1,624 research outputs found
Extended Hodge Theory for Fibred Cusp Manifolds
For a particular class of pseudo manifolds, we show that the intersection
cohomology groups for any perversity may be naturally represented by extended
weighted harmonic forms for a complete metric on the regular stratum with
respect to some weight determined by the perversity. Extended weighted
harmonic forms are harmonic forms that are almost in the given weighted
space for the metric in question, but not quite. This result is akin to the
representation of absolute and relative cohomology groups for a manifold with
boundary by extended harmonic forms on the associated manifold with cylindrical
ends. As in that setting, in the unweighted case, the boundary values of
the extended harmonic forms define a Lagrangian splitting of the boundary space
in the long exact sequence relating upper and lower middle perversity
intersection cohomology groups.Comment: 26 page
Self-Generated Magnetic Fields in Galactic Cooling Flows
Interstellar magnetic fields in elliptical galaxies are assumed to have their
origin in stellar fields that accompany normal mass loss from an evolving
population of old stars. The seed fields are amplified by interstellar
turbulence driven by stellar mass loss and supernova events. These disordered
fields are further amplified by time-dependent compression in the inward moving
galactic cooling flow and are expected to dominate near the galactic core.
Under favorable circumstances, fields similar in strength to those observed G can be generated solely from these natural
galactic processes. In general the interstellar field throughout elliptical
galaxies is determined by the outermost regions in the interstellar gas where
the turbulent dynamo process can occur. Because of the long hydrodynamic flow
times in galactic cooling flows, currently observed magnetic fields may result
from periods of intense turbulent field amplification that occurred in the
outer galaxy in the distant past. Particularly strong fields in ellipticals may
result from ancient galactic mergers or shear turbulence introduced at the
boundary between the interstellar gas and ambient cluster gas.Comment: 21 pages in AASTEX LaTeX with 2 figures; accepted by Astrophysical
Journa
Shear-Flow Driven Current Filamentation: Two-Dimensional Magnetohydrodynamic Simulations
The process of current filamentation in permanently externally driven,
initially globally ideal plasmas is investigated by means of two-dimensional
Magnetohydrodynamic (MHD)-simulations. This situation is typical for
astrophysical systems like jets, the interstellar and intergalactic medium
where the dynamics is dominated by external forces. Two different cases are
studied. In one case, the system is ideal permanently and dissipative processes
are excluded. In the second case, a system with a current density dependent
resistivity is considered. This resistivity is switched on self-consistently in
current filaments and allows for local dissipation due to magnetic
reconnection. Thus one finds tearing of current filaments and, besides, merging
of filaments due to coalescence instabilities. Energy input and dissipation
finally balance each other and the system reaches a state of constant magnetic
energy in time.Comment: 32 Pages, 13 Figures. accepted, to appear in Physics of Plasmas
(049012
A Radio Polarimetric Study of the Galactic Center Threads
Multi-frequency, polarimetric VLA observations of the non-thermal filaments
(NTF's), G0.08+0.15, and G359.96+0.09, also known as the Northern and Southern
Threads are presented at 20, 6, 3.6 and 2 cm, with high enough spatial
resolution to be resolved for the first time at 6 and 3.6 cm. The 20 cm image
reveals a wealth of new detail in the radio sources lying within the inner 60
pc of the Galaxy. The Southern Thread has a prominent split along its length,
similar to splitting at the ends of previously studied NTF's. With resolutions
as fine as 2'', the 3.6 and 6 cm images reveal a high degree of continuity and
little substructure internal to the filament. The spectral index of the
Northern Thread has been determined over a broad range of frequencies. Its flux
density falls with frequency, alpha=-0.5 between 90 and 6 cm, and becomes much
steeper (alpha=-2.0) between 6 and 2 cm. The spectral index does not vary
significantly along the length of the Northern Thread, which implies either
that the diffusion timescale for the emitting electrons is less than their
synchrotron lifetime, or that the emitting electrons are reaccelerated
continuously at multiple positions along the filament. Because of the lack of
spectral index variation, we have not located the source of relativistic
electrons. Polarization observations at 6 and 3.6 cm confirm the non-thermal
nature of the emission from the Northern Thread. The fractional polarization in
the Northern Thread reaches 70% in some regions, although the polarized
emission is patchy. Large rotation measures (RM > 2000 rad/m2) have been
observed with irregular variations across the filament.The intrinsic magnetic
field in the Northern Thread is predominantly aligned along its long axis.Comment: 19 pages, incl. 24 figs; to appear in the Astrophysical Journa
Particle Acceleration in three dimensional Reconnection Regions: A New Test Particle Approach
Magnetic Reconnection is an efficient and fast acceleration mechanism by
means of direct electric field acceleration parallel to the magnetic field.
