217,476 research outputs found
X-Ray Spectral Variability in Cygnus X-1
Spectral variability in different energy bands of X-rays from Cyg X-1 in
different states is studied with RXTE observations and time domain approaches.
In the hard tail of energy spectrum above keV, average peak aligned
shots are softer than the average steady emission and the hardness ratio
decreases when the flux increases during a shot for all states. In regard to a
soft band lower keV, the hardness in the soft state varies in an
opposite way: it peaks when the flux of the shot peaks. For the hard and
transition states, the hardness ratio in respect to a soft band during a shot
is in general lower than that of the steady component and a sharp rise is
observed at about the shot peak. For the soft state, the correlation
coefficient between the intensity and hardness ratio in the hard tail is
negative and decreases monotonically as the timescale increases from 0.01 s to
50 s, which is opposite to that in regard to a soft band. For the hard and
transition states, the correlation coefficients are in general negative and
have a trend of decrease with increasing timescale.Comment: 14 pages, 3 figures, accepted by Ap
Tropical Ocean and Global Atmosphere (TOGA) heat exchange project: A summary report
A pilot data center to compute ocean atmosphere heat exchange over the tropical ocean is prposed at the Jet Propulsion Laboratory (JPL) in response to the scientific needs of the Tropical Ocean and Global Atmosphere (TOGA) Program. Optimal methods will be used to estimate sea surface temperature (SET), surface wind speed, and humidity from spaceborne observations. A monthly summary of these parameters will be used to compute ocean atmosphere latent heat exchanges. Monthly fields of surface heat flux over tropical oceans will be constructed using estimations of latent heat exchanges and short wave radiation from satellite data. Verification of all satellite data sets with in situ measurements at a few locations will be provided. The data center will be an experimental active archive where the quality and quantity of data required for TOGA flux computation are managed. The center is essential to facilitate the construction of composite data sets from global measurements taken from different sensors on various satellites. It will provide efficient utilization and easy access to the large volume of satellite data available for studies of ocean atmosphere energy exchanges
An advanced meshless method for time fractional diffusion equation
Recently, because of the new developments in sustainable engineering and renewable energy, which are usually governed by a series of fractional partial differential equations (FPDEs), the numerical modelling and simulation for fractional calculus are attracting more and more attention from researchers. The current dominant numerical method for modeling FPDE is Finite Difference Method (FDM), which is based on a pre-defined grid leading to inherited issues or shortcomings including difficulty in simulation of problems with the complex problem domain and in using irregularly distributed nodes. Because of its distinguished advantages, the meshless method has good potential in simulation of FPDEs. This paper aims to develop an implicit meshless collocation technique for FPDE. The discrete system of FPDEs is obtained by using the meshless shape functions and the meshless collocation formulation. The stability and convergence of this meshless approach are investigated theoretically and numerically. The numerical examples with regular and irregular nodal distributions are used to validate and investigate accuracy and efficiency of the newly developed meshless formulation. It is concluded that the present meshless formulation is very effective for the modeling and simulation of fractional partial differential equations
Supersymmetry of consistent massive truncations of IIB supergravity
We discuss the supersymmetry and fermionic sector of the recently obtained
consistent truncations of IIB supergravity containing massive modes. In
particular, we present the general form of the five-dimensional N = 4
supersymmetry transformations and equations of motion for the fermions arising
in the reduction of IIB theory on T^{1,1} which contains all modes invariant
under the SU(2) x SU(2) isometry group. The N = 4 reduction can be further
truncated to two different N = 2 sub-sectors. For each of these, we present the
N = 2 fermionic supersymmetry transformations and corresponding
superpotentials. As an application, we obtain the explicit Killing spinors of
the Klebanov-Strassler solution and comment on the relation to the ansatz of
Papadopoulos and Tseytlin. We also demonstrate the applicability of consistent
truncations on squashed Sasaki-Einstein manifolds to a class of flux
compactifications, focusing on a recent solution describing the geometry of
gaugino condensation on wrapped D7 branes and which possesses dynamic SU(2)
structure.Comment: v2: minor typos corrected, references added, v3: significant
additions to include fermion equations of motion, journal versio
Two-stage Turing model for generating pigment patterns on the leopard and the jaguar
Based on the results of phylogenetic analysis, which showed that flecks are the primitive pattern of the felid family and all other patterns including rosettes and blotches develop from it, we construct a Turing reaction-diffusion model which generates spot patterns initially. Starting from this spotted pattern, we successfully generate patterns of adult leopards and jaguars by tuning parameters of the model in the subsequent phase of patterning
Theory of fishnet negative-index optical metamaterials
We theoretically study fishnet metamaterials at optical frequencies. In
contrast to earlier works, we provide a microscopic description by tracking the
transversal and longitudinal flows of energy through the fishnet mesh composed
of intersecting subwavelength plasmonic waveguides. The analysis is supported
by a semi-analytical model based on surface-plasmon coupled-mode equations,
which provides accurate formulas for the fishnet refractive index, including
the real-negative and imaginary parts. The model simply explains how the
surface plasmons couple at the waveguide intersections and it shines new light
on the fishnet negative-index paradigm at optical frequencies. Extension of the
theory for loss-compensated metamaterials with gain media is also presented.Comment: 4 figure
Asymmetric Avalanches in the Condensate of a Zeeman-limited Superconductor
We report the non-equilibrium behavior of disordered superconducting Al films
in high Zeeman fields. We have measured the tunneling density of states of the
films through the first-order Zeeman critical field transition. We find that
films with sheet resistances of a few hundred ohms exhibit large avalanche-like
collapses of the condensate on the superheating branch of the critical field
hysteresis loop. In contrast, the transition back into the superconducting
phase (i.e., along the supercooling branch) is always continuous. The fact that
the condensate follows an unstable trajectory to the normal state suggests that
the order parameter in the hysteretic regime is not homogeneous.Comment: 5 pages, 5 figures, to appear in PR
Inner product computation for sparse iterative solvers on\ud distributed supercomputer
Recent years have witnessed that iterative Krylov methods without re-designing are not suitable for distribute supercomputers because of intensive global communications. It is well accepted that re-engineering Krylov methods for prescribed computer architecture is necessary and important to achieve higher performance and scalability. The paper focuses on simple and practical ways to re-organize Krylov methods and improve their performance for current heterogeneous distributed supercomputers. In construct with most of current software development of Krylov methods which usually focuses on efficient matrix vector multiplications, the paper focuses on the way to compute inner products on supercomputers and explains why inner product computation on current heterogeneous distributed supercomputers is crucial for scalable Krylov methods. Communication complexity analysis shows that how the inner product computation can be the bottleneck of performance of (inner) product-type iterative solvers on distributed supercomputers due to global communications. Principles of reducing such global communications are discussed. The importance of minimizing communications is demonstrated by experiments using up to 900 processors. The experiments were carried on a Dawning 5000A, one of the fastest and earliest heterogeneous supercomputers in the world. Both the analysis and experiments indicates that inner product computation is very likely to be the most challenging kernel for inner product-based iterative solvers to achieve exascale
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