438 research outputs found
Poisson noise removal in multivariate count data
International audienceThe Multi-scale Variance Stabilization Transform (MSVST) has recently been proposed for 2D Poisson data denoising.1 In this work, we present an extension of the MSVST with the wavelet transform to multivariate data-each pixel is vector-valued-, where the vector field dimension may be the wavelength, the energy, or the time. Such data can be viewed naively as 3D data where the third dimension may be time, wavelength or energy (e.g. hyperspectral imaging). But this naive analysis using a 3D MSVST would be awkward as the data dimensions have different physical meanings. A more appropriate approach would be to use a wavelet transform, where the time or energy scale is not connected to the spatial scale. We show that our multivalued extension of MSVST can be used advantageously for approximately Gaussianizing and stabilizing the variance of a sequence of independent Poisson random vectors. This approach is shown to be fast and very well adapted to extremely low-count situations. We use a hypothesis testing framework in the wavelet domain to denoise the Gaussianized and stabilized coefficients, and then apply an iterative reconstruction algorithm to recover the estimated vector field of intensities underlying the Poisson data. Our approach is illustrated for the detection and characterization of astrophysical sources of high-energy gamma rays, using realistic simulated observations. We show that the multivariate MSVST permits efficient estimation across the time/energy dimension and immediate recovery of spectral properties
The spiral structure of our Milky Way Galaxy
The spiral structure of our Milky Way Galaxy is not yet known. HII regions
and giant molecular clouds are the most prominent spiral tracers. We collected
the spiral tracer data of our Milky Way from the literature, namely, HII
regions and giant molecular clouds (GMCs). With weighting factors based on the
excitation parameters of HII regions or the masses of GMCs, we fitted the
distribution of these tracers with models of two, three, four spiral-arms or
polynomial spiral arms. The distances of tracers, if not available from stellar
or direct measurements, were estimated kinetically from the standard rotation
curve of Brand & Blitz (1993) with =8.5 kpc, and =220 km
s or the newly fitted rotation curves with =8.0 kpc and
=220 km s or =8.4 kpc and =254 km s. We
found that the two-arm logarithmic model cannot fit the data in many regions.
The three- and the four-arm logarithmic models are able to connect most
tracers. However, at least two observed tangential directions cannot be matched
by the three- or four-arm model. We composed a polynomial spiral arm model,
which can not only fit the tracer distribution but also match observed
tangential directions. Using new rotation curves with =8.0 kpc and
=220 km s and =8.4 kpc and =254 km s for
the estimation of kinematic distances, we found that the distribution of HII
regions and GMCs can fit the models well, although the results do not change
significantly compared to the parameters with the standard and
.Comment: 34 Pages, 10 Figures, 5 Tables. Accepted for publication in A&A.
Edited
Application of a multidimensional wavelet denoising algorithm for the detection and characterization of astrophysical sources of gamma rays
International audienceZhang, Fadili, & Starck have recently developed a denoising procedure for Poisson data that offers advantages over other methods of intensity estimation in multiple dimensions. Their procedure, which is nonparametric, is based on thresholding wavelet coefficients. The restoration algorithm applied after thresholding provides good conservation of source flux. We present an investigation of the procedure of Zhang et al. for the detection and characterization of astrophysical sources of high-energy gamma rays, using realistic simulated observations with the Large Area Telescope (LAT). The LAT is to be launched in late 2007 on the Gamma-ray Large Area Space Telescope mission. Source detection in the LAT data is complicated by the low fluxes of point sources relative to the diffuse celestial background, the limited angular resolution, and the tremendous variation of that resolution with energy (from tens of degrees at 30 MeV to 0.1◦ at 10 GeV). The algorithm is very fast relative to traditional likelihood model fitting, and permits immediate estimation of spectral properties. Astrophysical sources of gamma rays, especially active galaxies, are typically quite variable, and our current work may lead to a reliable method to quickly characterize the flaring properties of newly-detected sources
Source detection using a 3D sparse representation: application to the Fermi gamma-ray space telescope
The multiscale variance stabilization Transform (MSVST) has recently been
proposed for Poisson data denoising. This procedure, which is nonparametric, is
based on thresholding wavelet coefficients. We present in this paper an
extension of the MSVST to 3D data (in fact 2D-1D data) when the third dimension
is not a spatial dimension, but the wavelength, the energy, or the time. We
show that the MSVST can be used for detecting and characterizing astrophysical
sources of high-energy gamma rays, using realistic simulated observations with
the Large Area Telescope (LAT). The LAT was launched in June 2008 on the Fermi
Gamma-ray Space Telescope mission. The MSVST algorithm is very fast relative to
traditional likelihood model fitting, and permits efficient detection across
the time dimension and immediate estimation of spectral properties.