Thus, acceleration of particles in reconnection regions is a very important
topic in plasma astrophysics. This paper shows that the conventional analytical
models and numerical test particle investigations can be misleading concerning
the energy distribution of the accelerated particles, since they oversimplify
the electric field structure by the assumption that the field is homogeneous.
These investigations of the acceleration of charged test particles are extended
by considering three-dimensional field configurations characterized by
localized field-aligned electric fields. Moreover, effects of radiative losses
are discussed. The comparison between homogeneous and inhomogeneous electric
field acceleration in reconnection regions shows dramatic differences
concerning both, the maximum particle energy and the form of the energy
distribution.Comment: 11 pages, 21 figure
Shuffle relations for regularised integrals of symbols
We prove shuffle relations which relate a product of regularised integrals of
classical symbols to regularised nested (Chen) iterated integrals, which hold
if all the symbols involved have non-vanishing residue. This is true in
particular for non-integer order symbols. In general the shuffle relations hold
up to finite parts of corrective terms arising from renormalisation on tensor
products of classical symbols, a procedure adapted from renormalisation
procedures on Feynman diagrams familiar to physicists. We relate the shuffle
relations for regularised integrals of symbols with shuffle relations for
multizeta functions adapting the above constructions to the case of symbols on
the unit circle.Comment: 40 pages,latex. Changes concern sections 4 and 5 : an error in
section 4 has been corrected, and the link between section 5 and the previous
ones has been precise
Inflation-Produced Magnetic Fields in Nonlinear Electrodynamics
We study the generation of primeval magnetic fields during inflation era in
nonlinear theories of electrodynamics. Although the intensity of the produced
fields strongly depends on characteristics of inflation and on the form of
electromagnetic Lagrangian, our results do not exclude the possibility that
these fields could be astrophysically interesting.Comment: 6 page
Non-thermal Processes in Black-Hole-Jet Magnetospheres
The environs of supermassive black holes are among the universe's most
extreme phenomena. Understanding the physical processes occurring in the
vicinity of black holes may provide the key to answer a number of fundamental
astrophysical questions including the detectability of strong gravity effects,
the formation and propagation of relativistic jets, the origin of the highest
energy gamma-rays and cosmic-rays, and the nature and evolution of the central
engine in Active Galactic Nuclei (AGN). As a step towards this direction, this
paper reviews some of the progress achieved in the field based on observations
in the very high energy domain. It particularly focuses on non-thermal particle
acceleration and emission processes that may occur in the rotating
magnetospheres originating from accreting, supermassive black hole systems.
Topics covered include direct electric field acceleration in the black hole's
magnetosphere, ultra-high energy cosmic ray production, Blandford-Znajek
mechanism, centrifugal acceleration and magnetic reconnection, along with the
relevant efficiency constraints imposed by interactions with matter, radiation
and fields. By way of application, a detailed discussion of well-known sources
(Sgr A*; Cen A; M87; NGC1399) is presented.Comment: invited review for International Journal of Modern Physics D, 49
pages, 15 figures; minor typos corrected to match published versio
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