19,920 research outputs found
Time-dependent radio emission from evolving jets
We investigated the time-dependent radiative and dynamical properties of
light supersonic jets launched into an external medium, using hydrodynamic
simulations and numerical radiative transfer calculations. These involved
various structural models for the ambient media, with density profiles
appropriate for galactic and extragalactic systems. The radiative transfer
formulation took full account of emission, absorption, re-emission, Faraday
rotation and Faraday conversion explicitly. High time-resolution intensity maps
were generated, frame-by-frame, to track the spatial hydrodynamical and
radiative properties of the evolving jets. Intensity light curves were computed
via integrating spatially over the emission maps. We apply the models to jets
in active galactic nuclei (AGN). From the jet simulations and the
time-dependent emission calculations we derived empirical relations for the
emission intensity and size for jets at various evolutionary stages. The
temporal properties of jet emission are not solely consequences of intrinsic
variations in the hydrodynamics and thermal properties of the jet. They also
depend on the interaction between the jet and the ambient medium. The
interpretation of radio jet morphology therefore needs to take account of
environmental factors. Our calculations have also shown that the environmental
interactions can affect specific emitting features, such as internal shocks and
hotspots. Quantification of the temporal evolution and spatial distribution of
these bright features, together with the derived relations between jet size and
emission, would enable us to set constraints on the hydrodynamics of AGN and
the structure of the ambient medium.Comment: 16 pages, 18 figures, MNRAS in press
Platonic model of mind as an approximation to neurodynamics
Hierarchy of approximations involved in simplification of microscopic theories, from sub-cellural to the whole brain level, is presented. A new approximation to neural dynamics is described, leading to a Platonic-like model of mind based on psychological spaces. Objects and events in these spaces correspond to quasi-stable states of brain dynamics and may be interpreted from psychological point of view. Platonic model bridges the gap between neurosciences and psychological sciences. Static and dynamic versions of this model are outlined and Feature Space Mapping, a neurofuzzy realization of the static version of Platonic model, described. Categorization experiments with human subjects are analyzed from the neurodynamical and Platonic model points of view
Trefftz Difference Schemes on Irregular Stencils
The recently developed Flexible Local Approximation MEthod (FLAME) produces
accurate difference schemes by replacing the usual Taylor expansion with
Trefftz functions -- local solutions of the underlying differential equation.
This paper advances and casts in a general form a significant modification of
FLAME proposed recently by Pinheiro & Webb: a least-squares fit instead of the
exact match of the approximate solution at the stencil nodes. As a consequence
of that, FLAME schemes can now be generated on irregular stencils with the
number of nodes substantially greater than the number of approximating
functions. The accuracy of the method is preserved but its robustness is
improved. For demonstration, the paper presents a number of numerical examples
in 2D and 3D: electrostatic (magnetostatic) particle interactions, scattering
of electromagnetic (acoustic) waves, and wave propagation in a photonic
crystal. The examples explore the role of the grid and stencil size, of the
number of approximating functions, and of the irregularity of the stencils.Comment: 28 pages, 12 figures; to be published in J Comp Phy
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