374 research outputs found
Segmentation of Loops from Coronal EUV Images
We present a procedure which extracts bright loop features from solar EUV
images. In terms of image intensities, these features are elongated ridge-like
intensity maxima. To discriminate the maxima, we need information about the
spatial derivatives of the image intensity. Commonly, the derivative estimates
are strongly affected by image noise. We therefore use a regularized estimation
of the derivative which is then used to interpolate a discrete vector field of
ridge points ``ridgels'' which are positioned on the ridge center and have the
intrinsic orientation of the local ridge direction. A scheme is proposed to
connect ridgels to smooth, spline-represented curves which fit the observed
loops. Finally, a half-automated user interface allows one to merge or split,
eliminate or select loop fits obtained form the above procedure. In this paper
we apply our tool to one of the first EUV images observed by the SECCHI
instrument onboard the recently launched STEREO spacecraft. We compare the
extracted loops with projected field lines computed from
almost-simultaneously-taken magnetograms measured by the SOHO/MDI Doppler
imager. The field lines were calculated using a linear force-free field model.
This comparison allows one to verify faint and spurious loop connections
produced by our segmentation tool and it also helps to prove the quality of the
magnetic-field model where well-identified loop structures comply with
field-line projections. We also discuss further potential applications of our
tool such as loop oscillations and stereoscopy.Comment: 13 pages, 9 figures, Solar Physics, online firs
A Compressed Sensing Approach to 3D Weak Lensing
(Abridged) Weak gravitational lensing is an ideal probe of the dark universe.
In recent years, several linear methods have been developed to reconstruct the
density distribution in the Universe in three dimensions, making use of
photometric redshift information to determine the radial distribution of lensed
sources. In this paper, we aim to address three key issues seen in these
methods; namely, the bias in the redshifts of detected objects, the line of
sight smearing seen in reconstructions, and the damping of the amplitude of the
reconstruction relative to the underlying density. We consider the problem
under the framework of compressed sensing (CS). Under the assumption that the
data are sparse in an appropriate dictionary, we construct a robust estimator
and employ state-of-the-art convex optimisation methods to reconstruct the
density contrast. For simplicity in implementation, and as a proof of concept
of our method, we reduce the problem to one-dimension, considering the
reconstruction along each line of sight independently. Despite the loss of
information this implies, we demonstrate that our method is able to accurately
reproduce cluster haloes up to a redshift of z=1, deeper than state-of-the-art
linear methods. We directly compare our method with these linear methods, and
demonstrate minimal radial smearing and redshift bias in our reconstructions,
as well as a reduced damping of the reconstruction amplitude as compared to the
linear methods. In addition, the CS framework allows us to consider an
underdetermined inverse problem, thereby allowing us to reconstruct the density
contrast at finer resolution than the input data.Comment: Submitted to A&A (6 July 2011
The relationship between substructure in 2D X-ray surface brightness images and weak lensing mass maps of galaxy clusters: A simulation study
In this paper, we undertake a study to determine what insight can be reliably
gleaned from the comparison of the X-ray and the weak lensing mass maps of
galaxy clusters. We do this by investigating the 2D substructure within three
high-resolution cosmological simulations of galaxy clusters. Our main results
focus on non-radiative gas dynamics, but we also consider the effects of
radiative cooling at high redshift. For our analysis, we use a novel approach,
based on unsharp-masking, to identify substructures in 2D surface mass density
and X-ray surface brightness maps. At full resolution (~ 15 h^-1 kpc), this
technique is capable of identifying almost all self-bound dark matter subhaloes
with M>10^12 h^-1 M_sun. We also report a correlation between the mass of a
subhalo and the area of its corresponding 2D detection; such a correlation,
once calibrated, could provide a useful estimator for substructure mass.
Comparing our 2D mass and X-ray substructures, we find a surprising number of
cases where the matching fails: around one third of galaxy-sized substructures
have no X-ray counterpart. Some interesting cases are also found at larger
masses, in particular the cores of merging clusters where the situation can be
complex. Finally, we degrade our mass maps to what is currently achievable with
weak-lensing observations (~100 h^-1 kpc at z=0.2). While the completeness mass
limit increases by around an order of magnitude, a mass-area correlation
remains. Our paper clearly demonstrates that the next generation of lensing
surveys should start to reveal a wealth of information on cluster substructure.
(Abridged)Comment: 30 pages, 27 figures, 3 tables. Accepted for publication in MNRAS.