Astrophysical sources of gamma rays, especially active galaxies, are typically
quite variable, and our current work may lead to a reliable method to quickly
characterize the flaring properties of newly-detected sources.Comment: Accepted. Full paper will figures available at
http://jstarck.free.fr/aa08_msvst.pd
Feasibility of an In-Situ Microbial Decontamination of an Ice-Melting Probe
Autonomous robotic systems for penetrating thick ice shells with simultaneous collecting of scientific data are very promising devices in both terrestrial (glacier, climate research) and extra-terrestrial applications. Technical challenges in development of such systems are numerous and include 3D-navigation, an appropriate energy source, motion control, etc. Not less important is the problem of forward contamination of the pristine glacial environments with microorganisms and biomolecules from the surface of the probe. This study was devoted to establishing a laboratory model for microbial contamination of a newlyconstructed
ice-melting probe called IceMole and to analyse the viability and amount of the contaminating microorganisms as a function of distance. The used bacterial strains were Bacillus subtilis (ATCC 6051) and Escherichia coli (ATCC 11775). The main objective was development of an efficient and reliable in-situ decontamination method of the melting probe. Therefore, several chemical substances were tested in respect of their efficacy to eliminate bacteria on the surface of the melting probe at low temperature (0 - 5 °C) and at continuous dilution by melted water. Our study has shown that at least 99.9% decontamination of the IceMole can be successfully achieved by the injection of 30% (v/v) hydrogen peroxide and 3% (v/v) sodium hypochlorite into the drilling site. We were able to reproduce this result in both time-dependent and depth-dependent experiments. The sufficient amount of 30% (v/v) H2O2 or 3% (v/v) NaClO has been found to be approximately 18 L per cm² of the probe’s surface
GLAST: Understanding the High Energy Gamma-Ray Sky
We discuss the ability of the GLAST Large Area Telescope (LAT) to identify,
resolve, and study the high energy gamma-ray sky. Compared to previous
instruments the telescope will have greatly improved sensitivity and ability to
localize gamma-ray point sources. The ability to resolve the location and
identity of EGRET unidentified sources is described. We summarize the current
knowledge of the high energy gamma-ray sky and discuss the astrophysics of
known and some prospective classes of gamma-ray emitters. In addition, we also
describe the potential of GLAST to resolve old puzzles and to discover new
classes of sources.Comment: To appear in Cosmic Gamma Ray Sources, Kluwer ASSL Series, Edited by
K.S. Cheng and G.E. Romer
A luminosity constraint on the origin of unidentified high energy sources
The identification of point sources poses a great challenge for the high
energy community. We present a new approach to evaluate the likelihood of a set
of sources being a Galactic population based on the simple assumption that
galaxies similar to the Milky Way host comparable populations of gamma-ray
emitters. We propose a luminosity constraint on Galactic source populations
which complements existing approaches by constraining the abundance and spatial
distribution of any objects of Galactic origin, rather than focusing on the
properties of a specific candidate emitter. We use M31 as a proxy for the Milky
Way, and demonstrate this technique by applying it to the unidentified EGRET
sources. We find that it is highly improbable that the majority of the
unidentified EGRET sources are members of a Galactic halo population (e.g.,
dark matter subhalos), but that current observations do not provide any
constraints on all of these sources being Galactic objects if they reside
entirely in the disk and bulge. Applying this method to upcoming observations
by the Fermi Gamma-ray Space Telescope has the potential to exclude association
of an even larger number of unidentified sources with any Galactic source
class.Comment: 18 pages, 4 figures, to appear in JPhys
HELIUM PHOTODISINTEGRATION AND NUCLEOSYNTHESIS: IMPLICATIONS FOR TOPOLOGICAL DEFECTS, HIGH ENERGY COSMIC RAYS, AND MASSIVE BLACK HOLES
We consider the production of He and H by He photodisintegration
initiated by non-thermal energy releases during early cosmic epochs. We find
that this process cannot be the predominant source of primordial H since it
would result in anomalously high He/D ratios in conflict with standard
chemical evolution assumptions. We apply this fact to constrain topological
defect models of highest energy cosmic ray (HECR) production. Such models have
been proposed as possible sources of ultrahigh energy particles and gamma-rays
with energies above eV. The constraints on these models derived from
He-photodisintegration are compared to corresponding limits from spectral
distortions of the cosmic microwave background radiation (CMBR) and from the
observed diffuse gamma-ray background. It is shown that for reasonable primary
particle injection spectra superconducting cosmic strings, unlike ordinary
strings or annihilating monopoles, cannot produce the HECR flux at the present
epoch without violating at least the He-photodisintegration bound. The
constraint from the diffuse gamma-ray background rules out the dominant
production of HECR by the decay of Grand Unification particles in models with
cosmological evolution assuming standard fragmentation functions. Constraints
on massive black hole induced photodisintegration are also discussed.Comment: 20 latex pages, 1 figure added via figures comman
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