1,350 research outputs found
Optimal Image Reconstruction in Radio Interferometry
We introduce a method for analyzing radio interferometry data which produces
maps which are optimal in the Bayesian sense of maximum posterior probability
density, given certain prior assumptions. It is similar to maximum entropy
techniques, but with an exact accounting of the multiplicity instead of the
usual approximation involving Stirling's formula. It also incorporates an Occam
factor, automatically limiting the effective amount of detail in the map to
that justified by the data. We use Gibbs sampling to determine, to any desired
degree of accuracy, the multi-dimensional posterior density distribution. From
this we can construct a mean posterior map and other measures of the posterior
density, including confidence limits on any well-defined function of the
posterior map.Comment: 41 pages, 11 figures. High resolution figures 8 and 9 available at
http://www.astro.uiuc.edu/~bwandelt/SuttonWandelt200
Displacement- and Timing-Noise Free Gravitational-Wave Detection
Motivated by a recently-invented scheme of displacement-noise-free
gravitational-wave detection, we demonstrate the existence of
gravitational-wave detection schemes insusceptible to both displacement and
timing (laser) noises, and are thus realizable by shot-noise-limited laser
interferometry. This is possible due to two reasons: first, gravitational waves
and displacement disturbances contribute to light propagation times in
different manners; second, for an N-detector system, the number of signal
channels is of the order O(N^2), while the total number of timing- and
displacement-noise channels is of the order O(N).Comment: 4 pages, 3 figures; mistake correcte
Shape Reconstruction and Weak Lensing Measurement with Interferometers: A Shapelet Approach
We present a new approach for image reconstruction and weak lensing
measurements with interferometers. Based on the shapelet formalism presented in
Refregier (2001), object images are decomposed into orthonormal Hermite basis
functions. The shapelet coefficients of a collection of sources are
simultaneously fit on the uv plane, the Fourier transform of the sky brightness
distribution observed by interferometers. The resulting chi-square fit is
linear in its parameters and can thus be performed efficiently by simple matrix
multiplications. We show how the complex effects of bandwidth smearing, time
averaging and non-coplanarity of the array can be easily and fully corrected
for in our method. Optimal image reconstruction, co-addition, astrometry, and
photometry can all be achieved using weighted sums of the derived coefficients.
As an example we consider the observing conditions of the FIRST radio survey
(Becker et al. 1995; White et al. 1997). We find that our method accurately
recovers the shapes of simulated images even for the sparse uv sampling of this
snapshot survey. Using one of the FIRST pointings, we find our method compares
well with CLEAN, the commonly used method for interferometric imaging. Our
method has the advantage of being linear in the fit parameters, of fitting all
sources simultaneously, and of providing the full covariance matrix of the
coefficients, which allows us to quantify the errors and cross-talk in image
shapes. It is therefore well-suited for quantitative shape measurements which
require high-precision. In particular, we show how our method can be combined
with the results of Refregier & Bacon (2001) to provide an accurate measurement
of weak lensing from interferometric data.Comment: 9 pages, 5 figures. To appear in ApJ; minor changes to match accepted
versio
Anisotropic angular broadening in the solar wind
We present Very Large Array observations at wavelengths of 2, 3.5, 6, and 20 cm, of angular broadening of radio sources due to the solar wind in the region 2-16 solar radii. Angular broadening is anisotropic with axial ratios in the range 2-16. Larger axial ratios are observed preferentially at smaller solar distances. Assuming that anisotropy is due to scattering blobs elongated along magnetic field lines, the distribution of position angles of the elliptically broadened images indicates that the field lines are non-radial even at the largest heliocentric distances observed here. At 5R ⊙, the major axis scattering angle is ~0.7" at λ=6 cm and it varies with heliocentric distance as R -1.6. The level of turbulence, characterized by the wave structure function at a scale of 10 km along the major axis, normalized to λ=20 cm, has a value 20±7 at 5R⊙ and varies with heliocentric distance as R -3. Comprison with earlier results suggest that the level of turbulence is higher during solar maximum. Assuming a power-law spectrum of electron density fluctuations, the fitted spectral exponents have values in the range 2.8-3.4 for scales sizes between 2-35 km. The data suggests temporal fluctuations (of up to 10%) in the spectral exponent on a time scale of a few tens of minutes. The observed structure functions at different solar distances do not show any evidence for an inner scale; the upper limits are 1 km at 2R⊙ and 4 km at 13R ⊙. These upper limits are in conflict with earlier determinations and may suggest a reduced inner scale during solar maximum
A Wide Field, Low Frequency Radio Survey of the Field of M31: I. Construction and Statistical Analysis of the Source Catalog
We present here the results of a 325 MHz radio survey of M31, conducted with
the A-configuration of the Very Large Array. The survey covered an area of 7.6
deg, and a total of 405 radio sources between \la6\arcsec and 170\arcsec
in extent were mapped with a resolution of 6\arcsec and a 1 sensitivity
of 0.6 \mjyb. For each source, its morphological class, major axis
, minor axis , position angle , peak flux ,
integrated flux density , spectral index and spectral curvature
parameter were calculated. A comparison of the flux and radial
distribution -- both in the plane of the sky and in the plane of M31 -- of
these sources with those of the XMM--LSS and WENSS radio surveys revealed that
a vast majority of sources detected are background radio galaxies. As a result
of this analysis, we expect that only a few sources are intrinsic to M31. These
sources are identified and discussed in an accompanying paper.Comment: 29 pages with 4 tables and 10 figures (JPEGs), accepted for
publication in ApJS. Full-resolution images available on reques
On the Enhanced Interstellar Scattering Toward B1849+005
(Abridged) This paper reports new Very Large Array (VLA) and Very Long
Baseline Array (VLBA) observations of the extragalactic source B1849+005 at
frequencies between 0.33 and 15 GHz and the re-analysis of archival VLA
observations at 0.33, 1.5, and 4.9 GHz. The structure of this source is complex
but interstellar scattering dominates the structure of the central component at
least to 15 GHz. An analysis of the phase structure functions of the
interferometric visibilities shows the density fluctuations along this line of
sight to be anisotropic (axial ratio = 1.3) with a frequency-independent
position angle, and having an inner scale of roughly a few hundred kilometers.
