4,820 research outputs found
Efficient implementation of the adaptive scale pixel decomposition algorithm
Context. Most popular algorithms in use to remove the effects of a
telescope's point spread function (PSF) in radio astronomy are variants of the
CLEAN algorithm. Most of these algorithms model the sky brightness using the
delta-function basis, which results in undesired artefacts when used on image
extended emission. The adaptive scale pixel decomposition (Asp-Clean) algorithm
models the sky brightness on a scale-sensitive basis and thus gives a
significantly better imaging performance when imaging fields that contain both
resolved and unresolved emission.
Aims. However, the runtime cost of Asp-Clean is higher than that of
scale-insensitive algorithms. In this paper, we identify the most expensive
step in the original Asp-Clean algorithm and present an efficient
implementation of it, which significantly reduces the computational cost while
keeping the imaging performance comparable to the original algorithm. The PSF
sidelobe levels of modern wide-band telescopes are significantly reduced,
allowing us to make approximations to reduce the computing cost, which in turn
allows for the deconvolution of larger images on reasonable timescales.
Methods. As in the original algorithm, scales in the image are estimated
through function fitting. Here we introduce an analytical method to model
extended emission, and a modified method for estimating the initial values used
for the fitting procedure, which ultimately leads to a lower computational
cost.
Results.The new implementation was tested with simulated EVLA data and the
imaging performance compared well with the original Asp-Clean algorithm. Tests
show that the current algorithm can recover features at different scales with
lower computational cost.Comment: 6 pages; 4 figure
Advances in Calibration and Imaging Techniques in Radio Interferometry
This paper summarizes some of the major calibration and image reconstruction
techniques used in radio interferometry and describes them in a common
mathematical framework. The use of this framework has a number of benefits,
ranging from clarification of the fundamentals, use of standard numerical
optimization techniques, and generalization or specialization to new
algorithms
Relic Abundances and the Boltzmann Equation
I discuss the validity of the quantum Boltzmann equation for the calculation
of WIMP relic densities.Comment: 5 pages, no figures; talk given at Dark Matter 2000; an important
reference is added in the revised versio
Confined Jet Impingement with Boiling on a Variety of Enhanced Surfaces
Confined jet impingement with boiling offers unique and attractive performance characteristics for thermal management of high heat flux components. Two-phase operation of jet impingement has been shown to provide high heat transfer coefficients while maintaining a uniform temperature over a target surface. This can be achieved with minimal increases in pumping power compared to single-phase operation. To investigate further enhancements in heat transfer coefficients and increases in the maximum heat flux supported by two-phase jet impingement, an experimental study of surface enhancements is performed using the dielectric working fluid HFE-7100. The performance of a single, 3.75 mm-diameter jet orifice is compared across four distinct copper target surfaces of varying enhancement scales: a baseline smooth flat surface, a flat surface coated with a microporous layer, a surface with macroscale area enhancement (extended square pin fins), and a hybrid surface on which the pin fins are coated with the microporous layer. The heat transfer performance of each surface is compared in single- and two-phase operation at three volumetric flow rates (450 ml/min, 900 ml/min, and 1800 ml/min); area-averaged heat transfer parameters and pressure drop are reported. The mechanisms resulting in enhanced performance for the different surfaces are identified, with a special focus on the coated pin fins. This hybrid surface showed the best enhancement of all those tested, and resulted in an extension of critical heat flux (CHF) by a maximum of 2.42 times compared to the smooth flat surface at the lowest flow rate investigated; no increase in the overall pressure drop was measured
Algebraic characterization of X-states in quantum information
A class of two-qubit states called X-states are increasingly being used to
discuss entanglement and other quantum correlations in the field of quantum
information. Maximally entangled Bell states and "Werner" states are subsets of
them. Apart from being so named because their density matrix looks like the
letter X, there is not as yet any characterization of them. The su(2) X su(2) X
u(1) subalgebra of the full su(4) algebra of two qubits is pointed out as the
underlying invariance of this class of states. X-states are a seven-parameter
family associated with this subalgebra of seven operators. This recognition
provides a route to preparing such states and also a convenient algebraic
procedure for analytically calculating their properties. At the same time, it
points to other groups of seven-parameter states that, while not at first sight
