149 research outputs found
A method for spatial deconvolution of spectra
A method for spatial deconvolution of spectra is presented. It follows the
same fundamental principles as the ``MCS image deconvolution algorithm''
(Magain, Courbin, Sohy, 1998) and uses information contained in the spectrum of
a reference Point Spread Function (PSF) to spatially deconvolve spectra of very
blended sources. An improved resolution rather than an infinite one is aimed
at, overcoming the well known problem of ``deconvolution artefacts''. As in the
MCS algorithm, the data are decomposed into a sum of analytical point sources
and a numerically deconvolved background, so that the spectrum of extended
sources in the immediate vicinity of bright point sources may be accurately
extracted and sharpened. The algorithm has been tested on simulated data
including seeing variation as a function of wavelength and atmospheric
refraction. It is shown that the spectra of severely blended point sources can
be resolved while fully preserving the spectrophotometric properties of the
data. Extended objects ``hidden'' by bright point sources (up to 4-5 magnitudes
brighter) can be accurately recovered as well, provided the data have a
sufficiently high total signal-to-noise ratio (200-300 per spectral resolution
element). Such spectra are relatively easy to obtain, even down to faint
magnitudes, within a few hours of integration time with 10m class telescopes.Comment: 18 pages, 6 postscript figures, in press in Ap
Inferring Core-Collapse Supernova Physics with Gravitational Waves
Stellar collapse and the subsequent development of a core-collapse supernova
explosion emit bursts of gravitational waves (GWs) that might be detected by
the advanced generation of laser interferometer gravitational-wave
observatories such as Advanced LIGO, Advanced Virgo, and LCGT. GW bursts from
core-collapse supernovae encode information on the intricate multi-dimensional
dynamics at work at the core of a dying massive star and may provide direct
evidence for the yet uncertain mechanism driving supernovae in massive stars.
Recent multi-dimensional simulations of core-collapse supernovae exploding via
the neutrino, magnetorotational, and acoustic explosion mechanisms have
predicted GW signals which have distinct structure in both the time and
frequency domains. Motivated by this, we describe a promising method for
determining the most likely explosion mechanism underlying a hypothetical GW
signal, based on Principal Component Analysis and Bayesian model selection.
Using simulated Advanced LIGO noise and assuming a single detector and linear
waveform polarization for simplicity, we demonstrate that our method can
distinguish magnetorotational explosions throughout the Milky Way (D <~ 10kpc)
and explosions driven by the neutrino and acoustic mechanisms to D <~ 2kpc.
Furthermore, we show that we can differentiate between models for rotating
accretion-induced collapse of massive white dwarfs and models of rotating iron
core collapse with high reliability out to several kpc.Comment: 22 pages, 9 figure
A Bayesian approach to strong lensing modelling of galaxy clusters
In this paper, we describe a procedure for modelling strong lensing galaxy
clusters with parametric methods, and to rank models quantitatively using the
Bayesian evidence. We use a publicly available Markov chain Monte-Carlo (MCMC)
sampler ('Bayesys'), allowing us to avoid local minima in the likelihood
functions. To illustrate the power of the MCMC technique, we simulate three
clusters of galaxies, each composed of a cluster-scale halo and a set of
perturbing galaxy-scale subhalos. We ray-trace three light beams through each
model to produce a catalogue of multiple images, and then use the MCMC sampler
to recover the model parameters in the three different lensing configurations.
We find that, for typical Hubble Space Telescope (HST)-quality imaging data,
the total mass in the Einstein radius is recovered with ~1-5% error according
to the considered lensing configuration. However, we find that the mass of the
galaxies is strongly degenerated with the cluster mass when no multiple images
appear in the cluster centre. The mass of the galaxies is generally recovered
with a 20% error, largely due to the poorly constrained cut-off radius.
Finally, we describe how to rank models quantitatively using the Bayesian
evidence. We confirm the ability of strong lensing to constrain the mass
profile in the central region of galaxy clusters in this way. Ultimately, such
a method applied to strong lensing clusters with a very large number of
multiple images may provide unique geometrical constraints on cosmology. The
implementation of the MCMC sampler used in this paper has been done within the
framework of the Lenstool software package, which is publicly available.Comment: Accepted to "Gravitational Lensing" Focus Issue of the New Journal of
Physics (invited), 35 pages, 11 figures at reduced resolutio
Annihilation Emission from the Galactic Black Hole
Both diffuse high energy gamma-rays and an extended electron-positron
annihilation line emission have been observed in the Galactic Center (GC)
region. Although X-ray observations indicate that the galactic black hole Sgr
A is inactive now, we suggest that Sgr A can become active when a
captured star is tidally disrupted and matter is accreted into the black hole.
