4,584 research outputs found
The Nature of the Hard-X-Ray Emitting Symbiotic Star RT Cru
We describe Chandra High-Energy Transmission Grating Spectrometer
observations of RT Cru, the first of a new sub-class of symbiotic stars that
appear to contain white dwarfs (WDs) capable of producing hard X-ray emission
out to greater than 50 keV. The production of such hard X-ray emission from the
objects in this sub-class (which also includes CD -57 3057, T CrB, and CH Cyg)
challenges our understanding of accreting WDs. We find that the 0.3 -- 8.0 keV
X-ray spectrum of RT Cru emanates from an isobaric cooling flow, as in the
optically thin accretion-disk boundary layers of some dwarf novae. The
parameters of the spectral fit confirm that the compact accretor is a WD, and
they are consistent with the WD being massive. We detect rapid, stochastic
variability from the X-ray emission below 4 keV. The combination of flickering
variability and a cooling-flow spectrum indicates that RT Cru is likely powered
by accretion through a disk. Whereas the cataclysmic variable stars with the
hardest X-ray emission are typically magnetic accretors with X-ray flux
modulated at the WD spin period, we find that the X-ray emission from RT Cru is
not pulsed. RT Cru therefore shows no evidence for magnetically channeled
accretion, consistent with our interpretation that the Chandra spectrum arises
from an accretion-disk boundary layer.Comment: 3 figures, accepted for publication in Ap
Chaotic Waveguide-Based Resonators for Microlasers
We propose the construction of highly directional emission microlasers using
two-dimensional high-index semiconductor waveguides as {\it open} resonators.
The prototype waveguide is formed by two collinear leads connected to a cavity
of certain shape. The proposed lasing mechanism requires that the shape of the
cavity yield mixed chaotic ray dynamics so as to have the appropiate (phase
space) resonance islands. These islands allow, via Heisenberg's uncertainty
principle, the appearance of quasi bound states (QBS) which, in turn,
propitiate the lasing mechanism. The energy values of the QBS are found through
the solution of the Helmholtz equation. We use classical ray dynamics to
predict the direction and intensity of the lasing produced by such open
resonators for typical values of the index of refraction.Comment: 5 pages, 5 figure
Classical versus Quantum Structure of the Scattering Probability Matrix. Chaotic wave-guides
The purely classical counterpart of the Scattering Probability Matrix (SPM)
of the quantum scattering matrix is defined for 2D
quantum waveguides for an arbitrary number of propagating modes . We compare
the quantum and classical structures of for a waveguide
with generic Hamiltonian chaos. It is shown that even for a moderate number of
channels, knowledge of the classical structure of the SPM allows us to predict
the global structure of the quantum one and, hence, understand important
quantum transport properties of waveguides in terms of purely classical
dynamics. It is also shown that the SPM, being an intensity measure, can give
additional dynamical information to that obtained by the Poincar\`{e} maps.Comment: 9 pages, 9 figure
Impurity effects on the band structure of one-dimensional photonic crystals: Experiment and theory
We study the effects of single impurities on the transmission in microwave
realizations of the photonic Kronig-Penney model, consisting of arrays of
Teflon pieces alternating with air spacings in a microwave guide. As only the
first propagating mode is considered, the system is essentially one dimensional
obeying the Helmholtz equation. We derive analytical closed form expressions
from which the band structure, frequency of defect modes, and band profiles can
be determined. These agree very well with experimental data for all types of
single defects considered (e.g. interstitial, substitutional) and shows that
our experimental set-up serves to explore some of the phenomena occurring in
more sophisticated experiments. Conversely, based on the understanding provided
by our formulas, information about the unknown impurity can be determined by
simply observing certain features in the experimental data for the
transmission. Further, our results are directly applicable to the closely
related quantum 1D Kronig-Penney model.Comment: 16 pages, 7 figure
One-Dimensional Kronig-Penney Model with Positional Disorder: Theory versus Experiment
We study the effects of random positional disorder in the transmission of
waves in a 1D Kronig-Penny model. For weak disorder we derive an analytical
expression for the localization length and relate it to the transmission
coefficient for finite samples. The obtained results describe very well the
experimental frequency dependence of the transmission in a microwave
realization of the model. Our results can be applied both to photonic crystals
and semiconductor super lattices.Comment: 9 pages, 6 figure
Formation of a disk-structure and jets in the symbiotic prototype Z And during its 2006-2010 active phase
We present an analysis of spectrophotometric observations of the latest cycle
of activity of the symbiotic binary Z And from 2006 to 2010. We estimate the
temperature of the hot component of Z And to be \approx 150000 - 170000 K at
minimum brightness, decreasing to \approx 90000 K at the brightness maximum.
Our estimate of the electron density in the gaseous nebula is
N_{e}=10^{10}-10^{12} cm^{-3} in the region of formation of lines of neutral
helium and 10^6-10^7 cm^{-3} in the region of formation of the [OIII] and
[NeIII] nebular lines. A trend for the gas density derived from helium lines to
increase and the gas density derived from [OIII] and [NeIII] lines to
simultaneously decrease with increasing brightness of the system was observed.
Our estimates show that the ratios of the theoretical and observed fluxes in
the [OIII] and [NeIII] lines agree best when the O/Ne ratio is similar to its
value for planetary nebulae. The model spectral energy distribution showed
that, in addition to a cool component and gaseous nebula, a relatively cool
pseudophotosphere (5250-11 500 K) is present in the system. The simultaneous
presence of a relatively cool pseudophotosphere and high-ionization spectral
lines is probably related to a disk-like structure of the pseudophotosphere.
The pseudophotosphere formed very rapidly, over several weeks, during a period
of increasing brightness of Z And. We infer that in 2009, as in 2006, the
activity of the system was accompanied by a collimated bipolar ejection of
matter. In contrast to the situation in 2006, the jets were detected even
before the system reached its maximum brightness. Moreover, components with
velocities close to 1200 km/s disappeared at the maximum, while those with
velocities close to 1800 km/s appeared.Comment: 18 pages, 19 figures, Accepted for publication in Astronomy Report
Nonlinear control of a nano-hexacopter carrying a manipulator arm
International audienceThis paper proposes a simple solution for stabilization of a nano-hexacopter carrying a manipulator arm in order to increase the type of missions achievable by these types of systems. The manipulator arm is attached to the lower part of the hexacopter. The motion of the arm induces a change of the center of mass of the whole body, which induces torques which can produce the loss of stability. The present work deals with the stabilization of the whole system-that is hexacopter and arm-by means of a set of nonlinear control laws. First, an attitude control, stabilizes the hexacopter to a desired attitude taking into account the movement of the arm. Then, a suitable virtual control and the translational dynamics allow the formulation of a nonlinear controller, which drives the aerial vehicle to a desired position. Both controls consist in saturation functions. Experimental results validate the proposed control strategy and compares the results when the motion of the arm is taken into account or not
Defining principles for mobile apps and platforms development in citizen science
Apps for mobile devices and web-based platforms are increasingly used in citizen science projects. While extensive research has been done in multiple areas of studies, from Human-Computer Interaction to public engagement in science, we are not aware of a collection of recommendations specific for citizen science that provides support and advice for planning, design and data management of mobile apps and platforms that will assist learning from best practice and successful implementations. In two workshops, citizen science practitioners with experience in mobile application and web-platform development and implementation came together to analyse, discuss and define recommendations for the initiators of technology based citizen science projects. Many of the recommendations produced during the two workshops are applicable to citizen science project that do not use mobile devices to collect data. Therefore, we propose to closely connect the results presented here with ECSA’s Ten Principles of Citizen Science
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