6,920 research outputs found
Observation of soliton explosions in a passively mode-locked fiber laser
Soliton explosions are among the most exotic dissipative phenomena studied in
mode-locked lasers. In this regime, a dissipative soliton circulating in the
laser cavity experiences an abrupt structural collapse, but within a few
roundtrips returns to its original quasi-stable state. In this work we report
on the first observation of such events in a fiber laser. Specifically, we
identify clear explosion signatures in measurements of shot-to-shot spectra of
an Yb-doped mode-locked fiber laser that is operating in a transition regime
between stable and noise-like emission. The comparatively long,
all-normal-dispersion cavity used in our experiments also permits direct
time-domain measurements, and we show that the explosions manifest themselves
as abrupt temporal shifts in the output pulse train. Our experimental results
are in good agreement with realistic numerical simulations based on an
iterative cavity map.Comment: 5 pages, 5 figures, submitte
Structural and spectroscopic characterisation of C4 oxygenates relevant to structure/activity relationships of the hydrogenation of α,β-unsaturated carbonyls
In the present work, we have investigated the conformational isomerism and calculated the vibrational spectra of the C4 oxygenates: 3-butyne-2-one, 3-butene-2-one, 2-butanone and 2-butanol using density functional theory. The calculations are validated by comparison to structural data where available and new, experimental inelastic neutron scattering and infrared spectra of the compounds. We find that for 3-butene-2-one and 2-butanol the spectra show clear evidence for the presence of conformational isomerism and this is supported by the calculations. Complete vibrational assignments for all four molecules are provided and this provides the essential information needed to generate structure/activity relationships for the sequential catalytic hydrogenation of 3-butyne-2-one to 2-butanol
Concepts for autonomous flight control for a balloon on Mars
Balloons operating as airborne rovers have been suggested as ideal candidates for early exploration of the Martian surface. An international study team composed of scientists from the U.S.S.R., France, and the U.S.A. is planning the launching in 1994 of a balloon system to fly on Mars. The current likely design is a dual thermal/gas balloon that consists of a gas balloon suspended above a solar-heated thermal balloon. At night, the thermal balloon provides no lift, and the balloon system drifts just above the Martian surface; the lift of the gas balloon is just sufficient to prevent the science payload from hitting the ground. During the day, the balloon system flies at an altitude of 4 to 5 kilometers, rising due to the added lift provided by the thermal balloon. Over the course of a single Martian day, there may be winds in several directions, and in fact it can be expected that there will be winds simultaneously in different directions at different altitudes. Therefore, a balloon system capable of controlling its own altitude, via an autonomous flight control system, can take advantage of these different winds to control its direction, thereby greatly increasing both its mission utility and its longevity
Effective Actions for Heterotic M-Theory
We discuss the moduli space approximation for heterotic M-theory, both for
the minimal case of two boundary branes only, and when a bulk brane is
included. The resulting effective actions may be used to describe the
cosmological dynamics in the regime where the branes are moving slowly, away
from singularities. We make use of the recently derived colliding branes
solution to determine the global structure of moduli space, finding a boundary
at which the trajectories undergo a hard wall reflection. This has important
consequences for the allowed moduli space trajectories, and for the behaviour
of cosmological perturbations in the model.Comment: 21 pages, 4 figures. References added and some discussions clarifie
Accelerating networks
Evolving out-of-equilibrium networks have been under intense scrutiny
recently. In many real-world settings the number of links added per new node is
not constant but depends on the time at which the node is introduced in the
system. This simple idea gives rise to the concept of accelerating networks,
for which we review an existing definition and -- after finding it somewhat
constrictive -- offer a new definition. The new definition provided here views
network acceleration as a time dependent property of a given system, as opposed
to being a property of the specific algorithm applied to grow the network. The
defnition also covers both unweighted and weighted networks. As time-stamped
network data becomes increasingly available, the proposed measures may be
easily carried out on empirical datasets. As a simple case study we apply the
concepts to study the evolution of three different instances of Wikipedia,
namely, those in English, German, and Japanese, and find that the networks
undergo different acceleration regimes in their evolution.Comment: 12 pages, 8 figure
Modeling Long- and Short-Term Temporal Patterns with Deep Neural Networks
Multivariate time series forecasting is an important machine learning problem
across many domains, including predictions of solar plant energy output,
electricity consumption, and traffic jam situation. Temporal data arise in
these real-world applications often involves a mixture of long-term and
short-term patterns, for which traditional approaches such as Autoregressive
models and Gaussian Process may fail. In this paper, we proposed a novel deep
learning framework, namely Long- and Short-term Time-series network (LSTNet),
to address this open challenge. LSTNet uses the Convolution Neural Network
(CNN) and the Recurrent Neural Network (RNN) to extract short-term local
dependency patterns among variables and to discover long-term patterns for time
series trends. Furthermore, we leverage traditional autoregressive model to
tackle the scale insensitive problem of the neural network model. In our
evaluation on real-world data with complex mixtures of repetitive patterns,
LSTNet achieved significant performance improvements over that of several
state-of-the-art baseline methods. All the data and experiment codes are
available online.Comment: Accepted by SIGIR 201
An upper mass limit for the progenitor of the TypeII-P supernova SN1999gi
Masses and progenitor evolutionary states of TypeII supernovae remain almost
unconstrained by direct observations. Only one robust observation of a
progenitor (SN1987A) and one plausible observation (SN1993J) are available.
Neither matched theoretical predictions and in this Letter we report limits on
a third progenitor (SN1999gi). The Hubble Space Telescope has imaged the site
of the TypeII-P supernova SN1999gi with the WFPC2 in two filters (F606W and
F300W) prior to explosion. The distance to the host galaxy (NGC3184) of 7.9Mpc
means that the most luminous, massive stars are resolved as single objects in
the archive images. The supernova occurred in a resolved, young OB association
2.3kpc from the centre of NGC3184 with an association age of about 4Myrs.
Follow-up images of SN1999gi with WFPC2 taken 14 months after discovery
determine the precise position of the SN on the pre-explosion frames. An upper
limit of the absolute magnitude of the progenitor is estimated (M_v >= -5.1).
By comparison with stellar evolutionary tracks this can be interpreted as a
stellar mass, and we determine an upper mass limit of 9(+3/-2)M_solar. We
discuss the possibility of determining the masses or mass limits for numerous
nearby core-collapse supernovae using the HST archive enhanced by our current
SNAP programme.Comment: To appear in ApJ Letters, 16 pages, 3 figure
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