The anisotropies occur on length scales of order 10^{15} cm (D/5 kpc), which
within the context of certain magnetohydrodynamic turbulence theories indicates
the length scale on which the kinetic and magnetic energy densities are
comparable. A conservative upper limit on the velocity of the scattering
material is 1800 km/s. In the 0.33 GHz field of view, there are a number of
other sources that might also be heavily scattered. Both B1849+005 and PSR
B1849+00 are highly scattered, and they are separated by only 13'. If the lines
of sight are affected by the same ``clump'' of scattering material, it must be
at least 2.3 kpc distant. However, a detailed attempt to account for the
scattering observables toward these sources does not produce a self-consistent
set of parameters for such a clump. A clump of H\alpha emission, possibly
associated with the H II region G33.418-0.004, lies between these two lines of
sight, but it seems unable to account for all of the required excess
scattering.Comment: 23 pages, LaTeX2e AASTeX, 13 figures in 14 PostScript files, accepted
for publication in Ap
A multiple-beam CLEAN for imaging intra-day variable radio sources
The CLEAN algorithm, widely used in radio interferometry for the
deconvolution of radio images, performs well only if the raw radio image (dirty
image) is, to good approximation, a simple convolution between the instrumental
point-spread function (dirty beam) and the true distribution of emission across
the sky. An important case in which this approximation breaks down is during
frequency synthesis if the observing bandwidth is wide enough for variations in
the spectrum of the sky to become significant. The convolution assumption also
breaks down, in any situation but snapshot observations, if sources in the
field vary significantly in flux density over the duration of the observation.
Such time-variation can even be instrumental in nature, for example due to
jitter or rotation of the primary beam pattern on the sky during an
observation. An algorithm already exists for dealing with the spectral
variation encountered in wide-band frequency synthesis interferometry. This
algorithm is an extension of CLEAN in which, at each iteration, a set of N
`dirty beams' are fitted and subtracted in parallel, instead of just a single
dirty beam as in standard CLEAN. In the wide-band algorithm the beams are
obtained by expanding a nominal source spectrum in a Taylor series, each term
of the series generating one of the beams. In the present paper this algorithm
is extended to images which contain sources which vary over both frequency and
time. Different expansion schemes (or bases) on the time and frequency axes are
compared, and issues such as Gibbs ringing and non-orthogonality are discussed.
It is shown that practical considerations make it often desirable to
orthogonalize the set of beams before commencing the cleaning. This is easily
accomplished via a Gram-Schmidt technique.Comment: 9 pages, 7 figures. Accepted for publication in A&
Influence of symmetry and Coulomb-correlation effects on the optical properties of nitride quantum dots
The electronic and optical properties of self-assembled InN/GaN quantum dots
(QDs) are investigated by means of a tight-binding model combined with
configuration interaction calculations. Tight-binding single particle wave
functions are used as a basis for computing Coulomb and dipole matrix elements.
Within this framework, we analyze multi-exciton emission spectra for two
different sizes of a lens-shaped InN/GaN QD with wurtzite crystal structure.
The impact of the symmetry of the involved electron and hole one-particle
states on the optical spectra is discussed in detail. Furthermore we show how
the characteristic features of the spectra can be interpreted using a
simplified Hamiltonian which provides analytical results for the interacting
multi-exciton complexes. We predict a vanishing exciton and biexciton ground
state emission for small lens-shaped InN/GaN QDs. For larger systems we report
a bright ground state emission but with drastically reduced oscillator
strengths caused by the quantum confined Stark effect.Comment: 15 pages, 17 figure
Development of a unified tensor calculus for the exceptional Lie algebras
The uniformity of the decomposition law, for a family F of Lie algebras which
includes the exceptional Lie algebras, of the tensor powers ad^n of their
adjoint representations ad is now well-known. This paper uses it to embark on
the development of a unified tensor calculus for the exceptional Lie algebras.
It deals explicitly with all the tensors that arise at the n=2 stage, obtaining
a large body of systematic information about their properties and identities
satisfied by them. Some results at the n=3 level are obtained, including a
simple derivation of the the dimension and Casimir eigenvalue data for all the
constituents of ad^3. This is vital input data for treating the set of all
tensors that enter the picture at the n=3 level, following a path already known
to be viable for a_1. The special way in which the Lie algebra d_4 conforms to
its place in the family F alongside the exceptional Lie algebras is described.Comment: 27 pages, LaTeX 2
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