appearing similar, are also invariant under the same subalgebra. And it opens
the way to analyzing invariant states of other subalgebras in bipartite
systems.Comment: 4 pages, 1 figur
Boiling Heat Transfer from an Array of Round Jets with Hybrid Surface Enhancements
The effect of a variety of surface enhancements on the heat transfer achieved with an array of impinging jets is experimentally investigated using the dielectric fluid HFE-7100 at different volumetric flow rates. The performance of a 5x5 array of jets, each 0.75mm in diameter, is compared to that of a single 3.75mm diameter jet with the same total open orifice area, in single-and two-phase operation. Four different target copper surfaces are evaluated: a baseline smooth flat surface, a flat surface coated with a microporous layer, a surface with macroscale area enhancement (extended square pin–fins), and a hybrid surface on which the pin–fins are coated with the microporous layer; area-averaged heat transfer and pressure drop measurements are reported. The array of jets enhances the single-phase heat transfer coefficients by 1.13–1.29 times and extends the critical heat flux (CHF) on all surfaces compared to the single jet at the same volumetric flow rates. Additionally, the array greatly enhances the heat flux dissipation capability of the hybrid coated pin–fin surface, extending CHF by 1.89–2.33 times compared to the single jet on this surface, with a minimal increase in pressure drop. The jet array coupled with the hybrid enhancement dissipates a maximum heat flux of 205.8 W/cm2 (heat input of 1.33 kW) at a flow rate of 1800 ml/min (corresponding to a jet diameter-based Reynolds number of 7800) with a pressure drop incurred of only 10.9 kPa. Compared to the single jet impinging on the smooth flat surface, the array of jets on the coated pin–fin enhanced surface increased CHF by a factor of over four at all flow rates
Local Single- and Two-Phase Heat Transfer from an Impinging Cross-Shaped Jet
Local single- and two-phase heat transfer distributions are measured under a confined impinging jet issuing from a cross-shaped orifice. Spatially resolved temperature maps and convection coefficients resulting from the impinging flow are obtained via infrared imaging of a thin-foil heat source. The cooling patterns in single- and two-phase operation are explained by an accompanying numerical investigation of the fluid flow issuing from the orifice; computed velocity magnitudes and turbulence intensities are presented. In single-phase operation, the coolest surface temperatures correspond to areas with high liquid velocities. High velocities and developing turbulence are also shown to increase convective heat transfer along the diagonal outflow directions from the impinging jet. During two-phase transport, boiling preferentially begins in regions of low velocity, providing enhanced heat transfer in the areas least affected by the impingement. The cross-shaped orifice achieves local heat transfer coefficients that exceed the stagnation-point value of a circular jet of equivalent open orifice area by up to 1.5 times, while resulting in an increased pressure drop only 1.1 times higher than that of the circular jet
Finding counterparts for All-sky X-ray surveys with Nway: a Bayesian algorithm for cross-matching multiple catalogues
We release the AllWISE counterparts and Gaia matches to 106,573 and 17,665
X-ray sources detected in the ROSAT 2RXS and XMMSL2 surveys with |b|>15. These
are the brightest X-ray sources in the sky, but their position uncertainties
and the sparse multi-wavelength coverage until now rendered the identification
of their counterparts a demanding task with uncertain results. New all-sky
multi-wavelength surveys of sufficient depth, like AllWISE and Gaia, and a new
Bayesian statistics based algorithm, NWAY, allow us, for the first time, to
provide reliable counterpart associations. NWAY extends previous distance and
sky density based association methods and, using one or more priors (e.g.,
colors, magnitudes), weights the probability that sources from two or more
catalogues are simultaneously associated on the basis of their observable
characteristics. Here, counterparts have been determined using a WISE
color-magnitude prior. A reference sample of 4524 XMM/Chandra and Swift X-ray
sources demonstrates a reliability of ~ 94.7% (2RXS) and 97.4% (XMMSL2).
Combining our results with Chandra-COSMOS data, we propose a new separation
between stars and AGN in the X-ray/WISE flux-magnitude plane, valid over six
orders of magnitude. We also release the NWAY code and its user manual. NWAY
was extensively tested with XMM-COSMOS data. Using two different sets of
priors, we find an agreement of 96% and 99% with published Likelihood Ratio
methods. Our results were achieved faster and without any follow-up visual
inspection. With the advent of deep and wide area surveys in X-rays (e.g.
SRG/eROSITA, Athena/WFI) and radio (ASKAP/EMU, LOFAR, APERTIF, etc.) NWAY will
provide a powerful and reliable counterpart identification tool.Comment: MNRAS, Paper accepted for publication. Updated catalogs are available
at www.mpe.mpg.de/XraySurveys/2RXS_XMMSL2 . NWAY available at
https://github.com/JohannesBuchner/nwa
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