327,593 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
Continuous-variable multipartite unlockable bound entangled Gaussian states
Continuous-variable (CV) multipartite unlockable bound-entangled states is
investigated in this paper. Comparing with the qubit multipartite unlockable
bound-entangled states, CV multipartite unlockable bound-entangled states
present the new and different properties. CV multipartite unlockable
bound-entangled states may serve as a useful quantum resource for new
multiparty communication schemes. The experimental protocol for generating CV
unlockable bound-entangled states is proposed with a setup that is at present
accessible.Comment: 6 pages, 4 figure
Observation of Terahertz Radiation via the Two-Color Laser Scheme with Uncommon Frequency Ratios
In the widely-studied two-color laser scheme for terahertz (THz) radiation
from a gas, the frequency ratio of the two lasers is usually fixed at
1:2. We investigate THz generation with uncommon frequency
ratios. Our experiments show, for the first time, efficient THz generation with
new ratios of 1:4 and 2:3. We observe that the THz
polarization can be adjusted by rotating the longer-wavelength laser
polarization and the polarization adjustment becomes inefficient by rotating
the other laser polarization; the THz energy shows similar scaling laws with
different frequency ratios. These observations are inconsistent with multi-wave
mixing theory, but support the gas-ionization model. This study pushes the
development of the two-color scheme and provides a new dimension to explore the
long-standing problem of the THz generation mechanism.Comment: 6 pages, 3 figure
The Galactic distribution of magnetic fields in molecular clouds and HII regions
{Magnetic fields exist on all scales in our Galaxy. There is a controversy
about whether the magnetic fields in molecular clouds are preserved from the
permeated magnetic fields in the interstellar medium (ISM) during cloud
formation. We investigate this controversy using available data in the light of
the newly revealed magnetic field structure of the Galactic disk obtained from
pulsar rotation measures (RMs).} % {We collected measurements of the magnetic
fields in molecular clouds, including Zeeman splitting data of OH masers in
clouds and OH or HI absorption or emission lines of clouds themselves.} % {The
Zeeman data show structures in the sign distribution of the line-of-sight
component of the magnetic field. Compared to the large-scale Galactic magnetic
fields derived from pulsar RMs, the sign distribution of the Zeeman data shows
similar large-scale field reversals. Previous such examinations were flawed in
the over-simplified global model used for the large-scale magnetic fields in
the Galactic disk.} % {We conclude that the magnetic fields in the clouds may
still ``remember'' the directions of magnetic fields in the Galactic ISM to
some extent, and could be used as complementary tracers of the large-scale
magnetic structure. More Zeeman data of OH masers in widely distributed clouds
are required.}Comment: Typo fixed in this new versio
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