High resolution version available at
http://www.physics.ox.ac.uk/users/powell/clustersubs_highres.pd
Probing the accelerating Universe with radio weak lensing in the JVLA Sky Survey
We outline the prospects for performing pioneering radio weak gravitational
lensing analyses using observations from a potential forthcoming JVLA Sky
Survey program. A large-scale survey with the JVLA can offer interesting and
unique opportunities for performing weak lensing studies in the radio band, a
field which has until now been the preserve of optical telescopes. In
particular, the JVLA has the capacity for large, deep radio surveys with
relatively high angular resolution, which are the key characteristics required
for a successful weak lensing study. We highlight the potential advantages and
unique aspects of performing weak lensing in the radio band. In particular, the
inclusion of continuum polarisation information can greatly reduce noise in
weak lensing reconstructions and can also remove the effects of intrinsic
galaxy alignments, the key astrophysical systematic effect that limits weak
lensing at all wavelengths. We identify a VLASS "deep fields" program (total
area ~10-20 square degs), to be conducted at L-band and with high-resolution
(A-array configuration), as the optimal survey strategy from the point of view
of weak lensing science. Such a survey will build on the unique strengths of
the JVLA and will remain unsurpassed in terms of its combination of resolution
and sensitivity until the advent of the Square Kilometre Array. We identify the
best fields on the JVLA-accessible sky from the point of view of overlapping
with existing deep optical and near infra-red data which will provide crucial
redshift information and facilitate a host of additional compelling
multi-wavelength science.Comment: Submitted in response to NRAO's recent call for community white
papers on the VLA Sky Survey (VLASS
The C313Y Piedmontese mutation decreases myostatin covalent dimerisation and stability
<p>Abstract</p> <p>Background</p> <p>Myostatin is a key negative regulator of muscle growth and development, whose activity has important implications for the treatment of muscle wastage disorders. Piedmontese cattle display a double-muscled phenotype associated with the expression of C313Y mutant myostatin. <it>In vivo</it>, C313Y myostatin is proteolytically processed, exported and circulated extracellularly but fails to correctly regulate muscle growth. The C313Y mutation removes the C313-containing disulphide bond, an integral part of the characteristic TGF-β cystine-knot structural motif.</p> <p>Results</p> <p>Here we present <it>in vitro </it>analysis of the structure and stability of the C313Y myostatin protein that reveals significantly decreased covalent dimerisation for C313Y myostatin accompanied by a loss of structural stability compared to wild type. The C313Y myostatin growth factor, processed from full length precursor protein, fails to inhibit C2C12 myoblast proliferation in contrast to wild type myostatin. Although structural modeling shows the substitution of tyrosine causes structural perturbation, biochemical analysis of additional disulphide mutants, C313A and C374A, indicates that an intact cystine-knot motif is a major determinant in myostatin growth factor stability and covalent dimerisation.</p> <p>Conclusions</p> <p>This research shows that the cystine-knot structure is important for myostatin dimerisation and stability, and that disruption of this structural motif perturbs myostatin signaling.</p
ISOCAM survey and dust models of 3CR radio galaxies and quasars
We present a survey of all 3CR sources imaged with ISOCAM onboard the
Infrared Space Observatory (ISO). For the source, we present spatially
integrated mid--infrared (MIR, 5 - 18mic.) fluxes measured from newly
calibrated ISOCAM images. In total, we detected 68 AGN of the 3CR catalogue, at
redshifts z < 2.5. The one with the highest redshift is 4C+72.26 at z = 3.53.
