619 research outputs found
Electromagnetic Casimir Forces of Parabolic Cylinder and Knife-Edge Geometries
An exact calculation of electromagnetic scattering from a perfectly
conducting parabolic cylinder is employed to compute Casimir forces in several
configurations. These include interactions between a parabolic cylinder and a
plane, two parabolic cylinders, and a parabolic cylinder and an ordinary
cylinder. To elucidate the effect of boundaries, special attention is focused
on the "knife-edge" limit in which the parabolic cylinder becomes a half-plane.
Geometrical effects are illustrated by considering arbitrary rotations of a
parabolic cylinder around its focal axis, and arbitrary translations
perpendicular to this axis. A quite different geometrical arrangement is
explored for the case of an ordinary cylinder placed in the interior of a
parabolic cylinder. All of these results extend simply to nonzero temperatures.Comment: 17 pages, 10 figures, uses RevTeX
Integrable vortex-type equations on the two-sphere
We consider the Yang-Mills instanton equations on the four-dimensional
manifold S^2xSigma, where Sigma is a compact Riemann surface of genus g>1 or
its covering space H^2=SU(1,1)/U(1). Introducing a natural ansatz for the gauge
potential, we reduce the instanton equations on S^2xSigma to vortex-type
equations on the sphere S^2. It is shown that when the scalar curvature of the
manifold S^2xSigma vanishes, the vortex-type equations are integrable, i.e. can
be obtained as compatibility conditions of two linear equations (Lax pair)
which are written down explicitly. Thus, the standard methods of integrable
systems can be applied for constructing their solutions. However, even if the
scalar curvature of S^2xSigma does not vanish, the vortex equations are well
defined and have solutions for any values of the topological charge N. We show
that any solution to the vortex equations on S^2 with a fixed topological
charge N corresponds to a Yang-Mills instanton on S^2xSigma of charge (g-1)N.Comment: 14 pages; v2: clarifying comments added, published versio
Casimir Force at a Knife's Edge
The Casimir force has been computed exactly for only a few simple geometries,
such as infinite plates, cylinders, and spheres. We show that a parabolic
cylinder, for which analytic solutions to the Helmholtz equation are available,
is another case where such a calculation is possible. We compute the
interaction energy of a parabolic cylinder and an infinite plate (both perfect
mirrors), as a function of their separation and inclination, and ,
and the cylinder's parabolic radius . As , the proximity force
approximation becomes exact. The opposite limit of corresponds to a
semi-infinite plate, where the effects of edge and inclination can be probed.Comment: 5 pages, 3 figures, uses RevTeX; v2: expanded conclusions; v3: fixed
missing factor in Eq. (3) and incorrect diagram label (no changes to
results); v4: fix similar factor in Eq. (16) (again no changes to results
One-zone models for spheroidal galaxies with a central supermassive black-hole. Self-regulated Bondi accretion
By means of a one-zone evolutionary model, we study the co-evolution of
supermassive black holes and their host galaxies, as a function of the
accretion radiative efficiency, dark matter content, and cosmological infall of
gas. In particular, the radiation feedback is computed by using the
self-regulated Bondi accretion. The models are characterized by strong
oscillations when the galaxy is in the AGN state with a high accretion
luminosity. We found that these one-zone models are able to reproduce two
important phases of galaxy evolution, namely an obscured-cold phase when the
bulk of star formation and black hole accretion occur, and the following
quiescent hot phase in which accretion remains highly sub-Eddington. A
Compton-thick phase is also found in almost all models, associated with the
cold phase. An exploration of the parameter space reveals that the closest
agreement with the present-day Magorrian relation is obtained, independently of
the dark matter halo mass, for galaxies with a low-mass seed black hole, and
the accretion radiative efficiency ~0.1.Comment: Accepted for publication in A&A, 12 pages, 5 figure
A multifrequency study of giant radio sources-II. Spectral ageing analysis of the lobes of selected sources
Multifrequency observations with the GMRT and the VLA are used to determine
the spectral breaks in consecutive strips along the lobes of a sample of
selected giant radio sources (GRSs) in order to estimate their spectral ages.
