7,740 research outputs found
Jet Acceleration by Tangled Magnetic Fields
We explore the possibility that extragalactic radio jets might be accelerated
by highly disorganized magnetic fields that are strong enough to dominate the
dynamics until the terminal Lorentz factor is reached. Following the
twin-exhaust model by Blandford & Rees (1974), the collimation under this
scenario is provided by the stratified thermal pressure from an external
medium. The acceleration efficiency then depends on the pressure gradient of
that medium. In order for this mechanism to work there must be continuous
tangling of the magnetic field, changing the magnetic equation of state away
from pure flux freezing (otherwise conversion of Poynting flux to kinetic
energy flux is suppressed). This is a complementary approach to models in which
the plasma is accelerated by large scale ordered fields. We include a simple
prescription for magnetic dissipation, which leads to tradeoffs among
conversion of magnetic energy into bulk kinetic energy, random particle energy,
and radiation. We present analytic dynamical solutions of such jets, assess the
effects of radiation drag, and comment on observational issues, such as the
predicted polarization and synchrotron brightness. Finally, we try to make the
connection to observed radio galaxies and gamma-ray bursts.Comment: 15 pages, 10 figures, accepted for publication in Ap
Categorisation and Detection of Dark Matter Candidates from String/M-theory Hidden Sectors
We study well-motivated dark matter candidates arising from weakly-coupled
hidden sectors in compactified string/-theory. Imposing generic top-down
constraints greatly restricts allowed candidates. By considering the possible
mechanisms for achieving the correct dark matter relic density, we compile
categories of viable dark matter candidates and annihilation mediators. We
consider the case where supersymmetry breaking occurs via moduli stabilisation
and is gravitationally mediated to the visible and other hidden sectors,
without assuming sequestering of the sector in which supersymmetry is broken.
We find that in this case, weakly-coupled hidden sectors only allow for
fermionic dark matter. Additionally, most of the mechanisms for obtaining the
full relic density only allow for a gauge boson mediator, such as a dark .
Given these considerations, we study the potential for discovering or
constraining the allowed parameter space given current and future direct
detection experiments, and direct production at the LHC. We also present a
model of a hidden sector which would contain a satisfactory dark matter
candidate.Comment: 29 pages, 10 figure
Role of anisotropy in the spin-dimer compound BaCuSi2O6
We present results of magnetisation and electron paramagnetic resonance
experiments on the spin-dimer system BaCuSi2O6. Evidence indicates that the
origin of anisotropic terms in the spin Hamiltonian is from magnetic dipolar
interactions. Axial symmetry-breaking is on a very small energy scale of ~11
mK, confirming Bose Einstein condensation critical scaling over an extended
temperature range in the vicinity of the quantum critical point.Comment: 4 pages, 4 figure
Holevo's bound from a general quantum fluctuation theorem
We give a novel derivation of Holevo's bound using an important result from
nonequilibrium statistical physics, the fluctuation theorem. To do so we
develop a general formalism of quantum fluctuation theorems for two-time
measurements, which explicitly accounts for the back action of quantum
measurements as well as possibly non-unitary time evolution. For a specific
choice of observables this fluctuation theorem yields a measurement-dependent
correction to the Holevo bound, leading to a tighter inequality. We conclude by
analyzing equality conditions for the improved bound.Comment: 5 page
On the shape of barchan dunes
Barchans are crescent-shaped sand dunes forming in aride regions with
unidirectional wind and limited sand supply. We report analytical and numerical
results for dune shapes under different environmental conditions as obtained
from the so-called `minimal model' of aeolian sand dunes. The profiles of
longitudinal vertical slices (i.e. along the wind direction) are analyzed as a
function of wind speed and sand supply. Shape transitions can be induced by
changes of mass, wind speed and sand supply. Within a minimal extension of the
model to the transverse direction the scale-invariant profile of transverse
vertical cuts can be derived analytically.Comment: to appear in J. Phys.: Condens. Matter 17 (2005
The Circumstellar Disk of the Butterfly Star in Taurus
We present a model of the circumstellar environment of the so-called
``Butterfly Star'' in Taurus (IRAS 04302+2247). The appearance of this young
stellar object is dominated by a large circumstellar disk seen edge-on and the
light scattering lobes above the disk. The model is based on multi-wavelength
continuum observations: Millimeter maps and high-resolution near-infrared
images obtained with HST/NICMOS.
