12,774 research outputs found
Modeling The Time Variability of Accreting Compact Sources
We present model light curves for accreting Black Hole Candidates (BHC) based
on a recently proposed model for their spectro-temporal properties. According
to this model, the observed light curves and aperiodic variability of BHC are
due to a series of soft photon injections at random (Poisson) intervals near
the compact object and their reprocessing into hard radiation in an extended
but non-uniform hot plasma corona surrounding the compact object. We argue that
the majority of the timing characteristics of these light curves are due to the
stochastic nature of the Comptonization process in the extended corona, whose
properties, most notably its radial density dependence, are imprinted in them.
We compute the corresponding Power Spectral Densities (PSD), autocorrelation
functions, time skewness of the light curves and time lags between the light
curves of the sources at different photon energies and compare our results to
observation. Our model light curves compare well with observations, providing
good fits to their overall morphology, as manifest by the autocorrelation and
skewness functions. The lags and PSDs of the model light curves are also in
good agreement with those observed (the model can even accommodate the presence
of QPOs). Finally, while most of the variability power resides at time scales
\gsim a few seconds, at the same time, the model allows also for shots of a
few msec in duration, in accordance with observation. We suggest that
refinements of this type of model along with spectral and phase lag information
can be used to probe the structure of this class of high energy sources.Comment: 23 pages Latex, 15 encapsulated postscript figures, to appear in the
Astrophysical Journa
Near-Optimal Distributed Approximation of Minimum-Weight Connected Dominating Set
This paper presents a near-optimal distributed approximation algorithm for
the minimum-weight connected dominating set (MCDS) problem. The presented
algorithm finds an approximation in rounds,
where is the network diameter and is the number of nodes.
MCDS is a classical NP-hard problem and the achieved approximation factor
is known to be optimal up to a constant factor, unless P=NP.
Furthermore, the round complexity is known to be
optimal modulo logarithmic factors (for any approximation), following [Das
Sarma et al.---STOC'11].Comment: An extended abstract version of this result appears in the
proceedings of 41st International Colloquium on Automata, Languages, and
Programming (ICALP 2014
Magnetic properties of a novel Pr Fe Ti phase
In a systematic study of the (Pr1âxTix)Fe5 alloy series, the (Pr0.65Ti0.35)Fe5 alloy has been
found to have a dominant phase with either the rhombohedral Th2Zn17 structure or the
newly discovered Nd2(Fe,Ti)19 (S. J. Collocott, R. K. Day, J. B. Dunlop, and R. L. Davis,
in Proceedings of the Seventh International Symposium on Magnetic Anisotropy and
Coercivity in RâT Alloys, Canberra, July 1992, p. 437) structure, depending on the
annealing procedure. Powderâxârayâdiffraction patterns and scanning electron
microscopy show that the sample annealed at a temperature of 850â°C followed by
1000â°C has the 2:17 structure whereas annealing at 1000â°C directly leads to the new
2:19 structure. Energyâdispersive xâray analysis yields Pr:Fe:Ti ratios of 10.7:86.2:3.1
for the Pr2(Fe,Ti)17 phase and 9.2:85.9:4.9 for the Pr2(Fe,Ti)19 phase.
57
Fe Mössbauer
spectroscopy (at 295 K) gives values for the average
57
Fe hyperfine field of 15.7 T for the
2:17 phase and 17.5 T for the 2:19 phase, respectively
Probing the Structure of Accreting Compact Sources Through X-Ray Time Lags and Spectra
We exhibit, by compiling all data sets we can acquire, that the Fourier
frequency dependent hard X-ray lags, first observed in the analysis of
aperiodic variability of the light curves of the black hole candidate Cygnus
X-1, appear to be a property shared by several other accreting black hole
candidate sources and also by the different spectral states of this source. We
then present both analytic and numerical models of these time lags resulting by
the process of Comptonization in a variety of hot electron configurations. We
argue that under the assumption that the observed spectra are due to
Comptonization, the dependence of the lags on the Fourier period provides a
means for mapping the spatial density profile of the hot electron plasma, while
the period at which the lags eventually level--off provides an estimate of the
size of the scattering cloud. We further examine the influence of the location
and spatial extent of the soft photon source on the form of the resulting lags
for a variety of configurations; we conclude that the study of the X-ray hard
lags can provide clues about these parameters of the Comptonization process
too. Fits of the existing data with our models indicate that the size of the
Comptonizing clouds are quite large in extent ( 1 light second) with
inferred radial density profiles which are in many instances inconsistent with
those of the standard dynamical models, while the extent of the source of soft
photons appears to be much smaller than those of the hot electrons by roughly
two orders of magnitude and its location consistent with the center of the hot
electron corona.Comment: 20 pages Latex, 11 postscript figures, to appear in the Astrophysical
Journal, Vol 512, Feb 20 issu
The Tensor Current Divergence Equation in U(1) Gauge Theories is Free of Anomalies
The possible anomaly of the tensor current divergence equation in U(1) gauge
theories is calculated by means of perturbative method. It is found that the
tensor current divergence equation is free of anomalies.Comment: Revtex4, 7 pages, 2 figure
Relative entropy of entanglement of a kind of two qubit entangled states
We in this paper strictly prove that some block diagonalizable two qubit
entangled state with six none zero elements reaches its quantum relative
entropy entanglement by the a separable state having the same matrix structure.
The entangled state comprises local filtering result state as a special case.Comment: 5 page
Spin and orbital angular momentum in gauge theories (II): QCD and nucleon spin structure
Parallel to the construction of gauge invariant spin and orbital angular
momentum for QED in paper (I) of this series, we present here an analogous but
non-trivial solution for QCD. Explicitly gauge invariant spin and orbital
angular momentum operators of quarks and gluons are obtained. This was
previously thought to be an impossible task, and opens a more promising avenue
towards the understanding of the nucleon spin structure.Comment: 3 pages, no figure; presented by F. Wang at NSTAR200
Improved lattice QCD with quarks: the 2 dimensional case
QCD in two dimensions is investigated using the improved fermionic lattice
Hamiltonian proposed by Luo, Chen, Xu, and Jiang. We show that the improved
theory leads to a significant reduction of the finite lattice spacing errors.
The quark condensate and the mass of lightest quark and anti-quark bound state
in the strong coupling phase (different from t'Hooft phase) are computed. We
find agreement between our results and the analytical ones in the continuum.Comment: LaTeX file (including text + 10 figures
On Gauge Theory and Topological String in Nekrasov-Shatashvili Limit
We study the Nekrasov-Shatashvili limit of the N=2 supersymmetric gauge
theory and topological string theory on certain local toric Calabi-Yau
manifolds. In this limit one of the two deformation parameters \epsilon_{1,2}
of the Omega background is set to zero and we study the perturbative expansion
of the topological amplitudes around the remaining parameter. We derive
differential equations from Seiberg-Witten curves and mirror geometries, which
determine the higher genus topological amplitudes up to a constant. We show
that the higher genus formulae previously obtained from holomorphic anomaly
equations and boundary conditions satisfy these differential equations. We also
provide a derivation of the holomorphic anomaly equations in the
Nekrasov-Shatashvili limit from these differential equations.Comment: 41 pages, no figure. v2: references adde
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