The maximum spectral ages estimated for the detected radio emission in the
lobes of our sources range from 6 to 36 Myr with a median value of
20 Myr using the classical equipartition fields. Using the magnetic field
estimates from the Beck & Krause formalism the spectral ages range from 5
to 38 Myr with a median value of 22 Myr. These ages are significantly
older than smaller sources. In all but one source (J1313+6937) the spectral age
gradually increases with distance from the hotspot regions, confirming that
acceleration of the particles mainly occurs in the hotspots. Most of the GRSs
do not exhibit zero spectral ages in the hotspots, as is the case in earlier
studies of smaller sources. This is likely to be largely due to contamination
by more extended emission due to relatively modest resolutions. The injection
spectral indices range from 0.55 to 0.88 with a median value of
0.6. We discuss these values in the light of theoretical expectations,
and show that the injection spectral index appears to be correlated with
luminosity and/or redshift as well as with linear size.Comment: 12 Pages, 13 Figures, 9 Tables, Accepted for publication in MNRA
Lambda Polarization in Polarized Proton-Proton Collisions at RHIC
We discuss Lambda polarization in semi-inclusive proton-proton collisions,
with one of the protons longitudinally polarized. The hyperfine interaction
responsible for the - and - mass splittings gives
rise to flavor asymmetric fragmentation functions and to sizable polarized
non-strange fragmentation functions. We predict large positive Lambda
polarization in polarized proton-proton collisions at large rapidities of the
produced Lambda, while other models, based on SU(3) flavor symmetric
fragmentation functions, predict zero or negative Lambda polarization. The
effect of and decays is also discussed. Forthcoming
experiments at RHIC will be able to differentiate between these predictions.Comment: 18 pages, 5 figure
Canonical Particle Acceleration in FRI Radio Galaxies
Matched resolution multi-frequency VLA observations of four radio galaxies
are used to derive the asymptotic low energy slope of the relativistic electron
distribution. Where available, low energy slopes are also determined for other
sources in the literature. They provide information on the acceleration physics
independent of radiative and other losses, which confuse measurements of the
synchrotron spectra in most radio, optical and X-ray studies. We find a narrow
range of inferred low energy electron energy slopes, n(E)=const*E^-2.1 for the
currently small sample of lower luminosity sources classified as FRI (not
classical doubles). This distribution is close to, but apparently inconsistent
with, the test particle limit of n(E)=const*E^-2.0 expected from strong
diffusive shock acceleration in the non-relativistic limit. Relativistic shocks
or those modified by the back-pressure of efficiently accelerated cosmic rays
are two alternatives to produce somewhat steeper spectra. We note for further
study the possiblity of acceleration through shocks, turbulence or shear in the
flaring/brightening regions in FRI jets as they move away from the nucleus.
Jets on pc scales and the collimated jets and hot spots of FRII (classical
double) sources would be governed by different acceleration sites and
mechanisms; they appear to show a much wider range of spectra than for FRI
sources.Comment: 16 figures, including 5 color. Accepted to Astrophysical Journa
Structure and Production of Lambda Baryons
We discuss the quark parton structure of the baryon and the
fragmentation of quarks into baryons. We show that the hyperfine
interaction, responsible for the - and - mass
splittings, leads not only to sizeable SU(3) and SU(6) symmetry breaking in the
quark distributions of the , but also to significant polarized
non-strange quark distributions. The same arguments suggest flavor asymmetric
quark fragmentation functions and non-zero polarized non-strange quark
fragmentation functions. The calculated fragmentation functions give a good
description of all measured observables. We predict significant positive
polarization in semi-inclusive DIS experiments while models based on
SU(3) flavor symmetry predict zero or negative polarization. Our
approach also provides a natural explanation for the dependence of the maximum
of the spectrum on the mass of the particles produced in
annihilation.Comment: 24 pages, 9 figures, minor change
The spectra and energies of classical double radio lobes
We compare two temporal properties of classical double radio sources: i)
radiative lifetimes of synchrotron-emitting particles and ii) dynamical source
ages. We discuss how these can be quite discrepant from one another, rendering
use of the traditional spectral ageing method inappropriate: we contend that
spectral ages give meaningful estimates of dynamical ages only when these ages
are << 10^7 years. In juxtaposing the fleeting radiative lifetimes with source
ages which are significantly longer, a refinement of the paradigm for radio
source evolution is required. The changing spectra along lobes are explained,
not predominantly by synchrotron ageing but, by gentle gradients in a magnetic
field mediated by a low-gamma matrix which illuminates an energy-distribution
of particles, controlled largely by classical synchrotron loss in the high
magnetic field of the hotspot. The energy in the particles is an order of
magnitude higher than that inferred from the minimum-energy estimate, implying
that the jet-power is of the same order as the accretion luminosity produced by
the quasar central engine. This refined paradigm points to a resolution of the
findings of Rudnick et al (1994) and Katz-Stone & Rudnick (1994) that both the
Jaffe-Perola and Kardashev-Pacholczyk model spectra are invariably poor
descriptions of the curved spectral shape of lobe emission, and indeed that for
Cygnus A all regions of the lobes are characterised by a `universal spectrum'.
[abridged]Comment: LaTeX, 4 figures. To appear in A
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