As a consequence the galactic black hole could be a powerful source of
relativistic protons. We are able to explain the current observed diffuse
gamma-rays and the very detailed 511 keV annihilation line of secondary
positrons by collisions of such protons, with appropriate injection times
and energy. Relativistic protons could have been injected into the ambient
material if the black hole captured a 50M star at several tens million
years ago. An alternative possibility is that the black hole continues to
capture stars with 1M every hundred thousand years. Secondary
positrons produced by collisions at energies \ga 30 MeV are cooled down
to thermal energies by Coulomb collisions, and annihilate in the warm neutral
and ionized phases of the interstellar medium with temperatures about several
eV, because the annihilation cross-section reaches its maximum at these
temperatures. It takes about ten million years for the positrons to cool down
to thermal temperatures so they can diffuse into a very large extended region
around the Galactic center. A much more recent star capture may be also able to
account for recent TeV observations within 10 pc of the galactic center as well
as for the unidentified GeV gamma-ray sources found by EGRET at GC. The
spectral difference between the GeV flux and the TeV flux could be explained
naturally in this model as well.Comment: Accepted by ApJ on March 24, 200
The Keck Aperture Masking Experiment: Near-Infrared Sizes of Dusty Wolf-Rayet Stars
We report the results of a high angular resolution near-infrared survey of
dusty Wolf-Rayet stars using the Keck-1 Telescope, including new
multi-wavelength images of the pinwheel nebulae WR 98a, WR 104, and WR 112.
Angular sizes were measured for an additional 8 dusty WR stars using aperture
masking interferometry, allowing us to probe characteristics sizes down to ~20
milliarcseconds (~40 AU for typical sources). With angular sizes and specific
fluxes, we can directly measure the wavelength-dependent surface brightness and
size relations for our sample. We discovered tight correlations of these
properties within our sample which could not be explained by simple
spherically-symmetric dust shells or even the more realistic ``pinwheel
nebula'' (3-D) radiative transfer model, when using optical constants of Zubko.
While the tightly-correlated surface brightness relations we uncovered offer
compelling indirect evidence of a shared and distinctive dust shell geometry
amongst our sample, long-baseline interferometers should target the
marginally-resolved objects in our sample in order to conclusively establish
the presence or absence of the putative underyling colliding wind binaries
thought to produce the dust shells around WC Wolf-Rayets.Comment: Accepted by Astrophysical Journa
Understanding hadronic gamma-ray emission from supernova remnants
We aim to test the plausibility of a theoretical framework in which the
gamma-ray emission detected from supernova remnants may be of hadronic origin,
i.e., due to the decay of neutral pions produced in nuclear collisions
involving relativistic nuclei. In particular, we investigate the effects
induced by magnetic field amplification on the expected particle spectra,
outlining a phenomenological scenario consistent with both the underlying
Physics and the larger and larger amount of observational data provided by the
present generation of gamma experiments, which seem to indicate rather steep
spectra for the accelerated particles. In addition, in order to study to study
how pre-supernova winds might affect the expected emission in this class of
sources, the time-dependent gamma-ray luminosity of a remnant with a massive
progenitor is worked out. Solid points and limitations of the proposed scenario
are finally discussed in a critical way.Comment: 30 pages, 5 figures; Several comments, references and a figure added.
Some typos correcte
Complexity Characterization in a Probabilistic Approach to Dynamical Systems Through Information Geometry and Inductive Inference
Information geometric techniques and inductive inference methods hold great
promise for solving computational problems of interest in classical and quantum
physics, especially with regard to complexity characterization of dynamical
systems in terms of their probabilistic description on curved statistical
manifolds. In this article, we investigate the possibility of describing the
macroscopic behavior of complex systems in terms of the underlying statistical
structure of their microscopic degrees of freedom by use of statistical
inductive inference and information geometry. We review the Maximum Relative
Entropy (MrE) formalism and the theoretical structure of the information
geometrodynamical approach to chaos (IGAC) on statistical manifolds. Special
focus is devoted to the description of the roles played by the sectional
curvature, the Jacobi field intensity and the information geometrodynamical
entropy (IGE). These quantities serve as powerful information geometric
complexity measures of information-constrained dynamics associated with
arbitrary chaotic and regular systems defined on the statistical manifold.
