3,143 research outputs found
Separating Gravitational Wave Signals from Instrument Artifacts
Central to the gravitational wave detection problem is the challenge of
separating features in the data produced by astrophysical sources from features
produced by the detector. Matched filtering provides an optimal solution for
Gaussian noise, but in practice, transient noise excursions or ``glitches''
complicate the analysis. Detector diagnostics and coincidence tests can be used
to veto many glitches which may otherwise be misinterpreted as gravitational
wave signals. The glitches that remain can lead to long tails in the matched
filter search statistics and drive up the detection threshold. Here we describe
a Bayesian approach that incorporates a more realistic model for the instrument
noise allowing for fluctuating noise levels that vary independently across
frequency bands, and deterministic ``glitch fitting'' using wavelets as
``glitch templates'', the number of which is determined by a trans-dimensional
Markov chain Monte Carlo algorithm. We demonstrate the method's effectiveness
on simulated data containing low amplitude gravitational wave signals from
inspiraling binary black hole systems, and simulated non-stationary and
non-Gaussian noise comprised of a Gaussian component with the standard
LIGO/Virgo spectrum, and injected glitches of various amplitude, prevalence,
and variety. Glitch fitting allows us to detect significantly weaker signals
than standard techniques.Comment: 21 pages, 18 figure
Enabling high confidence detections of gravitational-wave bursts
With the advanced LIGO and Virgo detectors taking observations the detection
of gravitational waves is expected within the next few years. Extracting
astrophysical information from gravitational wave detections is a well-posed
problem and thoroughly studied when detailed models for the waveforms are
available. However, one motivation for the field of gravitational wave
astronomy is the potential for new discoveries. Recognizing and characterizing
unanticipated signals requires data analysis techniques which do not depend on
theoretical predictions for the gravitational waveform. Past searches for
short-duration un-modeled gravitational wave signals have been hampered by
transient noise artifacts, or "glitches," in the detectors. In some cases, even
high signal-to-noise simulated astrophysical signals have proven difficult to
distinguish from glitches, so that essentially any plausible signal could be
detected with at most 2-3 level confidence. We have put forth the
BayesWave algorithm to differentiate between generic gravitational wave
transients and glitches, and to provide robust waveform reconstruction and
characterization of the astrophysical signals. Here we study BayesWave's
capabilities for rejecting glitches while assigning high confidence to
detection candidates through analytic approximations to the Bayesian evidence.
Analytic results are tested with numerical experiments by adding simulated
gravitational wave transient signals to LIGO data collected between 2009 and
2010 and found to be in good agreement.Comment: 15 pages, 6 figures, submitted to PR
Extreme distributions of ground winds /3 to 150 meters/ at Cape Kennedy, Florida
Statistical analysis of wind distribution probabilities at Cape Kenned
BayesWave: Bayesian Inference for Gravitational Wave Bursts and Instrument Glitches
A central challenge in Gravitational Wave Astronomy is identifying weak
signals in the presence of non-stationary and non-Gaussian noise. The
separation of gravitational wave signals from noise requires good models for
both. When accurate signal models are available, such as for binary Neutron
star systems, it is possible to make robust detection statements even when the
noise is poorly understood. In contrast, searches for "un-modeled" transient
signals are strongly impacted by the methods used to characterize the noise.
Here we take a Bayesian approach and introduce a multi-component, variable
dimension, parameterized noise model that explicitly accounts for
non-stationarity and non-Gaussianity in data from interferometric gravitational
wave detectors. Instrumental transients (glitches) and burst sources of
gravitational waves are modeled using a Morlet-Gabor continuous wavelet frame.
The number and placement of the wavelets is determined by a trans-dimensional
Reversible Jump Markov Chain Monte Carlo algorithm. The Gaussian component of
the noise and sharp line features in the noise spectrum are modeled using the
BayesLine algorithm, which operates in concert with the wavelet model.Comment: 36 pages, 15 figures, Version accepted by Class. Quant. Gra
Prototype Global Analysis of LISA Data with Multiple Source Types
The novel data analysis challenges posed by the Laser Interferometer Space
Antenna (LISA) arise from the overwhelmingly large number of astrophysical
sources in the measurement band and the density with which they are found in
the data. Robust detection and characterization of the numerous gravitational
wave sources in LISA data can not be done sequentially, but rather through a
simultaneous global fit of a data model containing the full suite of
astrophysical and instrumental features present in the data. While previous
analyses have focused on individual source types in isolation, here we present
the first demonstration of a LISA global fit analysis containing combined
astrophysical populations. The prototype pipeline uses a blocked Metropolis
Hastings algorithm to alternatingly fit to a population of ultra compact
galactic binaries, known ``verification binaries'' already identified by
electromagnetic observations, a population of massive black hole mergers, and
an instrument noise model. The Global LISA Analysis Software Suite (GLASS) is
assembled from independently developed samplers for the different model
components. The modular design enables flexibility to future development by
defining standard interfaces for adding new, or updating additional, components
to the global fit without being overly prescriptive for how those modules must
be internally designed. The GLASS pipeline is demonstrated on data simulated
for the LISA Data Challenge 2b. Results of the analysis and a road-map for
continued development are described in detail.Comment: 23 pages, 21 figures, submitted to Phys Rev
Validation and Calibration of Models for Reaction-Diffusion Systems
Space and time scales are not independent in diffusion. In fact, numerical
simulations show that different patterns are obtained when space and time steps
( and ) are varied independently. On the other hand,
anisotropy effects due to the symmetries of the discretization lattice prevent
the quantitative calibration of models. We introduce a new class of explicit
difference methods for numerical integration of diffusion and
reaction-diffusion equations, where the dependence on space and time scales
occurs naturally. Numerical solutions approach the exact solution of the
continuous diffusion equation for finite and , if the
parameter assumes a fixed constant value,
where is an odd positive integer parametrizing the alghorithm. The error
between the solutions of the discrete and the continuous equations goes to zero
as and the values of are dimension
independent. With these new integration methods, anisotropy effects resulting
from the finite differences are minimized, defining a standard for validation
and calibration of numerical solutions of diffusion and reaction-diffusion
equations. Comparison between numerical and analytical solutions of
reaction-diffusion equations give global discretization errors of the order of
in the sup norm. Circular patterns of travelling waves have a maximum
relative random deviation from the spherical symmetry of the order of 0.2%, and
the standard deviation of the fluctuations around the mean circular wave front
is of the order of .Comment: 33 pages, 8 figures, to appear in Int. J. Bifurcation and Chao
Design and operation of an autosampler controlled flow-injection preconcentration system for lead determination by flame atomic absorption spectrometry
Flow-injection manifolds are described which allow the preconcentration
of lead for flame atomic absorption determinations, using
columns contained within the sample loop of an injection valve. An
interface was designed which allowed the valves and pump in the
system to be controlled by an autosampler which enabled precise
timing of preconcentration and elution steps. The effects of sample
flow rate, buffer pH and buffer type for preconcentration and
eluent concentration and flow rate were investigated in order to
obtain optimum performance of the system. A 50-times improvement
in detection limits over conventional sample introduction was
obtained for a sample volume of approximately 12 ml, preconcentrated for
150 s. The injection of eluent, as opposed to the use of a
continuously flowing eluent stream, enabled this reagent to be
conserved
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