It was found that the disk and envelope parameters are comparable with those
of the circumstellar environment of other young stellar objects. A main result
is that the dust properties must be different in the circumstellar disk and in
the envelope: While a grain size distribution with grain radii up to 100 micron
is required to reproduce the millimeter observations of the disk, the envelope
is dominated by smaller grains similar to those of the interstellar medium.
Preprint with high figure quality available at:
http://spider.ipac.caltech.edu/staff/swolf/homepage/public/preprints/i04302.psComment: 32 pages, 9 figure
Counting Complex Disordered States by Efficient Pattern Matching: Chromatic Polynomials and Potts Partition Functions
Counting problems, determining the number of possible states of a large
system under certain constraints, play an important role in many areas of
science. They naturally arise for complex disordered systems in physics and
chemistry, in mathematical graph theory, and in computer science. Counting
problems, however, are among the hardest problems to access computationally.
Here, we suggest a novel method to access a benchmark counting problem, finding
chromatic polynomials of graphs. We develop a vertex-oriented symbolic pattern
matching algorithm that exploits the equivalence between the chromatic
polynomial and the zero-temperature partition function of the Potts
antiferromagnet on the same graph. Implementing this bottom-up algorithm using
appropriate computer algebra, the new method outperforms standard top-down
methods by several orders of magnitude, already for moderately sized graphs. As
a first application, we compute chromatic polynomials of samples of the simple
cubic lattice, for the first time computationally accessing three-dimensional
lattices of physical relevance. The method offers straightforward
generalizations to several other counting problems.Comment: 7 pages, 4 figure
Numerical Evidence for Multiplicative Logarithmic Corrections from Marginal Operators
Field theory calculations predict multiplicative logarithmic corrections to
correlation functions from marginally irrelevant operators. However, for the
numerically most suitable model - the spin-1/2 chain - these corrections have
been controversial. In this paper, the spin-spin correlation function of the
antiferromagnetic spin-1/2 chain is calculated numerically in the presence of a
next nearest neighbor coupling J2 for chains of up to 32 sites. By varying the
coupling strength J2 we can control the effect of the marginal operator, and
our results unambiguously confirm the field theory predictions. The critical
value at which the marginal operator vanishes has been determined to be at J2 =
0.241167 +/- 0.000005J.Comment: revised paper with extended data-analysis. 5 pages, using revtex with
4 embedded figures (included with macro). A complete postscript file with all
figures + text (5 pages) is available from
http://FY.CHALMERS.SE/~eggert/marginal.ps or by request from
[email protected]
Compact Lyman-alpha Emitting Candidates at z~2.4 in Deep Medium-band HST WFPC2 Images
Medium-band imaging with HST/WFPC2 in the F410M filter has previously
revealed a population of compact Lyman-alpha emission objects around the radio
galaxy 53W002 at z~2.4. We report detections of similar objects at z~2.4 in
random, high-latitude HST parallel observations of three additional fields,
lending support to the idea that they constitute a widespread population at
these redshifts. The three new fields contain 18 Lyman-alpha candidates, in
contrast to the 17 detected in the deeper exposure of the single WFPC2 field
around 53W002. We find substantial differences in the number of candidates from
field to field, suggesting that significant large-scale structure is already
present in the galaxy distribution at this cosmic epoch. The likely existence
of z~2.4 sub-galactic clumps in several random fields shows that these objects
may have been common in the early universe and strengthens the argument that
such objects may be responsible for the formation of a fraction of the luminous
present-day galaxies through hierarchical merging.Comment: Uses slightly modified AASTeX preprint style file (included).
Contains 22 pages, including 5 figures and 2 tables. Accepted for the
December issue of the Astronomical Journa
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