3,563 research outputs found
Search for Gamma-Ray Burst Classes with the RHESSI Satellite
A sample of 427 gamma-ray bursts (GRBs), measured by the RHESSI satellite, is
studied statistically with respect to duration and hardness ratio. Standard
statistical tests are used, such as , F-test and the maximum likelihood
ratio test, in order to compare the number of GRB groups in the RHESSI database
with that of the BATSE database. Previous studies based on the BATSE Catalog
claim the existence of an intermediate GRB group, besides the long and short
groups. Using only the GRB duration as information and or
F-test, we have not found any statistically significant intermediate group in
the RHESSI data. However, maximum likelihood ratio test reveals a significant
intermediate group. Also using the 2-dimensional hardness / plane, the
maximum likelihood analysis reveals a significant intermediate group. Contrary
to the BATSE database, the intermediate group in the RHESSI data-set is harder
than the long one. The existence of an intermediate group follows not only from
the BATSE data-set, but also from the RHESSI one.Comment: Accepted for publication in Astronomy and Astrophysics, 9 pages, 4
figure
Topological Charge Fluctuations and Low-Lying Dirac Eigenmodes
We discuss the utility of low-lying Dirac eigenmodes for studying the nature
of topological charge fluctuations in QCD. The implications of previous results
using the local chirality histogram method are discussed, and the new results
using the overlap Dirac operator in Wilson gauge backgrounds at lattice
spacings ranging from a~0.04 fm to a~0.12 fm are reported. While the degree of
local chirality does not change appreciably closer to the continuum limit, we
find that the size and density of local structures responsible for chiral
peaking do change significantly. The resulting values are in disagreement with
the assumptions of the Instanton Liquid Model. We conclude that the
fluctuations of topological charge in the QCD vacuum are not locally quantized.Comment: 3 pages, 4 figures, Lattice2001(confinement
Quantum corrections of Abelian Duality Transformations
A modification of the Abelian Duality transformations is proposed
guaranteeing that a (not necessarily conformally invariant) -model be
quantum equivalent (at least up to two loops in perturbation theory) to its
dual. This requires a somewhat non standard perturbative treatment of the {\sl
dual} -model. Explicit formulae of the modified duality transformation
are presented for a special class of block diagonal purely metric
-models.Comment: Latex 11 pages; remarks on a free model and references adde
Instanton dominance of topological charge fluctuations in QCD?
We consider the local chirality of near-zero eigenvectors from Wilson-Dirac
and clover improved Wilson-Dirac lattice operators as proposed recently by
Horv\'ath et al. We studied finer lattices and repaired for the loss of
orthogonality due to the non-normality of the Wilson-Dirac matrix. As a result
we do see a clear double peak structure on lattices with resolutions higher
than 0.1 fm. We found that the lattice artifacts can be considerably reduced by
exploiting the biorthogonal system of left and right eigenvectors. We conclude
that the dominance of instantons on topological charge fluctuations is not
ruled out by local chirality measurements.Comment: 10 pages, 6 figure
Low-Dimensional Long-Range Topological Charge Structure in the QCD Vacuum
While sign-coherent 4-dimensional structures cannot dominate topological
charge fluctuations in the QCD vacuum at all scales due to reflection
positivity, it is possible that enhanced coherence exists over extended
space-time regions of lower dimension. Using the overlap Dirac operator to
calculate topological charge density, we present evidence for such structure in
pure-glue SU(3) lattice gauge theory. It is found that a typical equilibrium
configuration is dominated by two oppositely-charged sign-coherent connected
structures (``sheets'') covering about 80% of space-time. Each sheet is built
from elementary 3-d cubes connected through 2-d faces, and approximates a
low-dimensional curved manifold (or possibly a fractal structure) embedded in
the 4-d space. At the heart of the sheet is a ``skeleton'' formed by about 18%
of the most intense space-time points organized into a global long-range
structure, involving connected parts spreading over maximal possible distances.
We find that the skeleton is locally 1-dimensional and propose that its
geometrical properties might be relevant for understanding the possible role of
topological charge fluctuations in the physics of chiral symmetry breaking.Comment: 4 pages RevTeX, 4 figures; v2: 6 pages, 5 figures, more explanations
provided, figure and references added, published versio
Detectability of gravitational wave bursts from a class of neutron star starquake GRB models
A large class of gamma-ray burst (GRB) models (overwhelming until recently) involve the release of energy in a neutron star quake. Even though the extreme isotropy of the GRB sky established by the BATSE experiment has now shifted the interest to cosmological models, the former starquake scenarios are still attractive and may naturally produce a gravitational wave burst that carries most of the released energy. We discuss the prospects for detection of these high-frequency bursts by the forthcoming interferometers and spheroidal antennas, emphasizing the most recent results on the distribution and nature of GRB sources. We find that, even if the overall picture is correct, the positive detection of GRB-associated gravitational wave bursts is unlikely and therefore these events will not be a prime target for the detectors
Topological Charge Membranes in 2D and 4D Gauge Theory
Local topological charge structure in the 2D CP(N-1) sigma models is studied
using the overlap Dirac operator. Long-range coherence of topological charge
along locally 1D regions in 2D space-time is observed. We discuss the
connection between these results and the recent discovery of coherent 3D sheets
of topological charge in 4D QCD. In both cases, coherent regions of topological
charge form along surfaces of approximmate codimension 1.Comment: Lattice2004(topology
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