ISOCAM data are combined with other photometric measurements to construct the
spectral energy distribution (SED) from optical to radio wavelengths. In order
to describe dust emission we apply new radiative transfer models. By varying
three parameters, luminosity, effective size and extinction, we obtain a fit to
the SED for our objects. In the MIR the hot dust component is mainly due to
small grains and PAHs. In the models, a type 1 AGN is represented by a compact
dust distribution, the dust is therefore very warm and emission of PAHs is weak
because of photo--destruction. In AGNs of type 2, the dust is relatively colder
but PAH bands are strong.Comment: Accepted by A&A, 30pages, 10 Figures, available at:
http://www.eso.org/~rsiebenm/FTP/3CRisocam.pd
Low Complexity Regularization of Linear Inverse Problems
Inverse problems and regularization theory is a central theme in contemporary
signal processing, where the goal is to reconstruct an unknown signal from
partial indirect, and possibly noisy, measurements of it. A now standard method
for recovering the unknown signal is to solve a convex optimization problem
that enforces some prior knowledge about its structure. This has proved
efficient in many problems routinely encountered in imaging sciences,
statistics and machine learning. This chapter delivers a review of recent
advances in the field where the regularization prior promotes solutions
conforming to some notion of simplicity/low-complexity. These priors encompass
as popular examples sparsity and group sparsity (to capture the compressibility
of natural signals and images), total variation and analysis sparsity (to
promote piecewise regularity), and low-rank (as natural extension of sparsity
to matrix-valued data). Our aim is to provide a unified treatment of all these
regularizations under a single umbrella, namely the theory of partial
smoothness. This framework is very general and accommodates all low-complexity
regularizers just mentioned, as well as many others. Partial smoothness turns
out to be the canonical way to encode low-dimensional models that can be linear
spaces or more general smooth manifolds. This review is intended to serve as a
one stop shop toward the understanding of the theoretical properties of the
so-regularized solutions. It covers a large spectrum including: (i) recovery
guarantees and stability to noise, both in terms of -stability and
model (manifold) identification; (ii) sensitivity analysis to perturbations of
the parameters involved (in particular the observations), with applications to
unbiased risk estimation ; (iii) convergence properties of the forward-backward
proximal splitting scheme, that is particularly well suited to solve the
corresponding large-scale regularized optimization problem
Fermi Discovery of Gamma-Ray Emission from NGC 1275
We report the discovery of high-energy (E>100 MeV) gamma-ray emission from
NGC 1275, a giant elliptical galaxy lying at the center of the Perseus cluster
of galaxies, based on observations made with the Large Area Telescope (LAT) of
the Fermi Gamma ray Space Telescope. The positional center of the gamma-ray
source is only ~3' away from the NGC 1275 nucleus, well within the 95% LAT
error circle of ~5'.The spatial distribution of gamma-ray photons is consistent
with a point source. The average flux and power-law photon index measured with
the LAT from 2008 August 4 to 2008 December 5 are F_gamma = (2.10+-0.23)x
10^{-7} ph (>100 MeV) cm^{-2} s^{-1} and Gamma = 2.17+-0.05, respectively. The
measurements are statistically consistent with constant flux during the
four-month LAT observing period.Previous EGRET observations gave an upper limit
of F_gamma 100 MeV) cm^{-2} s^{-1} to the gamma-ray flux
from NGC 1275. This indicates that the source is variable on timescales of
years to decades, and therefore restricts the fraction of emission that can be
produced in extended regions of the galaxy cluster. Contemporaneous and
historical radio observations are also reported. The broadband spectrum of NGC
1275 is modeled with a simple one-zone synchrotron/synchrotron self-Compton
model and a model with a decelerating jet flow.Comment: 27 pages, 7 figures, Accepted for publication in the Astrophysical
Journa
Bright AGN Source List from the First Three Months of the Fermi Large Area Telescope All-Sky Survey
The first three months of sky-survey operation with the Fermi Gamma Ray Space
Telescope (Fermi) Large Area Telescope (LAT) reveals 132 bright sources at
|b|>10 deg with test statistic greater than 100 (corresponding to about 10
sigma). Two methods, based on the CGRaBS, CRATES and BZCat catalogs, indicate
high-confidence associations of 106 of these sources with known AGNs. This
sample is referred to as the LAT Bright AGN Sample (LBAS). It contains two
radio galaxies, namely Centaurus A and NGC 1275, and 104 blazars consisting of
57 flat spectrum radio quasars (FSRQs), 42 BL Lac objects, and 5 blazars with
uncertain classification. Four new blazars were discovered on the basis of the
LAT detections. Remarkably, the LBAS includes 10 high-energy peaked BL Lacs
(HBLs), sources which were so far hard to detect in the GeV range. Another 10
lower-confidence associations are found. Only thirty three of the sources, plus
two at |b|>10 deg, were previously detected with EGRET, probably due to the
variable nature of these sources. The analysis of the gamma-ray properties of
the LBAS sources reveals that the average GeV spectra of BL Lac objects are
significantly harder than the spectra of FSRQs. No significant correlation
between radio and peak gamma-ray fluxes is observed. Blazar log N - log S and
luminosity functions are constructed to investigate the evolution of the
different blazar classes, with positive evolution indicated for FSRQs but none
for BLLacs. The contribution of LAT-blazars to the total extragalactic
gamma-ray intensity is estimated.Comment: Submitted to ApJ. Not yet refereed. 61 pages, 26 figure
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