2,681 research outputs found
A local Gaussian filter and adaptive morphology as tools for completing partially discontinuous curves
This paper presents a method for extraction and analysis of curve--type
structures which consist of disconnected components. Such structures are found
in electron--microscopy (EM) images of metal nanograins, which are widely used
in the field of nanosensor technology.
The topography of metal nanograins in compound nanomaterials is crucial to
nanosensor characteristics. The method of completing such templates consists of
three steps. In the first step, a local Gaussian filter is used with different
weights for each neighborhood. In the second step, an adaptive morphology
operation is applied to detect the endpoints of curve segments and connect
them. In the last step, pruning is employed to extract a curve which optimally
fits the template
Models of Galaxy Clusters with Thermal Conduction
We present a simple model of hot gas in galaxy clusters, assuming hydrostatic
equilibrium and energy balance between radiative cooling and thermal
conduction. For five clusters, A1795, A1835, A2199, A2390 and RXJ1347.5-1145,
the model gives a good description of the observed radial profiles of electron
density and temperature, provided we take the thermal conductivity to
be about 30% of the Spitzer conductivity. Since the required is
consistent with the recent theoretical estimate of Narayan & Medvedev (2001)
for a turbulent magnetized plasma, we consider a conduction-based equilibrium
model to be viable for these clusters. We further show that the hot gas is
thermally stable because of the presence of conduction. For five other
clusters, A2052, A2597, Hydra A, Ser 159-03 and 3C295, the model requires
unphysically large values of to fit the data. These clusters must have
some additional source of heat, most likely an active galactic nucleus since
all the clusters have strong radio galaxies at their centers. We suggest that
thermal conduction, though not dominant in these clusters, may nevertheless
play a significant role by preventing the gas from becoming thermally unstable.Comment: Published in ApJ; 22 pages, including 2 tables, 4 figures; typos
corrected to match the published versio
Delay of Disorder by Diluted Polymers
We study the effect of diluted flexible polymers on a disordered capillary
wave state. The waves are generated at an interface of a dyed water sugar
solution and a low viscous silicon oil. This allows for a quantitative
measurement of the spatio-temporal Fourier spectrum. The primary pattern after
the first bifurcation from the flat interface are squares. With increasing
driving strength we observe a melting of the square pattern. It is replaced by
a weak turbulent cascade. The addition of a small amount of polymers to the
water layer does not affect the critical acceleration but shifts the disorder
transition to higher driving strenghs and the short wave length - high
frequency fluctuations are suppressed
On vacuum gravitational collapse in nine dimensions
We consider the vacuum gravitational collapse for cohomogeneity-two solutions
of the nine dimensional Einstein equations. Using combined numerical and
analytical methods we give evidence that within this model the
Schwarzschild-Tangherlini black hole is asymptotically stable. In addition, we
briefly discuss the critical behavior at the threshold of black hole formation.Comment: 4 pages, 4 figure
The RANLUX generator: resonances in a random walk test
Using a recently proposed directed random walk test, we systematically
investigate the popular random number generator RANLUX developed by Luescher
and implemented by James. We confirm the good quality of this generator with
the recommended luxury level. At a smaller luxury level (for instance equal to
1) resonances are observed in the random walk test. We also find that the
lagged Fibonacci and Subtract-with-Carry recipes exhibit similar failures in
the random walk test. A revised analysis of the corresponding dynamical systems
leads to the observation of resonances in the eigenvalues of Jacobi matrix.Comment: 18 pages with 14 figures, Essential addings in the Abstract onl
Quantum chaos of a mixed, open system of kicked cold atoms
The quantum and classical dynamics of particles kicked by a gaussian
attractive potential are studied. Classically, it is an open mixed system (the
motion in some parts of the phase space is chaotic, and in some parts it is
regular). The fidelity (Lochshmidt echo) is found to exhibit oscillations that
can be determined from classical considerations but are sensitive to phase
space structures that are smaller than Planck's constant. Families of
quasi-energies are determined from classical phase space structures.
Substantial differences between the classical and quantum dynamics are found
for time dependent scattering. It is argued that the system can be
experimentally realized by cold atoms kicked by a gaussian light beam.Comment: 19 pages, 21 figures, (accepted for publication in Phys. Rev. E
Asymptotic Spectroscopy of Rotating Black Holes
We calculate analytically the transmission and reflection amplitudes for
waves incident on a rotating black hole in d=4, analytically continued to
asymptotically large, nearly imaginary frequency. These amplitudes determine
the asymptotic resonant frequencies of the black hole, including quasinormal
modes, total-transmission modes and total-reflection modes. We identify these
modes with semiclassical bound states of a one-dimensional Schrodinger
equation, localized along contours in the complexified r-plane which connect
turning points of corresponding null geodesics. Each family of modes has a
characteristic temperature and chemical potential. The relations between them
provide hints about the microscopic description of the black hole in this
asymptotic regime.Comment: References adde
Lie systems and integrability conditions for t-dependent frequency harmonic oscillators
Time-dependent frequency harmonic oscillators (TDFHO's) are studied through
the theory of Lie systems. We show that they are related to a certain kind of
equations in the Lie group SL(2,R). Some integrability conditions appear as
conditions to be able to transform such equations into simpler ones in a very
specific way. As a particular application of our results we find t-dependent
constants of the motion for certain one-dimensional TDFHO's. Our approach
provides an unifying framework which allows us to apply our developments to all
Lie systems associated with equations in SL(2,R) and to generalise our methods
to study any Lie system
An XMM-Newton and Chandra investigation of the nuclear accretion in the Sombrero Galaxy (NGC4594)
We present an analysis of the XMM-Newton and Chandra ACIS-S observations of
the LINER nucleus of the Sombrero galaxy and we discuss possible explanations
for its very sub-Eddington luminosity by complementing the X-ray results with
high angular resolution observations in other bands. The X-ray investigation
shows a hard (Gamma=1.89) and moderately absorbed (N_H=1.8 10^21 cm^-2) nuclear
source of 1.5 10^40 erg s^-1 in the 2-10 keV band, surrounded by hot gas at a
temperature of \sim 0.6 keV. The bolometric nuclear luminosity is at least \sim
200 times lower than expected if mass accreted on the supermassive black hole,
that HST shows to reside at the center of this galaxy, at the rate predicted by
the spherical and adiabatic Bondi accretion theory and with the high radiative
efficiency of a standard accretion disc. The low luminosity, coupled to the
observed absence of Fe-K emission in the nuclear spectrum, indicates that such
a disc is not present. This nucleus differs from bright unobscured AGNs also
for the lack of high flux variability and of prominent broad Halpha emission.
However, it is also too faint for the predictions of simple radiatively
inefficient accretion taking place at the Bondi rate; it could be too radio
bright, instead, for radiatively inefficient accretion that includes strong
mass outflows or convection. This discrepancy could be solved by the possible
presence of nuclear radio jets. An alternative explanation of the low
luminosity, in place of radiative inefficiency, could be unsteady accretion.Comment: 24 pages, including 7 figures; to be published in the Ap
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