23,643 research outputs found
Spherical Orbifolds for Cosmic Topology
Harmonic analysis is a tool to infer cosmic topology from the measured
astrophysical cosmic microwave background CMB radiation. For overall positive
curvature, Platonic spherical manifolds are candidates for this analysis. We
combine the specific point symmetry of the Platonic manifolds with their deck
transformations. This analysis in topology leads from manifolds to orbifolds.
We discuss the deck transformations of the orbifolds and give eigenmodes for
the harmonic analysis as linear combinations of Wigner polynomials on the
3-sphere. These provide new tools for detecting cosmic topology from the CMB
radiation.Comment: 17 pages, 9 figures. arXiv admin note: substantial text overlap with
arXiv:1011.427
A Bayesian method for pulsar template generation
Extracting Times of Arrival from pulsar radio signals depends on the
knowledge of the pulsars pulse profile and how this template is generated. We
examine pulsar template generation with Bayesian methods. We will contrast the
classical generation mechanism of averaging intensity profiles with a new
approach based on Bayesian inference. We introduce the Bayesian measurement
model imposed and derive the algorithm to reconstruct a "statistical template"
out of noisy data. The properties of these "statistical templates" are analysed
with simulated and real measurement data from PSR B1133+16. We explain how to
put this new form of template to use in analysing secondary parameters of
interest and give various examples: We implement a nonlinear filter for
determining ToAs of pulsars. Applying this method to data from PSR J1713+0747
we derive ToAs self consistently, meaning all epochs were timed and we used the
same epochs for template generation. While the average template contains
fluctuations and noise as unavoidable artifacts, we find that the "statistical
template" derived by Bayesian inference quantifies fluctuations and remaining
uncertainty. This is why the algorithm suggested turns out to reconstruct
templates of statistical significance from ten to fifty single pulses. A moving
data window of fifty pulses, taking out one single pulse at the beginning and
adding one at the end of the window unravels the characteristics of the methods
to be compared. It shows that the change induced in the classical
reconstruction is dominated by random fluctuations for the average template,
while statistically significant changes drive the dynamics of the proposed
method's reconstruction. The analysis of phase shifts with simulated data
reveals that the proposed nonlinear algorithm is able to reconstruct correct
phase information along with an acceptable estimation of the remaining
uncertainty.Comment: 21 pages, 16 figures, submitted to MNRA
The double torus as a 2D cosmos: groups, geometry and closed geodesics
The double torus provides a relativistic model for a closed 2D cosmos with
topology of genus 2 and constant negative curvature. Its unfolding into an
octagon extends to an octagonal tessellation of its universal covering, the
hyperbolic space H^2. The tessellation is analysed with tools from hyperbolic
crystallography. Actions on H^2 of groups/subgroups are identified for SU(1,
1), for a hyperbolic Coxeter group acting also on SU(1, 1), and for the
homotopy group \Phi_2 whose extension is normal in the Coxeter group. Closed
geodesics arise from links on H^2 between octagon centres. The direction and
length of the shortest closed geodesics is computed.Comment: Latex, 27 pages, 5 figures (late submission to arxiv.org
On measuring alpha in B(t)-> rho^\pm pi^\mp
Defining a most economical parametrization of time-dependent B-> rho^\pm
pi^\mp decays, including a measurable phase alpha_{eff} which equals the weak
phase alpha in the limit of vanishing penguin amplitudes, we propose two ways
for determining alpha in this processes. We explain the limitation of one
method, assuming only that two relevant tree amplitudes factorize and that
their relative strong phase, delta_t, is negligible. The other method, based on
broken flavor SU(3), permits a determination of alpha in B^0-> rho^\pm pi^\mp
in an overconstrained system using also rate measurements of B^{0,+}-> K^* pi
and B^{0,+}->rho K. Current data are shown to restrict two ratios of penguin
and tree amplitudes, r_\pm, to a narrow range around 0.2, and to imply an upper
bound |alpha_{eff} - alpha| < 15 degrees. Assuming that delta_t is much smaller
than 90 degrees, we find alpha =(93\pm 16) degrees and (102 \pm 20) degrees
using BABAR and BELLE results for B(t)-> rho^\pm pi^mp. Avoiding this
assumption for completeness, we demonstrate the reduction of discrete
ambiguities in alpha with increased statistics, and show that SU(3) breaking
effects are effectively second order in r_\pm.Comment: 23 pages, 2 figures, data and references updated, to be published in
Phys. Rev.