Finally, the application of such information geometric techniques to several
theoretical models are presented.Comment: 29 page
Treating Solar Model Uncertainties: A Consistent Statistical Analysis of Solar Neutrino Models and Data
We describe how to consistently incorporate solar model uncertainties, along
with experimental errors and correlations, when analyzing solar neutrino data
to derive confidence limits on parameter space for proposed solutions of the
solar neutrino problem. Our work resolves ambiguities and inconsistencies in
the previous literature. As an application of our methods we calculate the
masses and mixing angles allowed by the current data for the proposed MSW
solution using both Bayesian and frequentist methods, allowing purely for solar
model flux variations, to compare with previous work. We consider the effects
of including metal diffusion in the solar models and also discuss implications
for future experiments.Comment: 29 pages (incl figs), latex, 6 figures (appended as separate
uuencoded file. To embed figures in text, uncomment 6 \epsfysize lines which
appear before bibliography), CWRU-P5-94, CfPA-94-TTH-29,
Fermilab-Pub-94/176-
The Parker instability under a linear gravity
A linear stability analysis has been done to a magnetized disk under a linear gravity. We have reduced the linearized perturbation equations to a second-order differential equation that resembles the Schrodinger equation with the potential of a harmonic oscillator. Depending on the signs of energy and potential terms, eigensolutions can be classified into ''continuum'' and ''discrete'' families. When the magnetic field is ignored, the continuum family is identified as the convective mode, while the discrete family is identified as acoustic-gravity waves. If the effective adiabatic index gamma is less than unity, the former develops into the convective instability. When a magnetic field is included, the continuum and discrete families further branch into several solutions with different characters. The continuum family is divided into two modes: one is the original Parker mode, which is a slow MHD mode modulated by the gravity, and the other is a stable Alfven mode. The Parker modes can be either stable or unstable depending on gamma. When gamma is smaller than a critical value gamma(cr), the Parker mode becomes unstable. The discrete family is divided into three modes: a stable fast MHD mode modulated by the gravity, a stable slow MHD mode modulated by the gravity, and an unstable mode that is also attributed to a slow MHD mode. The unstable discrete mode does not always exist. Even though the unstable discrete mode exists, the Parker mode dominates it if the Parker mode is unstable. However, if gamma greater than or equal to gamma(cr), then the discrete mode could be the only unstable one. When gamma is equal gamma(cr), the minimum growth time of the unstable discrete mode is 1.3 x 10(8) yr, with a corresponding length scale of 2.4 kpc. It is suggestive that the corrugated features seen in the Galaxy and external galaxies are related to the unstable discrete modeopen131
Exact Expressions for the Critical Mach Numbers in the Two-Fluid Model of Cosmic-Ray Modified Shocks
The acceleration of relativistic particles due to repeated scattering across
a shock wave remains the most attractive model for the production of energetic
cosmic rays. This process has been analyzed extensively during the past two
decades using the ``two-fluid'' model of diffusive shock acceleration. It is
well known that 1, 2, or 3 distinct solutions for the flow structure can be
found depending on the upstream parameters. The precise nature of the critical
conditions delineating the number and character of shock transitions has
remained unclear, mainly due to the inappropriate choice of parameters used in
the determination of the upstream boundary conditions. We derive the exact
critical conditions by reformulating the upstream boundary conditions in terms
of two individual Mach numbers defined with respect to the cosmic-ray and gas
sound speeds, respectively. The gas and cosmic-ray adiabatic indices are
assumed to remain constant throughout the flow, although they may have
arbitrary, independent values. Our results provide for the first time a
complete, analytical classification of the parameter space of shock transitions
in the two-fluid model. When multiple solutions are possible, we propose using
the associated entropy distributions as a means for indentifying the most
stable configuration.Comment: Accepted for publication in ApJ; corrected a few typos; added journal
re
- âŠ