16,763 research outputs found
Fast Inverse Nonlinear Fourier Transform For Generating Multi-Solitons In Optical Fiber
The achievable data rates of current fiber-optic
wavelength-division-multiplexing (WDM) systems are limited by nonlinear
interactions between different subchannels. Recently, it was thus proposed to
replace the conventional Fourier transform in WDM systems with an appropriately
defined nonlinear Fourier transform (NFT). The computational complexity of NFTs
is a topic of current research. In this paper, a fast inverse NFT algorithm for
the important special case of multi-solitonic signals is presented. The
algorithm requires only floating point operations to
compute samples of a multi-soliton. To the best of our knowledge, this is
the first algorithm for this problem with -linear complexity. The
paper also includes a many samples analysis of the generated nonlinear Fourier
spectra.Comment: Submitted to IEEE ISIT 2015 (fixed a few typos
Simulations of the formation and evolution of isolated dwarf galaxies
We present new fully self-consistent models of the formation and evolution of
isolated dwarf galaxies. We have used the publicly available N-body/SPH code
HYDRA, to which we have added a set of star formation criteria, and
prescriptions for chemical enrichment (taking into account contributions from
both SNIa and SNII), supernova feedback, and gas cooling. The models follow the
evolution of an initially homogeneous gas cloud collapsing in a pre-existing
dark-matter halo. These simplified initial conditions are supported by the
merger trees of isolated dwarf galaxies extracted from the milli-Millennium
Simulation.
The star-formation histories of the model galaxies exhibit burst-like
behaviour. These bursts are a consequence of the blow-out and subsequent
in-fall of gas. The amount of gas that leaves the galaxy for good is found to
be small, in absolute numbers, ranging between 3x10^7 Msol and 6x10^7 Msol .
For the least massive models, however, this is over 80 per cent of their
initial gas mass. The local fluctuations in gas density are strong enough to
trigger star-bursts in the massive models, or to inhibit anything more than
small residual star formation for the less massive models. Between these
star-bursts there can be time intervals of several Gyrs.
We have compared model predictions with available data for the relations
between luminosity and surface brightness profile, half-light radius, central
velocity dispersion, broad band colour (B-V) and metallicity, as well as the
location relative to the fundamental plane. The properties of the model dwarf
galaxies agree quite well with those of observed dwarf galaxies.Comment: 16 pages, 20 figures, accepted for publication in MNRA
Matched wideband low-noise amplifiers for radio astronomy
Two packaged low noise amplifiers for the 0.3–4 GHz frequency range are described. The amplifiers can be operated at temperatures of 300–4 K and achieve noise temperatures in the 5 K range (<0.1 dB noise figure) at 15 K physical temperature. One amplifier utilizes commercially available, plastic-packaged SiGe transistors for first and second stages; the second amplifier is identical except it utilizes an experimental chip transistor as the first stage. Both amplifiers use resistive feedback to provide input reflection coefficient S11<−10 dB over a decade bandwidth with gain over 30 dB. The amplifiers can be used as rf amplifiers in very low noise radio astronomy systems or as i.f. amplifiers following superconducting mixers operating in the millimeter and submillimeter frequency range
Efficient Computation in Adaptive Artificial Spiking Neural Networks
Artificial Neural Networks (ANNs) are bio-inspired models of neural
computation that have proven highly effective. Still, ANNs lack a natural
notion of time, and neural units in ANNs exchange analog values in a
frame-based manner, a computationally and energetically inefficient form of
communication. This contrasts sharply with biological neurons that communicate
sparingly and efficiently using binary spikes. While artificial Spiking Neural
Networks (SNNs) can be constructed by replacing the units of an ANN with
spiking neurons, the current performance is far from that of deep ANNs on hard
benchmarks and these SNNs use much higher firing rates compared to their
biological counterparts, limiting their efficiency. Here we show how spiking
neurons that employ an efficient form of neural coding can be used to construct
SNNs that match high-performance ANNs and exceed state-of-the-art in SNNs on
important benchmarks, while requiring much lower average firing rates. For
this, we use spike-time coding based on the firing rate limiting adaptation
phenomenon observed in biological spiking neurons. This phenomenon can be
captured in adapting spiking neuron models, for which we derive the effective
transfer function. Neural units in ANNs trained with this transfer function can
be substituted directly with adaptive spiking neurons, and the resulting
Adaptive SNNs (AdSNNs) can carry out inference in deep neural networks using up
to an order of magnitude fewer spikes compared to previous SNNs. Adaptive
spike-time coding additionally allows for the dynamic control of neural coding
precision: we show how a simple model of arousal in AdSNNs further halves the
average required firing rate and this notion naturally extends to other forms
of attention. AdSNNs thus hold promise as a novel and efficient model for
neural computation that naturally fits to temporally continuous and
asynchronous applications
Modelling with measures: Approximation of a mass-emitting object by a point source
We consider a linear diffusion equation on
, where
is a bounded domain. The time-dependent flux on the
boundary is prescribed. The aim of the
paper is to approximate the dynamics by the solution of the diffusion equation
on the whole of with a measure-valued point source in the origin
and provide estimates for the quality of approximation. For all time , we
derive an -bound on the difference in flux on the
boundary. Moreover, we derive for all an -bound and an
-bound for the difference of the solutions to the two
models
Proving Termination of Graph Transformation Systems using Weighted Type Graphs over Semirings
We introduce techniques for proving uniform termination of graph
transformation systems, based on matrix interpretations for string rewriting.
We generalize this technique by adapting it to graph rewriting instead of
string rewriting and by generalizing to ordered semirings. In this way we
obtain a framework which includes the tropical and arctic type graphs
introduced in a previous paper and a new variant of arithmetic type graphs.
These type graphs can be used to assign weights to graphs and to show that
these weights decrease in every rewriting step in order to prove termination.
We present an example involving counters and discuss the implementation in the
tool Grez
The metallicity dependence of WR winds
Wolf-Rayet (WR) stars are the most advanced stage in the evolution of the
most massive stars. The strong feedback provided by these objects and their
subsequent supernova (SN) explosions are decisive for a variety of
astrophysical topics such as the cosmic matter cycle. Consequently,
understanding the properties of WR stars and their evolution is indispensable.
A crucial but still not well known quantity determining the evolution of WR
stars is their mass-loss rate. Since the mass loss is predicted to increase
with metallicity, the feedback provided by these objects and their spectral
appearance are expected to be a function of the metal content of their host
galaxy. This has severe implications for the role of massive stars in general
and the exploration of low metallicity environments in particular. Hitherto,
the metallicity dependence of WR star winds was not well studied. In this
contribution, we review the results from our comprehensive spectral analyses of
WR stars in environments of different metallicities, ranging from slightly
super-solar to SMC-like metallicities. Based on these studies, we derived
empirical relations for the dependence of the WN mass-loss rates on the
metallicity and iron abundance, respectively.Comment: 5 pages, 4 figures, to be published in the Proceedings of the IAU
Symposium No. 329 "The lives and death-throes of massive stars
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