11,481 research outputs found
Jet-Disc coupling in the accreting black hole XTEJ1118+480
We interpret the rapid correlated UV/optical/ X-ray variability of
XTEJ1118+480 as a signature of the coupling between the X-ray corona and a jet
emitting synchrotron radiation in the optical band.We propose a scenario in
which the jet and the X-ray corona are fed by the same energy reservoir where
large amounts of accretion power are stored before being channelled into either
the jet or the high energy radiation. This time dependent model reproduces the
main features of the rapid multi-wavelength variability of XTEJ1118+480. A
strong requirement of the model is that the total jet power should be at least
a few times larger than the observed X-ray luminosity. This would be consistent
with the overall low radiative efficiency of the source. We present independent
arguments showing that the jet probably dominates the energetic output of all
accreting black holes in the low-hard state.Comment: 8 pages, 1 figure, to appear in the proceedings of "From X-ray
binaries to quasars: Black hole accretion on all mass scales, (Amsterdam,
July 2004)", Eds. T. Maccarone, R. Fender, L. H
Selective phenotyping, entropy reduction, and the mastermind game.
BACKGROUND: With the advance of genome sequencing technologies, phenotyping, rather than genotyping, is becoming the most expensive task when mapping genetic traits. The need for efficient selective phenotyping strategies, i.e. methods to select a subset of genotyped individuals for phenotyping, therefore increases. Current methods have focused either on improving the detection of causative genetic variants or their precise genomic location separately. RESULTS: Here we recognize selective phenotyping as a Bayesian model discrimination problem and introduce SPARE (Selective Phenotyping Approach by Reduction of Entropy). Unlike previous methods, SPARE can integrate the information of previously phenotyped individuals, thereby enabling an efficient incremental strategy. The effective performance of SPARE is demonstrated on simulated data as well as on an experimental yeast dataset. CONCLUSIONS: Using entropy reduction as an objective criterion gives a natural way to tackle both issues of detection and localization simultaneously and to integrate intermediate phenotypic data. We foresee entropy-based strategies as a fruitful research direction for selective phenotyping
Optimization of quantum Monte Carlo wave functions by energy minimization
We study three wave function optimization methods based on energy
minimization in a variational Monte Carlo framework: the Newton, linear and
perturbative methods. In the Newton method, the parameter variations are
calculated from the energy gradient and Hessian, using a reduced variance
statistical estimator for the latter. In the linear method, the parameter
variations are found by diagonalizing a non-symmetric estimator of the
Hamiltonian matrix in the space spanned by the wave function and its
derivatives with respect to the parameters, making use of a strong
zero-variance principle. In the less computationally expensive perturbative
method, the parameter variations are calculated by approximately solving the
generalized eigenvalue equation of the linear method by a nonorthogonal
perturbation theory. These general methods are illustrated here by the
optimization of wave functions consisting of a Jastrow factor multiplied by an
expansion in configuration state functions (CSFs) for the C molecule,
including both valence and core electrons in the calculation. The Newton and
linear methods are very efficient for the optimization of the Jastrow, CSF and
orbital parameters. The perturbative method is a good alternative for the
optimization of just the CSF and orbital parameters. Although the optimization
is performed at the variational Monte Carlo level, we observe for the C
molecule studied here, and for other systems we have studied, that as more
parameters in the trial wave functions are optimized, the diffusion Monte Carlo
total energy improves monotonically, implying that the nodal hypersurface also
improves monotonically.Comment: 18 pages, 8 figures, final versio
The x-ray corona and jet of cygnus x-1
Evidence is presented indicating that in the hard state of Cygnus X-1, the
coronal mag- netic field might be below equipartition with radiation
(suggesting that the corona is not powered by magnetic field dissipation) and
that the ion temperature in the corona is significantly lower than what
predicted by ADAF like models. It is also shown that the current estimates of
the jet power set interesting contraints on the jet velocity (which is at least
mildly relativistic), the accretion efficiency (which is large in both spectral
states), and the nature of the X-ray emitting region (which is unlikely to be
the jet).Comment: 8 pages, 1 figure. Accepted for publication in Journal of Modern
