818 research outputs found
Poisson noise reduction with non-local PCA
Photon-limited imaging arises when the number of photons collected by a
sensor array is small relative to the number of detector elements. Photon
limitations are an important concern for many applications such as spectral
imaging, night vision, nuclear medicine, and astronomy. Typically a Poisson
distribution is used to model these observations, and the inherent
heteroscedasticity of the data combined with standard noise removal methods
yields significant artifacts. This paper introduces a novel denoising algorithm
for photon-limited images which combines elements of dictionary learning and
sparse patch-based representations of images. The method employs both an
adaptation of Principal Component Analysis (PCA) for Poisson noise and recently
developed sparsity-regularized convex optimization algorithms for
photon-limited images. A comprehensive empirical evaluation of the proposed
method helps characterize the performance of this approach relative to other
state-of-the-art denoising methods. The results reveal that, despite its
conceptual simplicity, Poisson PCA-based denoising appears to be highly
competitive in very low light regimes.Comment: erratum: Image man is wrongly name pepper in the journal versio
Dark Matter Search Using XMM-Newton Observations of Willman 1
We report the results of a search for an emission line from radiatively
decaying dark matter in the ultra-faint dwarf spheroidal galaxy Willman 1 based
on analysis of spectra extracted from XMM-Newton X-ray Observatory data. The
observation follows up our analysis of Chandra data of Willman 1 that resulted
in line flux upper limits over the Chandra bandpass and evidence of a 2.5 keV
feature at a significance below the 99% confidence threshold used to define the
limits. The higher effective area of the XMM-Newton detectors, combined with
application of recently developing methods for extended-source analysis, allow
us to derive improved constraints on the combination of mass and mixing angle
of the sterile neutrino dark matter candidate. We do not confirm the Chandra
evidence for a 2.5 keV emission line.Comment: 23 pages, including 17 figures; accepted for publication in Ap
Invertibility of Multi-Energy X-ray Transform
Purpose: The goal is to provide a sufficient condition on the invertibility
of a multi-energy (ME) X-ray transform. The energy-dependent X-ray attenuation
profiles can be represented by a set of coefficients using the Alvarez-Macovski
(AM) method. An ME X-ray transform is a mapping from AM coefficients to
noise-free energy-weighted measurements, where .
Methods: We apply a general invertibility theorem which tests whether the
Jacobian of the mapping has zero values over the support of the
mapping. The Jacobian of an arbitrary ME X-ray transform is an integration over
all spectral measurements. A sufficient condition of for
all is that the integrand of is (or )
everywhere. Note that the trivial case of the integrand equals to zero
everywhere is ignored. With symmetry, we simplified the integrand of the
Jacobian into three factors that are determined by the total attenuation, the
basis functions, and the energy-weighting functions, respectively. The factor
related to total attenuation is always positive, hence the invertibility of the
X-ray transform can be determined by testing the signs of the other two
factors. Furthermore, we use the Cramer-Rao lower bound (CRLB) to characterize
the noise-induced estimation uncertainty and provide a maximum-likelihood (ML)
estimator.
Conclusions: We have provided a framework to study the invertibility of an
arbitrary ME X-ray transform and proved the global invertibility for four types
of systems
The Effects of Extending the Spectral Information Acquired by a Photon-counting Detector for Spectral CT
Photon-counting x-ray detectors with pulse-height analysis provide spectral information that may improve material decomposition and contrast-to-noise ratio (CNR) in CT images. The number of energy measurements that can be acquired simultaneously on a detector pixel is equal to the number of comparator channels. Some spectral CT designs have a limited number of comparator channels, due to the complexity of readout electronics. The spectral information could be extended by changing the comparator threshold levels over time, sub pixels, or view angle. However, acquiring more energy measurements than comparator channels increases the noise and/or dose, due to differences in noise correlations across energy measurements and decreased dose utilisation. This study experimentally quantified the effects of acquiring more energy measurements than comparator channels using a bench-top spectral CT system. An analytical and simulation study modeling an ideal detector investigated whether there was a net benefit for material decomposition or optimal energy weighting when acquiring more energy measurements than comparator channels. Experimental results demonstrated that in a two-threshold acquisition, acquiring the high-energy measurement independently from the low-energy measurement increased noise standard deviation in material-decomposition basis images by factors of 1.5–1.7 due to changes in covariance between energy measurements. CNR in energy-weighted images decreased by factors of 0.92–0.71. Noise standard deviation increased by an additional factor of due to reduced dose utilisation. The results demonstrated no benefit for two-material decomposition noise or energy-weighted CNR when acquiring more energy measurements than comparator channels. Understanding the noise penalty of acquiring more energy measurements than comparator channels is important for designing spectral detectors and for designing experiments and interpreting data from prototype systems with a limited number of comparator channels
A spatial likelihood analysis for MAGIC telescope data
Context. The increase in sensitivity of Imaging Atmospheric Cherenkov
Telescopes (IACTs) has lead to numerous detections of extended -ray
sources at TeV energies, sometimes of sizes comparable to the instrument's
field of view (FoV). This creates a demand for advanced and flexible data
analysis methods, able to extract source information by utilising the photon
counts in the entire FoV.
