121 research outputs found
Transfer function approach to collective mode dynamics in a Nd:YAG laser
For a multimode laser operating in a steady state regime with small-signal gain modulation, transfer functions are measured for both total intensity and intensities of individual modes. A quantitative picture of the phase clustering of contributions from each cavity mode to the collective mode dynamics is obtained from the transfer functions in the pole-residue representation.Stamatescu, L. Hamilton, M. W
A two step algorithm for learning from unspecific reinforcement
We study a simple learning model based on the Hebb rule to cope with
"delayed", unspecific reinforcement. In spite of the unspecific nature of the
information-feedback, convergence to asymptotically perfect generalization is
observed, with a rate depending, however, in a non- universal way on learning
parameters. Asymptotic convergence can be as fast as that of Hebbian learning,
but may be slower. Moreover, for a certain range of parameter settings, it
depends on initial conditions whether the system can reach the regime of
asymptotically perfect generalization, or rather approaches a stationary state
of poor generalization.Comment: 13 pages LaTeX, 4 figures, note on biologically motivated stochastic
variant of the algorithm adde
Noisy excitation of nonlinear oscillator arrays
M. W. Hamilton, T. Hill, and L. Stamatesc
Method for determining antiphase dynamics in a multimode laser
We measure the cross spectrum of the intensity fluctuations of pairs of modes for a multilongitudinal mode neodymium-doped yttrium aluminum garnet laser operating in the steady state regime. From the data we build up a picture of how the longitudinal mode fluctuations interfere and directly show the antiphase dynamics of the intensity fluctuations.T. Hill, L. Stamatescu and M. W. Hamilto
Fermions in three-dimensional spinfoam quantum gravity
We study the coupling of massive fermions to the quantum mechanical dynamics
of spacetime emerging from the spinfoam approach in three dimensions. We first
recall the classical theory before constructing a spinfoam model of quantum
gravity coupled to spinors. The technique used is based on a finite expansion
in inverse fermion masses leading to the computation of the vacuum to vacuum
transition amplitude of the theory. The path integral is derived as a sum over
closed fermionic loops wrapping around the spinfoam. The effects of quantum
torsion are realised as a modification of the intertwining operators assigned
to the edges of the two-complex, in accordance with loop quantum gravity. The
creation of non-trivial curvature is modelled by a modification of the pure
gravity vertex amplitudes. The appendix contains a review of the geometrical
and algebraic structures underlying the classical coupling of fermions to three
dimensional gravity.Comment: 40 pages, 3 figures, version accepted for publication in GER
CANGAROO-III Search for Gamma Rays from SN 1987A and the Surrounding Field
Optical images of SN 1987A show a triple ring structure. The inner (dust)
ring has recently increased in brightness and in the number of hot spots
suggesting that the supernova shock wave has collided with the dense
pre-existing circumstellar medium, a scenario supported by radio and X-ray
observations. Such a shocked environment is widely expected to result in the
acceleration of charged particles, and the accompanying emission of very high
energy gamma-rays. Here, we report the results of observations made in 2004 and
2006 which yield upper limits on the TeV gamma-ray flux, which are compared
with a theoretical prediction. In addition, we set upper limits on the TeV flux
for four high energy objects which are located within the same field of view of
the observation: the super-bubble 30 Dor C, the Crab-like pulsar PSR
B054069, the X-ray binary LMC X-1, and the supernova remnant N157B.Comment: 5 pages, 5 figures, Accepted for publication in Ap
Observation of an extended VHE gamma-ray emission from MSH 15-52 with CANGAROO-III
We have observed the supernova remnant MSH 15-52 (G320.4-1.2), which contains
the gamma-ray pulsar PSR B1509-58, using the CANGAROO-III imaging atmospheric
Cherenkov telescope array from April to June in 2006. We detected gamma rays
above 810 GeV at the 7 sigma level during a total effective exposure of 48.4
hours. We obtained a differential gamma-ray flux at 2.35 TeV of
(7.9+/-1.5_{stat}+/-1.7_{sys}) \times 10^{-13} cm^{-2}s^{-1}TeV^{-1} with a
photon index of 2.21+/-0.39_{stat}+/-0.40_{sys}, which is compatible with that
of the H.E.S.S. observation in 2004. The morphology shows extended emission
compared to our Point Spread Function. We consider the plausible origin of the
high energy emission based on a multi-wavelength spectral analysis and
energetics arguments.Comment: 9 pages, 9 figures, Accepted in Ap
Observation of Very High Energy Gamma Rays from HESS J1804-216 with CANGAROO-III Telescopes
We observed the unidentified TeV gamma-ray source HESS J1804-216 with the
CANGAROO-III atmospheric Cerenkov telescopes from May to July in 2006. We
detected very high energy gamma rays above 600 GeV at the 10 sigma level in an
effective exposure of 76 hr. We obtained a differential flux of
(5.0+/-1.5_{stat}+/-1.6_{sys})\times 10^{-12}(E/1 TeV)^{-\alpha}
cm^{-2}s^{-1}TeV^{-1} with a photon index \alpha of 2.69 +/- 0.30_{stat} +/-
0.34_{sys}, which is consistent with that of the H.E.S.S. observation in 2004.
We also confirm the extended morphology of the source. By combining our result
with multi-wavelength observations, we discuss the possible counterparts of
HESS J1804-216 and the radiation mechanism based on leptonic and hadronic
processes for a supernova remnant and a pulsar wind nebula.Comment: 11 pages, 12 figures, Accepted in Ap
CANGAROO-III Observation of TeV Gamma Rays from the vicinity of PSR B1 706-44
Observation by the CANGAROO-III stereoscopic system of the Imaging Cherenkov
Telescope has detected extended emission of TeV gamma rays in the vicinity of
the pulsar PSR B170644. The strength of the signal observed as
gamma-ray-like events varies when we apply different ways of emulating
background events. The reason for such uncertainties is argued in relevance to
gamma-rays embedded in the "off-source data", that is, unknown sources and
diffuse emission in the Galactic plane, namely, the existence of a complex
structure of TeV gamma-ray emission around PSR B170644.Comment: 10 pages, 13 figures, to be published in Ap
CANGAROO-III search for TeV Gamma-rays from two clusters of galaxies
Because accretion and merger shocks in clusters of galaxies may accelerate
particles to high energies, clusters are candidate sites for the origin of
ultra-high-energy (UHE) cosmic-rays. A prediction was presented for gamma-ray
emission from a cluster of galaxies at a detectable level with the current
generation of imaging atmospheric Cherenkov telescopes. The gamma-ray emission
was produced via inverse Compton upscattering of cosmic microwave background
(CMB) photons by electron-positron pairs generated by collisions of UHE cosmic
rays in the cluster. We observed two clusters of galaxies, Abell 3667 and Abell
4038, searching for very-high-energy gamma-ray emission with the CANGAROO-III
atmospheric Cherenkov telescope system in 2006. The analysis showed no
significant excess around these clusters, yielding upper limits on the
gamma-ray emission. From a comparison of the upper limit for the north-west
radio relic region of Abell 3667 with a model prediction, we derive a lower
limit for the magnetic field of the region of ~0.1 micro G. This shows the
potential of gamma-ray observations in studies of the cluster environment. We
also discuss the flux upper limit from cluster center regions using a model of
gamma-ray emission from neutral pions produced in hadronic collisions of
cosmic-ray protons with the intra-cluster medium (ICM). The derived upper limit
of the cosmic-ray energy density within this framework is an order of magnitude
higher than that of our Galaxy.Comment: 7 pages, 6 figures, Accepted in Ap
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