Physics D, Proceedings of HEPRO II conference, Buenos Aires, Argentina,
October 26-30, 200
Two-dimensional structures in the quintic Ginzburg-Landau equation
By using ZEUS cluster at Embry-Riddle Aeronautical University we perform
extensive numerical simulations based on a two-dimensional Fourier spectral
method Fourier spatial discretization and an explicit scheme for time
differencing) to find the range of existence of the spatiotemporal solitons of
the two-dimensional complex Ginzburg-Landau equation with cubic and quintic
nonlinearities. We start from the parameters used by Akhmediev {\it et. al.}
and slowly vary them one by one to determine the regimes where solitons exist
as stable/unstable structures. We present eight classes of dissipative solitons
from which six are known (stationary, pulsating, vortex spinning, filament,
exploding, creeping) and two are novel (creeping-vortex propellers and spinning
"bean-shaped" solitons). By running lengthy simulations for the different
parameters of the equation, we find ranges of existence of stable structures
(stationary, pulsating, circular vortex spinning, organized exploding), and
unstable structures (elliptic vortex spinning that leads to filament,
disorganized exploding, creeping). Moreover, by varying even the two initial
conditions together with vorticity, we find a richer behavior in the form of
creeping-vortex propellers, and spinning "bean-shaped" solitons. Each class
differentiates from the other by distinctive features of their energy
evolution, shape of initial conditions, as well as domain of existence of
parameters.Comment: 19 pages, 19 figures, 8 tables, updated text and reference
DOLPHIn - Dictionary Learning for Phase Retrieval
We propose a new algorithm to learn a dictionary for reconstructing and
sparsely encoding signals from measurements without phase. Specifically, we
consider the task of estimating a two-dimensional image from squared-magnitude
measurements of a complex-valued linear transformation of the original image.
Several recent phase retrieval algorithms exploit underlying sparsity of the
unknown signal in order to improve recovery performance. In this work, we
consider such a sparse signal prior in the context of phase retrieval, when the
sparsifying dictionary is not known in advance. Our algorithm jointly
reconstructs the unknown signal - possibly corrupted by noise - and learns a
dictionary such that each patch of the estimated image can be sparsely
represented. Numerical experiments demonstrate that our approach can obtain
significantly better reconstructions for phase retrieval problems with noise
than methods that cannot exploit such "hidden" sparsity. Moreover, on the
theoretical side, we provide a convergence result for our method
Compact and Flexible Basis Functions for Quantum Monte Carlo Calculations
Molecular calculations in quantum Monte Carlo frequently employ a mixed basis
consisting of contracted and primitive Gaussian functions. While standard basis
sets of varying size and accuracy are available in the literature, we
demonstrate that reoptimizing the primitive function exponents within quantum
Monte Carlo yields more compact basis sets for a given accuracy. Particularly
large gains are achieved for highly excited states. For calculations requiring
non-diverging pseudopotentials, we introduce Gauss-Slater basis functions that
behave as Gaussians at short distances and Slaters at long distances. These
basis functions further improve the energy and fluctuations of the local energy
for a given basis size. Gains achieved by exponent optimization and
Gauss-Slater basis use are exemplified by calculations for the ground state of
carbon, the lowest lying excited states of carbon with , ,
, symmetries, carbon dimer, and naphthalene. Basis size
reduction enables quantum Monte Carlo treatment of larger molecules at high
accuracy.Comment: 8 Pages, 2 Figures, 9 Table
Direct observation of quantum phonon fluctuations in a one dimensional Bose gas
We report the first direct observation of collective quantum fluctuations in
a continuous field. Shot-to-shot atom number fluctuations in small sub-volumes
of a weakly interacting ultracold atomic 1D cloud are studied using \textit{in
situ} absorption imaging and statistical analysis of the density profiles. In
the cloud centers, well in the \textit{quantum quasicondensate} regime, the
ratio of chemical potential to thermal energy is , and,
owing to high resolution, up to 20% of the microscopically observed
fluctuations are quantum phonons. Within a non-local analysis at variable
observation length, we observe a clear deviation from a classical field
prediction, which reveals the emergence of dominant quantum fluctuations at
short length scales, as the thermodynamic limit breaks down.Comment: 4 pages, 3 figures (Supplementary material 3 pages, 3 figures
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