5,831 research outputs found
BRST Structures and Symplectic Geometry on a Class of Supermanifolds
By investigating the symplectic geometry and geometric quantization on a
class of supermanifolds, we exhibit BRST structures for a certain kind of
algebras. We discuss the undeformed and q-deformed cases in the classical as
well as in the quantum cases.Comment: 14 pages, Late
Computer simulation of a single pilot flying a modern high-performance helicopter
Presented is a computer simulation of a human response pilot model able to execute operational flight maneuvers and vehicle stabilization of a modern high-performance helicopter. Low-order, single-variable, human response mechanisms, integrated to form a multivariable pilot structure, provide a comprehensive operational control over the vehicle. Evaluations of the integrated pilot were performed by direct insertion into a nonlinear, total-force simulation environment provided by NASA Lewis. Comparisons between the integrated pilot structure and single-variable pilot mechanisms are presented. Static and dynamically alterable configurations of the pilot structure are introduced to simulate pilot activities during vehicle maneuvers. These configurations, in conjunction with higher level, decision-making processes, are considered for use where guidance and navigational procedures, operational mode transfers, and resource sharing are required
Computer simulation of multiple pilots flying a modern high performance helicopter
A computer simulation of a human response pilot mechanism within the flight control loop of a high-performance modern helicopter is presented. A human response mechanism, implemented by a low order, linear transfer function, is used in a decoupled single variable configuration that exploits the dominant vehicle characteristics by associating cockpit controls and instrumentation with specific vehicle dynamics. Low order helicopter models obtained from evaluations of the time and frequency domain responses of a nonlinear simulation model, provided by NASA Lewis Research Center, are presented and considered in the discussion of the pilot development. Pilot responses and reactions to test maneuvers are presented and discussed. Higher level implementation, using the pilot mechanisms, are discussed and considered for their use in a comprehensive control structure
A complex environment around Cir X-1
We present the results of an archival 54 ks long Chandra observation of the
peculiar source Cir X--1 during the phase passage 0.223-0.261. A comparative
analysis of X-ray spectra, selected at different flux levels of the source,
allows us to distinguish between a very hard state, at a low countrate, and a
brighter, softer, highly absorbed spectrum during episodes of flaring activity,
when the unabsorbed source luminosity is about three times the value in the
hard state. The spectrum of the hard state clearly shows emission lines of
highly ionized elements, while, during the flaring state, the spectrum also
shows strong resonant absorption lines. The most intense and interesting
feature in this latter state is present in the Fe K alpha region: a very
broadened absorption line at energies ~ 6.5 keV that could result from a
smeared blending of resonant absorption lines of moderately ionized iron ions
(Fe XX - Fe XXIV). We also observe strong resonant absorption lines of Fe XXV
and Fe XXVI, together with a smeared absorption edge above 7 keV. We argue that
the emitting region during the quiescent/hard state is constituted of a purely
photo-ionized medium, possibly present above an accretion disk, or of a
photo-ionized plasma present in a beamed outflow. During the flaring states the
source undergoes enhanced turbulent accretion that modifies both the accretion
geometry and the optical depth of the gas surrounding the primary X-ray source.Comment: Accepted for publication in Ap
Constraining Alternate Models of Black Holes: Type I X-ray Bursts on Accreting Fermion-Fermion and Boson-Fermion Stars
The existence of black holes remains open to doubt until other conceivable
options are excluded. With this motivation, we consider a model of a compact
star in which most of the mass consists of dark particles of some kind, and a
small fraction of the mass is in the form of ordinary nucleonic gas. The gas
does not interact with the dark matter other than via gravity, but collects at
the center as a separate fermionic fluid component. Depending on whether the
dark mass is made of fermions or bosons, the objects may be called
fermion-fermion stars or boson-fermion stars, respectively. For appropriate
choices of the mass of the dark matter particles, these objects are viable
models of black hole candidates in X-ray binaries. We consider models with a
dark mass of 10 solar masses and a range of gas mass from 10^{-6} to nearly one
solar mass, and analyse the bursting properties of the models when they accrete
gas. We show that all the models would experience thermonuclear Type I X-ray
bursts at appropriate mass accretion rates. Since no Type I bursts have been
reported from black hole candidates, the models are ruled out. The case for
identifying black hole candidates in X-ray binaries as true black holes is thus
strengthened.Comment: 29 pages, 7 figures, to appear in The Astrophysical Journa