Aims. We present a new software package, "SkyPrism", aimed at performing 2D
(3D if energy is considered) fits of IACT data, possibly containing multiple
and extended sources, based on sky images binned in energy. Though the
development of this package was focused on the analysis of data collected with
the MAGIC telescopes, it can further be adapted to other instruments, such as
the future Cherenkov Telescope Array (CTA).
Methods. We have developed a set of tools that, apart from sky images (count
maps), compute the instrument response functions (IRFs) of MAGIC (effective
exposure throughout the FoV, point spread function (PSF), energy resolution and
background shape), based on the input data, Monte-Carlo simulations and the
pointing track of the telescopes. With this information, the presented package
can perform a simultaneous maximum likelihood fit of source models of arbitrary
morphology to the sky images providing energy spectra, detection significances,
and upper limits.
Results. We demonstrate that the SkyPrism tool accurately reconstructs the
MAGIC PSF, on and off-axis performance as well as the underlying background. We
further show that for a point source analysis with MAGIC's default
observational settings, SkyPrism gives results compatible with those of the
standard tools while being more flexible and widely applicable.Comment: 13 pages, 10 figure
Cold fronts and multi-temperature structures in the core of Abell 2052
The physics of the coolest phases in the hot Intra-Cluster Medium (ICM) of
clusters of galaxies is yet to be fully unveiled. X-ray cavities blown by the
central Active Galactic Nucleus (AGN) contain enough energy to heat the
surrounding gas and stop cooling, but locally blobs or filaments of gas appear
to be able to cool to low temperatures of 10^4 K. In X-rays, however, gas with
temperatures lower than 0.5 keV is not observed. Using a deep XMM-Newton
observation of the cluster of galaxies Abell 2052, we derive 2D maps of the
temperature, entropy, and iron abundance in the core region. About 130 kpc
South-West of the central galaxy, we discover a discontinuity in the surface
brightness of the hot gas which is consistent with a cold front. Interestingly,
the iron abundance jumps from ~0.75 to ~0.5 across the front. In a smaller
region to the North-West of the central galaxy we find a relatively high
contribution of cool 0.5 keV gas, but no X-ray emitting gas is detected below
that temperature. However, the region appears to be associated with much cooler
H-alpha filaments in the optical waveband. The elliptical shape of the cold
front in the SW of the cluster suggests that the front is caused by sloshing of
the hot gas in the clusters gravitational potential. This effect is probably an
important mechanism to transport metals from the core region to the outer parts
of the cluster. The smooth temperature profile across the sharp jump in the
metalicity indicates the presence of heat conduction and the lack of mixing
across the discontinuity. The cool blob of gas NW of the central galaxy was
probably pushed away from the core and squeezed by the adjacent bubble, where
it can cool efficiently and relatively undisturbed by the AGN. Shock induced
mixing between the two phases may cause the 0.5 keV gas to cool non-radiatively
and explain our non-detection of gas below 0.5 keV.Comment: 11 pages, 9 figures, A&A, in pres
What Fraction of Gravitational Lens Galaxies Lie in Groups?
We predict how the observed variations in galaxy populations with environment
affect the number and properties of gravitational lenses in different
environments. Two trends dominate: lensing strongly favors early-type galaxies,
which tend to lie in dense environments, but dense environments tend to have a
larger ratio of dwarf to giant galaxies than the field. The two effects nearly
cancel, and the distribution of environments for lens and non-lens galaxies are
not substantially different (lens galaxies are slightly less likely than
non-lens galaxies to lie in groups and clusters). We predict that about 20% of
lens galaxies are in bound groups (defined as systems with a line-of-sight
velocity dispersion sigma in the range 200 < sigma < 500 km/s), and another
roughly 3% are in rich clusters (sigma > 500 km/s). Therefore at least roughly
25% of lenses are likely to have environments that significantly perturb the
lensing potential. If such perturbations do not significantly increase the
image separation, we predict that lenses in groups have a mean image separation
that is about 0.2'' smaller than that for lenses in the field and estimate that
20-40 lenses in groups are required to test this prediction with significance.
