7,411 research outputs found
HESS J1825-137: A pulsar wind nebula associated with PSR B1823-13?
HESS J1825-137 was detected with a significance of 8.1 in the
Galactic Plane survey conducted with the H.E.S.S. instrument in 2004. Both HESS
J1825-137 and the X-ray pulsar wind nebula G18.0--0.7 (associated with the
Vela-like pulsar PSR B1823-13) are offset south of the pulsar, which may be the
result of the SNR expanding into an inhomogeneous medium. The TeV size ( pc, for a distance of 4 kpc) is times larger than the X-ray size,
which may be the result of propagation effects as a result of the longer
lifetime of TeV emitting electrons, compared to the relatively short lifetime
of keV synchrotron emitting electrons. The TeV photon spectral index of can also be related to the extended PWN X-ray synchrotron photon index of
, if this spectrum is dominated by synchrotron cooling. The
anomalously large size of the pulsar wind nebula can be explained if the pulsar
was born with a relatively large initial spindown power and braking index
, provided that the SNR expanded into the hot ISM with relatively low
density ( cm).Comment: 4 pages, 4 figures, to appear in the Proc. of the 29th International
Cosmic Ray Conference, OG Sessio
Preasymptotic multiscaling in the phase-ordering dynamics of the kinetic Ising model
The evolution of the structure factor is studied during the phase-ordering
dynamics of the kinetic Ising model with conserved order parameter. A
preasymptotic multiscaling regime is found as in the solution of the
Cahn-Hilliard-Cook equation, revealing that the late stage of phase-ordering is
always approached through a crossover from multiscaling to standard scaling,
independently from the nature of the microscopic dynamics.Comment: 11 pages, 3 figures, to be published in Europhys. Let
Pushing Stochastic Gradient towards Second-Order Methods -- Backpropagation Learning with Transformations in Nonlinearities
Recently, we proposed to transform the outputs of each hidden neuron in a
multi-layer perceptron network to have zero output and zero slope on average,
and use separate shortcut connections to model the linear dependencies instead.
We continue the work by firstly introducing a third transformation to normalize
the scale of the outputs of each hidden neuron, and secondly by analyzing the
connections to second order optimization methods. We show that the
transformations make a simple stochastic gradient behave closer to second-order
optimization methods and thus speed up learning. This is shown both in theory
and with experiments. The experiments on the third transformation show that
while it further increases the speed of learning, it can also hurt performance
by converging to a worse local optimum, where both the inputs and outputs of
many hidden neurons are close to zero.Comment: 10 pages, 5 figures, ICLR201
The GeV-TeV Connection in Galactic gamma-ray sources
Recent observations with atmospheric Cherenkov telescope systems such as
H.E.S.S. and MAGIC have revealed a large number of new sources of
very-high-energy (VHE) gamma-rays from 100 GeV - 100 TeV, mostly concentrated
along the Galactic plane. At lower energies (100 MeV - 10 GeV) the
satellite-based instrument EGRET revealed a population of sources clustering
along the Galactic Plane. Given their adjacent energy bands a systematic
correlation study between the two source catalogues seems appropriate. Here,
the populations of Galactic sources in both energy domains are characterised on
observational as well as on phenomenological grounds. Surprisingly few common
sources are found in terms of positional coincidence and spectral consistency.
These common sources and their potential counterparts and emission mechanisms
will be discussed in detail. In cases of detection only in one energy band, for
the first time consistent upper limits in the other energy band have been
derived. The EGRET upper limits are rather unconstraining due to the
sensitivity mismatch to current VHE instruments. The VHE upper limits put
strong constraints on simple power-law extrapolation of several of the EGRET
spectra and thus strongly suggest cutoffs in the unexplored energy range from
10 GeV - 100 GeV. Physical reasons for the existence of cutoffs and for
differences in the source population at GeV and TeV energies will be discussed.
Finally, predictions will be derived for common GeV - TeV sources for the
upcoming GLAST mission bridging for the first time the energy gap between
current GeV and TeV instruments.Comment: (1) Kavli Institute for Particle Astrophysics and Cosmology (KIPAC),
Stanford, USA (2) Stanford University, W.W. Hansen Experimental Physics Lab
(HEPL) and KIPAC, Stanford, USA (3) ICREA & Institut de Ciencies de l'Espai
(IEEC-CSIC) Campus UAB, Fac. de Ciencies, Barcelona, Spain. (4) School of
Physics and Astronomy, University of Leeds, UK. Paper Submitted to Ap
On the origin of \gamma-ray emission in \eta\ Carina
\eta\ Car is the only colliding-wind binary for which high-energy \gamma\
rays are detected. Although the physical conditions in the shock region change
on timescales of hours to days, the variability seen at GeV energies is weak
and on significantly longer timescales. The \gamma-ray spectrum exhibits two
features that can be interpreted as emission from the shocks on either side of
the contact discontinuity. Here we report on the first time-dependent modelling
of the non-thermal emission in \eta\ Car. We find that emission from primary
electrons is likely not responsible for the \gamma-ray emission, but
accelerated protons interacting with the dense wind material can explain the
observations. In our model, efficient acceleration is required at both shocks,
with the primary side acting as a hadron calorimeter, whilst on the companion
side acceleration is limited by the flow time out of the system, resulting in
changing acceleration conditions. The system therefore represents a unique
laboratory for the exploration of hadronic particle acceleration in
non-relativistic shocks.Comment: 5 pages, 4 figures, 1 table, accepted for publication in MNRAS
Letter
Multiple Quantum-Well Technology Takes SEED
Progress in the development of self-electrooptic-effect devices (SEEDs) is discussed. The devices include the resistor-SEED (R-SEED) device, which can be viewed as a simple NOR gate. The symmetric SEED (S-SEED) and the logic-SEED (L-SEED) devices with improved features, functionality, and performance are also considered. The integration of FETs with multiple quantum well (MQW) modulators (FET-SEED), enables optical interconnections of electronic circuits. Where the SEED technology can be used is discussed, and an experimental optical switching fabric made using these devices is described
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