Jamming under tension in polymer crazes
Molecular dynamics simulations are used to study a unique expanded jammed
state. Tension transforms many glassy polymers from a dense glass to a network
of fibrils and voids called a craze. Entanglements between polymers and
interchain friction jam the system after a fixed increase in volume. As in
dense jammed systems, the distribution of forces is exponential, but they are
tensile rather than compressive. The broad distribution of forces has important
implications for fibril breakdown and the ultimate strength of crazes.Comment: 4 pages, 4 figure
CII, CI, and CO in the massive star forming region W3 Main
We have used the KOSMA 3m telescope to map the core 7'x5' of the Galactic
massive star forming region W3Main in the two fine structure lines of atomic
carbon and four mid-J transitions of CO and 13CO. In combination with a map of
singly ionized carbon (Howe et al. 1991), and FIR fine structure line data
observed by ISO/LWS at the center position, these data sets allow to study in
detail the physical structure of the photon dominated cloud interface regions
(PDRs) where the occurance of carbon changes from CII to CI, and to CO.Comment: 4 pages, 4 figures, to appear in "Proceedings of the 4th
Cologne-Bonn-Zermatt-Symposium, The dense interstellar medium in galaxies",
eds. S. Pfalzner, C. Kramer, C. Straubmeier, and A. Heithausen (Springer
Verlag
Overlapping Unit Cells in 3d Quasicrystal Structure
A 3-dimensional quasiperiodic lattice, with overlapping unit cells and
periodic in one direction, is constructed using grid and projection methods
pioneered by de Bruijn. Each unit cell consists of 26 points, of which 22 are
the vertices of a convex polytope P, and 4 are interior points also shared with
other neighboring unit cells. Using Kronecker's theorem the frequencies of all
possible types of overlapping are found.Comment: LaTeX2e, 11 pages, 5 figures (8 eps files), uses iopart.class. Final
versio
On some geometric features of the Kramer interior solution for a rotating perfect fluid
Geometric features (including convexity properties) of an exact interior
gravitational field due to a self-gravitating axisymmetric body of perfect
fluid in stationary, rigid rotation are studied. In spite of the seemingly
non-Newtonian features of the bounding surface for some rotation rates, we
show, by means of a detailed analysis of the three-dimensional spatial
geodesics, that the standard Newtonian convexity properties do hold. A central
role is played by a family of geodesics that are introduced here, and provide a
generalization of the Newtonian straight lines parallel to the axis of
rotation.Comment: LaTeX, 15 pages with 4 Poscript figures. To be published in Classical
and Quantum Gravit
Long term monitoring of mode switching for PSR B0329+54
The mode switching phenomenon of PSR B0329+54 is investigated based on the
long-term monitoring from September 2003 to April 2009 made with the Urumqi 25m
radio telescope at 1540 MHz. At that frequency, the change of relative
intensity between the leading and trailing components is the predominant
feature of mode switching. The intensity ratios between the leading and
trailing components are measured for the individual profiles averaged over a
few minutes. It is found that the ratios follow normal distributions, where the
abnormal mode has a wider typical width than the normal mode, indicating that
the abnormal mode is less stable than the normal mode. Our data show that 84.9%
of the time for PSR B0329+54 was in the normal mode and 15.1% was in the
abnormal mode. From the two passages of eight-day quasi-continuous observations
in 2004, and supplemented by the daily data observed with 15 m telescope at 610
MHz at Jodrell Bank Observatory, the intrinsic distributions of mode timescales
are constrained with the Bayesian inference method. It is found that the gamma
distribution with the shape parameter slightly smaller than 1 is favored over
the normal, lognormal and Pareto distributions. The optimal scale parameters of
the gamma distribution is 31.5 minutes for the abnormal mode and 154 minutes
for the normal mode. The shape parameters have very similar values, i.e.
0.75^{+0.22}_{-0.17} for the normal mode and 0.84^{+0.28}_{-0.22} for the
abnormal mode, indicating the physical mechanisms in both modes may be the
same. No long-term modulation of the relative intensity ratios was found for
both the modes, suggesting that the mode switching was stable. The intrinsic
timescale distributions, for the first time constrained for this pulsar,
provide valuable information to understand the physics of mode switching.Comment: 31 pages,12 figures, Accepted by the Ap
A Note on Segre Types of Second Order Symmetric Tensors in 5-D Brane-world Cosmology
Recent developments in string theory suggest that there might exist extra
spatial dimensions, which are not small nor compact. The framework of most
brane cosmological models is that in which the matter fields are confined on a
brane-world embedded in five dimensions (the bulk). Motivated by this we
reexamine the classification of the second order symmetric tensors in 5--D, and
prove two theorems which collect together some basic results on the algebraic
structure of these tensors in 5-dimensional space-times. We also briefly
indicate how one can obtain, by induction, the classification of symmetric
two-tensors (and the corresponding canonical forms) on n-dimensional spaces
from the classification on 4-dimensional spaces. This is important in the
context of 11--D supergravity and 10--D superstrings.Comment: 12 pages, to appear in Mod. Phys. Lett. A (2003) in the present for
- …