The tail of the distribution of image separations is already illuminating.
Although lensing by galactic potential wells should rarely produce lenses with
image separations theta >~ 6'', two such lenses are seen among 49 known lenses,
suggesting that environmental perturbations of the lensing potential can be
significant. Further comparison of theory and data will offer a direct probe of
the dark halos of galaxies and groups and reveal the extent to which they
affect lensing estimates of cosmological parameters.Comment: 32 pages, 6 embedded figures; accepted for publication in Ap
Hard X-ray imaging facility for space shuttle: A scientific and conceptual engineering study
A shuttle-accommodated instrument for imaging hard X-rays in the study of nonthermal particles and high temperature particles in various solar and cosmic phenomena was defined and its feasibility demonstrated. The imaging system configuration is described as well as the electronics, aspect systems, mechanical and thermal properties and the ground support equipment
Innovations in the Analysis of Chandra-ACIS Observations
As members of the instrument team for the Advanced CCD Imaging Spectrometer
(ACIS) on NASA's Chandra X-ray Observatory and as Chandra General Observers, we
have developed a wide variety of data analysis methods that we believe are
useful to the Chandra community, and have constructed a significant body of
publicly-available software (the ACIS Extract package) addressing important
ACIS data and science analysis tasks. This paper seeks to describe these data
analysis methods for two purposes: to document the data analysis work performed
in our own science projects, and to help other ACIS observers judge whether
these methods may be useful in their own projects (regardless of what tools and
procedures they choose to implement those methods).
The ACIS data analysis recommendations we offer here address much of the
workflow in a typical ACIS project, including data preparation, point source
detection via both wavelet decomposition and image reconstruction, masking
point sources, identification of diffuse structures, event extraction for both
point and diffuse sources, merging extractions from multiple observations,
nonparametric broad-band photometry, analysis of low-count spectra, and
automation of these tasks. Many of the innovations presented here arise from
several, often interwoven, complications that are found in many Chandra
projects: large numbers of point sources (hundreds to several thousand), faint
point sources, misaligned multiple observations of an astronomical field, point
source crowding, and scientifically relevant diffuse emission.Comment: Accepted by the ApJ, 2010 Mar 10 (\#343576) 39 pages, 16 figure
A Comparison of X-ray and Strong Lensing Properties of Simulated X-ray Clusters
We use gas-dynamical simulations of galaxy clusters to compare their X-ray
and strong lensing properties. Special emphasis is laid on mass estimates. The
cluster masses range between 6 x 10^14 solar masses and 4 x 10^15 solar masses,
and they are examined at redshifts between 1 and 0. We compute the X-ray
emission of the intracluster gas by thermal bremsstrahlung, add background
contamination, and mimic imaging and spectral observations with current X-ray
telescopes. Although the beta model routinely provides excellent fits to the
X-ray emission profiles, the derived masses are typically biased low because of
the restricted range of radii within which the fit can be done. For beta values
of ~ 2/3, which is the average in our numerically simulated sample, the mass is
typically underestimated by ~ 40 per cent. The masses of clusters which exhibit
pronounced substructure are often substantially underestimated. We suggest that
the ratio between peak temperature and emission-weighted average cluster
temperature may provide a good indicator for ongoing merging and, therefore,
for unreliable mass estimates. X-ray mass estimates are substantially improved
if we fit a King density profile rather than the beta model to the X-ray
emission, thereby dropping the degree of freedom associated with beta. Clusters
selected for their strong lensing properties are typically dynamically more
active than typical clusters. Bulk flows in the intracluster gas contain a
larger than average fraction of the internal energy of the gas in such objects,
hence the measured gas temperatures are biased low. The bulk of the optical
depth for arc formation is contributed by clusters with intermediate rather
than high X-ray luminosity. Arcs occur predominantly in clusters which exhibit
substructure and are not in an equilibrium state. Finally we explain why theComment: 22 pages including figures, submitted to MNRA
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