On the existence of internal modes of sine-Gordon kinks
We study whether or not sine-Gordon kinks exhibit internal modes or
``quasimodes.'' By considering the response of the kinks to ac forces and
initial distortions, we show that neither intrinsic internal modes nor
``quasimodes'' exist in contrast to previous reports. However, we do identify a
different kind of internal mode bifurcating from the bottom edge of the phonon
band which arises from the discretization of the system in the numerical
simulations, thus confirming recent predictions.Comment: 4 pages, 2 figures, REVTeX, to appear as a Rapid Communication in
Phys Rev E (July 1st
Logarithmic oscillators: ideal Hamiltonian thermostats
A logarithmic oscillator (in short, log-oscillator) behaves like an ideal
thermostat because of its infinite heat capacity: when it weakly couples to
another system, time averages of the system observables agree with ensemble
averages from a Gibbs distribution with a temperature T that is given by the
strength of the logarithmic potential. The resulting equations of motion are
Hamiltonian and may be implemented not only in a computer but also with
real-world experiments, e.g., with cold atoms.Comment: 5 pages, 3 figures. v4: version accepted in Phys. Rev. Let
Detecting Singleton Review Spammers Using Semantic Similarity
Online reviews have increasingly become a very important resource for
consumers when making purchases. Though it is becoming more and more difficult
for people to make well-informed buying decisions without being deceived by
fake reviews. Prior works on the opinion spam problem mostly considered
classifying fake reviews using behavioral user patterns. They focused on
prolific users who write more than a couple of reviews, discarding one-time
reviewers. The number of singleton reviewers however is expected to be high for
many review websites. While behavioral patterns are effective when dealing with
elite users, for one-time reviewers, the review text needs to be exploited. In
this paper we tackle the problem of detecting fake reviews written by the same
person using multiple names, posting each review under a different name. We
propose two methods to detect similar reviews and show the results generally
outperform the vectorial similarity measures used in prior works. The first
method extends the semantic similarity between words to the reviews level. The
second method is based on topic modeling and exploits the similarity of the
reviews topic distributions using two models: bag-of-words and
bag-of-opinion-phrases. The experiments were conducted on reviews from three
different datasets: Yelp (57K reviews), Trustpilot (9K reviews) and Ott dataset
(800 reviews).Comment: 6 pages, WWW 201
Dynamic disorder in receptor-ligand forced dissociation experiments
Recently experiments showed that some biological noncovalent bonds increase
their lifetimes when they are stretched by an external force, and their
lifetimes will decrease when the force increases further. Several specific
quantitative models have been proposed to explain the intriguing transitions
from the "catch-bond" to the "slip-bond". Different from the previous efforts,
in this work we propose that the dynamic disorder of the force-dependent
dissociation rate can account for the counterintuitive behaviors of the bonds.
A Gaussian stochastic rate model is used to quantitatively describe the
transitions observed recently in the single bond P-selctin glycoprotein ligand
1(PSGL-1)P-selectin force rupture experiment [Marshall, {\it et al.}, (2003)
Nature {\bf 423}, 190-193]. Our model agrees well to the experimental data. We
conclude that the catch bonds could arise from the stronger positive
correlation between the height of the intrinsic energy barrier and the distance
from the bound state to the barrier; classical pathway scenario or {\it a
priori} catch bond assumption is not essential.Comment: 4 pages, 2 figure
Topology of Entanglement Evolution of Two Qubits
The dynamics of a two-qubit system is considered with the aim of a general
categorization of the different ways in which entanglement can disappear in the
course of the evolution, e.g., entanglement sudden death. The dynamics is
described by the function ~n(t), where ~n is the 15-dimensional polarization
vector. This representation is particularly useful because the components of ~n
are direct physical observables, there is a meaningful notion of orthogonality,
and the concurrence C can be computed for any point in the space. We analyze
the topology of the space S of separable states (those having C = 0), and the
often lower-dimensional linear dynamical subspace D that is characteristic of a
specific physical model. This allows us to give a rigorous characterization of
the four possible kinds of entanglement evolution. Which evolution is realized
depends on the dimensionality of D and of D \cap S, the position of the
asymptotic point of the evolution, and whether or not the evolution is
"distance-Markovian", a notion we define. We give several examples to
illustrate the general principles, and to give a method to compute critical
points. We construct a model that shows all four behaviors.Comment: 15 pages, 11 figures, 2 tabl
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