124,967 research outputs found
Accelerated filtering on graphs using Lanczos method
Signal-processing on graphs has developed into a very active field of
research during the last decade. In particular, the number of applications
using frames constructed from graphs, like wavelets on graphs, has
substantially increased. To attain scalability for large graphs, fast
graph-signal filtering techniques are needed. In this contribution, we propose
an accelerated algorithm based on the Lanczos method that adapts to the
Laplacian spectrum without explicitly computing it. The result is an accurate,
robust, scalable and efficient algorithm. Compared to existing methods based on
Chebyshev polynomials, our solution achieves higher accuracy without increasing
the overall complexity significantly. Furthermore, it is particularly well
suited for graphs with large spectral gaps
The Kernel Polynomial Method
Efficient and stable algorithms for the calculation of spectral quantities
and correlation functions are some of the key tools in computational condensed
matter physics. In this article we review basic properties and recent
developments of Chebyshev expansion based algorithms and the Kernel Polynomial
Method. Characterized by a resource consumption that scales linearly with the
problem dimension these methods enjoyed growing popularity over the last decade
and found broad application not only in physics. Representative examples from
the fields of disordered systems, strongly correlated electrons,
electron-phonon interaction, and quantum spin systems we discuss in detail. In
addition, we illustrate how the Kernel Polynomial Method is successfully
embedded into other numerical techniques, such as Cluster Perturbation Theory
or Monte Carlo simulation.Comment: 32 pages, 17 figs; revised versio
RXTE Observation of Cygnus X-1: II. Timing Analysis
We present timing analysis for a Rossi X-ray Timing Explorer observation of
Cygnus X-1 in its hard/low state. This was the first RXTE observation of Cyg
X-1 taken after it transited back to this state from its soft/high state.
RXTE's large effective area, superior timing capabilities, and ability to
obtain long, uninterrupted observations have allowed us to obtain measurements
of the power spectral density (PSD), coherence function, and Fourier time lags
to a decade lower in frequency and half a decade higher in frequency than
typically was achieved with previous instruments. Notable aspects of our
observations include a weak 0.005 Hz feature in the PSD coincident with a
coherence recovery; a `hardening' of the high-frequency PSD with increasing
energy; a broad frequency range measurement of the coherence function,
revealing rollovers from unity coherence at both low and high frequency; and an
accurate determination of the Fourier time lags over two and a half decades in
frequency. As has been noted in previous similar observations, the time delay
is approximately proportional to f^(-0.7), and at a fixed Fourier frequency the
time delay of the hard X-rays compared to the softest energy channel tends to
increase logarithmically with energy. Curiously, the 0.01-0.2 Hz coherence
between the highest and lowest energy bands is actually slightly greater than
the coherence between the second highest and lowest energy bands. We carefully
describe all of the analysis techniques used in this paper, and we make
comparisons of the data to general theoretical expectations. In a companion
paper, we make specific comparisons to a Compton corona model that we have
successfully used to describe the energy spectral data from this observation.Comment: To Be Published in the Astrophysical Journal. 18 pages. Uses
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Silicon optical modulators
Optical technology is poised to revolutionise short reach interconnects. The leading candidate technology is silicon photonics, and the workhorse of such interconnect is the optical modulator. Modulators have been improved dramatically in recent years. Most notably the bandwidth has increased from the MHz to the multi GHz regime in little more than half a decade. However, the demands of optical interconnect are significant, and many questions remain unanswered as to whether silicon can meet the required performance metrics. Minimising metrics such as the energy per bit, and device footprint, whilst maximising bandwidth and modulation depth are non trivial demands. All of this must be achieved with acceptable thermal tolerance and optical spectral width, using CMOS compatible fabrication processes. Here we discuss the techniques that have, and will, be used to implement silicon optical modulators, as well as the outlook for these devices, and the candidate solutions of the future
Using Time-Series Analysis to Assess the Extent of Climate Variability and Climate Change in Bayelsa State, Nigeria
