision of 0.1\%, an order of magnitude
improvement over all earlier work. Using our advanced reflection model {\tt
relxill}, we target the strong features in the component of emission reflected
from the disk, namely, the relativistically-broadened Fe K emission line, the
Fe K edge and the Compton hump. We report results for two joint fits to the six
spectra: For the first fit, we fix the spin parameter to its maximal value
() and allow the inner disk radius to vary. Results
include (i) precise measurements of , with evidence that the disk
becomes slightly truncated at a few percent of Eddington; and (ii) an
order-of-magnitude swing with luminosity in the high energy cutoff, which
reaches keV at our lowest luminosity. For the second fit, we make the
standard assumption in estimating spin that the inner edge of the accretion
disk is located at the innermost stable circular orbit () and find (90\% confidence,
statistical). For both fits, and at the same level of statistical confidence,
we estimate that the disk inclination is deg and that the Fe
abundance is super-solar, .Comment: Accepted for publication in ApJ, 20 pages, 13 figure
A Practical Environment to Apply Model-Driven Web Engineering
The application of a model-driven paradigm in the development of Web Systems has yielded very good research
results. Several research groups are defining metamodels, transformations, and tools which offer a suitable environment,
known as model-driven Web engineering (MDWE). However, there are very few practical experiences in real
Web system developments using real development teams. This chapter presents a practical environment of MDWE
based on the use of NDT (navigational development techniques) and Java Web systems, and it provides a practical
evaluation of its application within a real project: specialized Diraya.Ministerio de Educación y Ciencia TIN2007-67843-C06-03Ministerio de Educación y Ciencia TIN2007-30391-
1020 steel coated with Ti/TiN by Cathodic Arc and Ion Implantation
TiN coatings have been widely studied in order to improve mechanical properties of steels. In this work, thin Ti/TiN films were prepared by plasma based immersion ion implantation and deposition (PBII&D) with a cathodic arc on AISI 1020 steel substrates. Substrates were exposed to the discharge during 1 min in vacuum for the deposition of a Tiunderlayer with the aim of improving the adhesion to the substrate. Then, a TiN layer was deposited during 6 min in a nitrogen environment at a pressure of 3xl0-4 mbar. Samples were obtained at room temperature and at 300 °C, and with or without ion implantation in order to analyze differences between the effects of each treatment on the tribological properties. The mechanical and tribological properties of the films were characterized. The coatings deposited by PBII&D at 300 °C presented the highest hardness and young modulus, the best wear resistance and corrosion performance.Fil: Bermeo, Diego Fernando. Universidad Santiago de Cali; ColombiaFil: Quintana, Juan Pablo. Consejo Nacional de Investigaciones CientÃficas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Instituto de FÃsica del Plasma. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Instituto de FÃsica del Plasma; ArgentinaFil: Kleiman, Ariel Javier. Consejo Nacional de Investigaciones CientÃficas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Instituto de FÃsica del Plasma. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Instituto de FÃsica del Plasma; ArgentinaFil: Sequeda, F.. Universidad del Valle; ColombiaFil: Márquez, A.. Consejo Nacional de Investigaciones CientÃficas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Instituto de FÃsica del Plasma. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Instituto de FÃsica del Plasma; Argentin
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