5,993 research outputs found
Multidimensional relativistic MHD simulations of Pulsar Wind Nebulae: dynamics and emission
Pulsar Wind Nebulae, and the Crab nebula in particular, are the best cosmic
laboratories to investigate the dynamics of magnetized relativistic outflows
and particle acceleration up to PeV energies. Multidimensional MHD modeling by
means of numerical simulations has been very successful at reproducing, to the
very finest details, the innermost structure of these synchrotron emitting
nebulae, as observed in the X-rays. Therefore, the comparison between the
simulated source and observations can be used as a powerful diagnostic tool to
probe the physical conditions in pulsar winds, like their composition,
magnetization, and degree of anisotropy. However, in spite of the wealth of
observations and of the accuracy of current MHD models, the precise mechanisms
for magnetic field dissipation and for the acceleration of the non-thermal
emitting particles are mysteries still puzzling theorists to date. Here we
review the methodologies of the computational approach to the modeling of
Pulsar Wind Nebulae, discussing the most relevant results and the recent
progresses achieved in this fascinating field of high-energy astrophysics.Comment: 29 pages review, preliminary version. To appear in the book
"Modelling Nebulae" edited by D. Torres for Springer, based on the invited
contributions to the workshop held in Sant Cugat (Barcelona), June 14-17,
201
Detection of dirt impairments from archived film sequences : survey and evaluations
Film dirt is the most commonly encountered artifact in archive restoration applications. Since dirt usually appears as a temporally impulsive event, motion-compensated interframe processing is widely applied for its detection. However, motion-compensated prediction requires a high degree of complexity and can be unreliable when motion estimation fails. Consequently, many techniques using spatial or spatiotemporal filtering without motion were also been proposed as alternatives. A comprehensive survey and evaluation of existing methods is presented, in which both qualitative and quantitative performances are compared in terms of accuracy, robustness, and complexity. After analyzing these algorithms and identifying their limitations, we conclude with guidance in choosing from these algorithms and promising directions for future research
A Hybrid Godunov Method for Radiation Hydrodynamics
From a mathematical perspective, radiation hydrodynamics can be thought of as
a system of hyperbolic balance laws with dual multiscale behavior (multiscale
behavior associated with the hyperbolic wave speeds as well as multiscale
behavior associated with source term relaxation). With this outlook in mind,
this paper presents a hybrid Godunov method for one-dimensional radiation
hydrodynamics that is uniformly well behaved from the photon free streaming
(hyperbolic) limit through the weak equilibrium diffusion (parabolic) limit and
to the strong equilibrium diffusion (hyperbolic) limit. Moreover, one finds
that the technique preserves certain asymptotic limits. The method incorporates
a backward Euler upwinding scheme for the radiation energy density and flux as
well as a modified Godunov scheme for the material density, momentum density,
and energy density. The backward Euler upwinding scheme is first-order accurate
and uses an implicit HLLE flux function to temporally advance the radiation
components according to the material flow scale. The modified Godunov scheme is
second-order accurate and directly couples stiff source term effects to the
hyperbolic structure of the system of balance laws. This Godunov technique is
composed of a predictor step that is based on Duhamel's principle and a
corrector step that is based on Picard iteration. The Godunov scheme is
explicit on the material flow scale but is unsplit and fully couples matter and
radiation without invoking a diffusion-type approximation for radiation
hydrodynamics. This technique derives from earlier work by Miniati & Colella
2007. Numerical tests demonstrate that the method is stable, robust, and
accurate across various parameter regimes.Comment: accepted for publication in Journal of Computational Physics; 61
pages, 15 figures, 11 table
2D Phase Unwrapping via Graph Cuts
Phase imaging technologies such as interferometric synthetic aperture radar (InSAR),
magnetic resonance imaging (MRI), or optical interferometry, are nowadays widespread
and with an increasing usage. The so-called phase unwrapping, which consists in the in-
ference of the absolute phase from the modulo-2Ď€ phase, is a critical step in many of their
processing chains, yet still one of its most challenging problems. We introduce an en-
ergy minimization based approach to 2D phase unwrapping. In this approach we address
the problem by adopting a Bayesian point of view and a Markov random field (MRF)
to model the phase. The maximum a posteriori estimation of the absolute phase gives
rise to an integer optimization problem, for which we introduce a family of efficient algo-
rithms based on existing graph cuts techniques. We term our approach and algorithms
PUMA, for Phase Unwrapping MAx flow. As long as the prior potential of the MRF
is convex, PUMA guarantees an exact global solution. In particular it solves exactly all
the minimum L p norm (p ≥ 1) phase unwrapping problems, unifying in that sense, a set
of existing independent algorithms. For non convex potentials we introduce a version of
PUMA that, while yielding only approximate solutions, gives very useful phase unwrap-
ping results. The main characteristic of the introduced solutions is the ability to blindly
preserve discontinuities. Extending the previous versions of PUMA, we tackle denoising by
exploiting a multi-precision idea, which allows us to use the same rationale both for phase
unwrapping and denoising. Finally, the last presented version of PUMA uses a frequency
diversity concept to unwrap phase images having large phase rates. A representative set
of experiences illustrates the performance of PUMA
Three dimensional evolution of differentially rotating magnetized neutron stars
We construct a new three-dimensional general relativistic
magnetohydrodynamics code, in which a fixed mesh refinement technique is
implemented. To ensure the divergence-free condition as well as the magnetic
flux conservation, we employ the method by Balsara (2001). Using this new code,
we evolve differentially rotating magnetized neutron stars, and find that a
magnetically driven outflow is launched from the star exhibiting a kink
instability. The matter ejection rate and Poynting flux are still consistent
with our previous finding (Shibata et al., 2011) obtained in axisymmetric
simulations.Comment: 12 pages, 14 figures, accepted by PR
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