Time series analysis is a useful statistical tool in the assessment of climate variability and climate change. This study applied a time series analysis to rainfall and temperature data in the Bayelsa State of Nigeria. Since NIMET has only one gauging station in the state, Climate Research (CRU 0.5Ă 0.5) gridded data for 28 locations from 1956 to 2016 were used. They were sorted, validated with NiMet data, and utilized for analyses of various time series techniques such as Mann-Kendal, Spearmanâs Rho, Linear Regression, Thei-Sen Slope Cumulative sum, Cumulative Deviation, Rank Sum, Studentâs (t-test) and spectral analysis. The results obtained revealed that there had been increasing temperature and abrupt climatic changes in the state, especially in the 1976-1985 decade, with 1980 as the most probable year of abrupt change. The hottest decade was 1986-1995, with an average temperature change of 0.14856 oC/decade, while the coolest decade was 1976-1985 with an average Temperature change of -0.01723 oC/decade. Also, there had been some changes in rainfall, with the wettest decade occurring in 1986-1995 with an average rainfall change of 61mm/decade, while the driest decade occurred in 1976-1985 with an average rainfall change of 14.08 mm/decade. The output of spectral analysis showed that the most Significant Periodicity for Rainfall and Temperature was 15 years. The result further revealed that there was high rainfall variability with a coefficient of variability of 62.74%. These rainfall fluctuations have implications for coastal flooding, quality, and quantity of available groundwater in the state. These results are useful to planners and policymakers in creating awareness of climate change's impact on rainfall in the study are
Quenching of Star Formation
In the last decade we have seen an enormous increase in the size and quality
of spectroscopic galaxy surveys, both at low and high redshift. New statistical
techniques to analyse large portions of galaxy spectra are now finding favour
over traditional index based methods. Here we will review a new robust and
iterative Principal Component Analysis (PCA) algorithm, which solves several
common issues with classic PCA. Application to the 4000AA break region of
galaxies in the VIMOS VLT Deep Survey (VVDS) and Sloan Digital Sky Survey
(SDSS) gives new high signal-to-noise ratio spectral indices easily
interpretable in terms of recent star formation history. In particular, we
identify a sample of post-starburst galaxies at z~0.7 and z~0.07. We quantify
for the first time the importance of post-starburst galaxies, consistent with
being descendants of gas-rich major mergers, for building the red sequence.
Finally, we present a comparison with new low and high redshift "mock
spectroscopic surveys" derived from a Millennium Run semi-analytic model.Comment: 7 pages, 3 figures. Conference proceedings in "Classification and
Discovery in Large Astronomical Surveys", 2008, C.A.L. Bailer-Jones (ed.
Integrating Light Curve and Atmospheric Modeling of Transiting Exoplanets
Spectral retrieval techniques are currently our best tool to interpret the observed exoplanet atmospheric data. Said techniques retrieve the optimal atmospheric components and parameters by identifying the best fit to an observed transmission/emission spectrum. Over the past decade, our understanding of remote worlds in our galaxy has flourished thanks to the use of increasingly sophisticated spectral retrieval techniques and the collective effort of the community working on exoplanet atmospheric models. A new generation of instruments in space and from the ground is expected to deliver higher quality data in the next decade; it is therefore paramount to upgrade current models and improve their reliability, their completeness, and the numerical speed with which they can be run. In this paper, we address the issue of reliability of the results provided by retrieval models in the presence of systematics of unknown origin. More specifically, we demonstrate that if we fit directly individual light curves at different wavelengths (L-retrieval), instead of fitting transit or eclipse depths, as it is currently done (S-retrieval), the said methodology is more sensitive against astrophysical and instrumental noise. This new approach is tested, in particular, when discrepant simulated observations from Hubble Space Telescope/Wide Field Camera 3 and Spitzer/IRAC are combined. We find that while S-retrievals converge to an incorrect solution without any warning, L-retrievals are able to flag potential discrepancies between the data sets
Stellar populations -- the next ten years
The study of stellar populations is a discipline that is highly dependent on
both imaging and spectroscopy. I discuss techniques in different regimes of
resolving power: broadband imaging (R~4), intermediate band imaging (R~16, 64),
narrowband spectral imaging (R~256, 1024, 4096). In recent years, we have seen
major advances in broadband all-sky surveys that are set to continue across
optical and IR bands, with the added benefit of the time domain, higher
sensitivity, and improved photometric accuracy. Tunable filters and integral
field spectrographs are poised to make further inroads into intermediate and
narrowband imaging studies of stellar populations. Further advances will come
from AO-assisted imaging and imaging spectroscopy, although photometric
accuracy will be challenging. Integral field spectroscopy will continue to have
a major impact on future stellar population studies, extending into the near
infrared once the OH suppression problem is finally resolved. A sky rendered
dark will allow a host of new ideas to be explored, and old ideas to be
revisited.Comment: Invited review, IAUS 241, "Stellar Populations as Building Blocks of
Galaxies," eds. Vazdekis, Peletier. 12 pages, 1 table. (The sideways table
should print ok; there are 10 columns.
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