13,172 research outputs found
An improved expanding and shift scheme for the construction of fourth-order difference co-arrays
An improved expanding and shift (IEAS) scheme for efficient fourth-order difference co-array construction is proposed. Similar to the previously proposed expanding and shift (EAS) scheme, it consists of two sparse sub-arrays, but one of them is modified and shifted according to a new rule. Examples are provided with the second sub-array being a two-level nested array (IEAS-NA), as such a choice can generate more fourth-order difference lags (FODLs), although with the same number of consecutive lags. Furthermore, the array aperture of IEAS-NA is always greater than the corresponding EAS structure, which helps improving the DOA estimation result. Simulations results are provided to show the improved performance by the proposed new scheme
An Expanding and Shift Scheme for Constructing Fourth-Order Difference Co-Arrays
An expanding and shift (EAS) scheme for efficient fourth-order difference co-array construction is proposed. It consists of two sparse sub-arrays, where one of them is modified and shifted according to the analysis provided. The number of consecutive lags of the proposed structure at the fourth order is consistently larger than two previously proposed methods. Two effective construction examples are provided with the second sparse sub-array chosen to be a two-level nested array, as such a choice can increase the number of consecutive lags further. Simulations are performed to show the improved performance by the proposed method in comparison with existing structures
Real-time evolution for weak interaction quenches in quantum systems
Motivated by recent experiments in ultracold atomic gases that explore the
nonequilibrium dynamics of interacting quantum many-body systems, we
investigate the nonequilibrium properties of a Fermi liquid. We apply an
interaction quench within the Fermi liquid phase of the Hubbard model by
switching on a weak interaction suddenly; then we follow the real-time dynamics
of the momentum distribution by a systematic expansion in the interaction
strength based on the flow equation method. In this paper we derive our main
results, namely the applicability of a quasiparticle description, the
observation of a new type of quasi-stationary nonequilibrium Fermi liquid like
state and a delayed thermalization of the momentum distribution. We explain the
physical origin of the delayed relaxation as a consequence of phase space
constraints in fermionic many-body systems. This brings about a close relation
to similar behavior of one-particle systems which we illustrate by a discussion
of the squeezed oscillator; we generalize to an extended class of systems with
discrete energy spectra and point out the generic character of the
nonequilibrium Fermi liquid results for weak interaction quenches. Both for
discrete and continuous systems we observe that particular nonequilibrium
expectation values are twice as large as their corresponding analogues in
equilibrium. For a Fermi liquid, this shows up as an increased
correlation-induced reduction of the quasiparticle residue in nonequilibrium.Comment: 54 page
Simplified and enhanced multiple level nested arrays exploiting high order difference co-arrays
Based on the high order difference co-array concept, an enhanced four level nested array (E-FL-NA) is first proposed, which optimizes the consecutive lags at the fourth order difference co-array stage. To simplify the formulations for sensor locations for comprehensive illustration and also convenient structure construction, a simplified and enhanced four level nested array (SE-FL-NA) is then proposed, whose performance is compromised but still better than the four level nested array (FL-NA). This simplified structure is further extended to the higher order case with multiple sub-arrays, referred to as simplified and enhanced multiple level nested arrays (SE-ML-NAs), where significantly increased degrees of freedom (DOFs) can be provided and exploited for underdetermined DOA estimation. Simulation results are provided to verify the superior performance of the proposed E-FL-NA, while a higher number of detectable sources is achieved by the SE-ML-NA with a limited number of physical sensors
HI Lightcones for LADUMA using Gadget-3 : performance profiling and application of an HPC code
Includes bibliographical references.This project concerns the investigation, performance profiling and optimisation of the high performance cosmological code, GADGET-3. This code was used to develop a synthetic field-of-view, or lightcone, for the MeerKAT telescope to replicate what it will observe when it conducts the LADUMA ultra-deep HI survey. This lightcone will assist in the planning process of the survey. The deliverables for this project are summarised as follows: * Provide an up-to-date performance evaluation and optimisation report for the cosmological simulation code GADGET-3. * Use GADGET-3 to produce an sufficiently high resolution simulation of a region of the Universe. • Develop a Python code to produce a lightcone which represents the MeerKAT telescope's field-of-view, by post-processing simulation output snapshots. * Extract relevant metadata from the simulation snapshots to provide additional insight into the simulated observation. * Produce an efficiently written and well documented software package to enable other researchers to produce synthetic lightcones
CalFUSE v3: A Data-Reduction Pipeline for the Far Ultraviolet Spectroscopic Explorer
Since its launch in 1999, the Far Ultraviolet Spectroscopic Explorer (FUSE)
has made over 4600 observations of some 2500 individual targets. The data are
reduced by the Principal Investigator team at the Johns Hopkins University and
archived at the Multimission Archive at Space Telescope (MAST). The
data-reduction software package, called CalFUSE, has evolved considerably over
the lifetime of the mission. The entire FUSE data set has recently been
reprocessed with CalFUSE v3.2, the latest version of this software. This paper
describes CalFUSE v3.2, the instrument calibrations upon which it is based, and
the format of the resulting calibrated data files.Comment: To appear in PASP; 29 pages, 13 figures, uses aastex, emulateap
Canonical Hamiltonian for an extended test body in curved spacetime: To quadratic order in spin
We derive a Hamiltonian for an extended spinning test body in a curved
background spacetime, to quadratic order in the spin, in terms of
three-dimensional position, momentum, and spin variables having canonical
Poisson brackets. This requires a careful analysis of how changes of the spin
supplementary condition are related to shifts of the body's representative
worldline and transformations of the body's multipole moments, and we employ
bitensor calculus for a precise framing of this analysis. We apply the result
to the case of the Kerr spacetime and thereby compute an explicit canonical
Hamiltonian for the test-body limit of the spinning two-body problem in general
relativity, valid for generic orbits and spin orientations, to quadratic order
in the test spin. This fully relativistic Hamiltonian is then expanded in
post-Newtonian orders and in powers of the Kerr spin parameter, allowing
comparisons with the test-mass limits of available post-Newtonian results. Both
the fully relativistic Hamiltonian and the results of its expansion can inform
the construction of waveform models, especially effective-one-body models, for
the analysis of gravitational waves from compact binaries.Comment: RevTeX, 25 pages, 2 figures. v2: Updated to match PRD version;
further references added; some changes in presentation and notation;
typographical errors corrected, most notably in Eqs. (7.51) and (7.58
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