435 research outputs found
GEO600: building a gravitational wave observatory
An international network of laser-interferometric gravitational wave detectors is nearing the end of the main commissioning phase and will soon enter into an extended observational period, collecting data for a duration of order one year. This article outlines the construction, design and commissioning of the GEO 600 gravitational wave observatory. GEO 600 is a laser-interferometric gravitational wave detector which has 600 m long arms and uses advanced optical and suspension techniques to achieve a sensitivity comparable to the longer base-line detectors currently operating in the U.S.A and Europe
Data Analysis Methods for Testing Alternative Theories of Gravity with LISA Pathfinder
In this paper we present a data analysis approach applicable to the potential
saddle-point fly-by mission extension of LISA Pathfinder (LPF). At the peak of
its sensitivity, LPF will sample the gravitational field in our Solar System
with a precision of several at frequencies
around . Such an accurate accelerometer will allow us to test
alternative theories of gravity that predict deviations from Newtonian dynamics
in the non-relativistic limit. As an example, we consider the case of the
Tensor-Vector-Scalar theory of gravity and calculate, within the
non-relativistic limit of this theory, the signals that anomalous tidal
stresses generate in LPF. We study the parameter space of these signals and
divide it into two subgroups, one related to the mission parameters and the
other to the theory parameters that are determined by the gravity model. We
investigate how the mission parameters affect the signal detectability
concluding that these parameters can be determined with the sufficient
precision from the navigation of the spacecraft and fixed during our analysis.
Further, we apply Bayesian parameter estimation and determine the accuracy to
which the gravity theory parameters may be inferred. We evaluate the portion of
parameter space that may be eliminated in case of no signal detection and
estimate the detectability of signals as a function of parameter space
location. We also perform a first investigation of non-Gaussian
"noise-glitches" that may occur in the data. The analysis we develop is
universal and may be applied to anomalous tidal stress induced signals
predicted by any theory of gravity
Calibrating spectral estimation for the LISA Technology Package with multichannel synthetic noise generation
The scientific objectives of the Lisa Technology Package (LTP) experiment, on
board of the LISA Pathfinder mission, demand for an accurate calibration and
validation of the data analysis tools in advance of the mission launch. The
levels of confidence required on the mission outcomes can be reached only with
an intense activity on synthetically generated data. A flexible procedure
allowing the generation of cross-correlated stationary noise time series was
set-up. Multi-channel time series with the desired cross correlation behavior
can be generated once a model for a multichannel cross-spectral matrix is
provided. The core of the procedure is the synthesis of a noise coloring
multichannel filter through a frequency-by-frequency eigendecomposition of the
model cross-spectral matrix and a Z-domain fit. The common problem of initial
transients in noise time series is solved with a proper initialization of the
filter recursive equations. The noise generator performances were tested in a
two dimensional case study of the LTP dynamics along the two principal channels
of the sensing interferometer.Comment: Accepted for publication in Physical Review D (http://prd.aps.org/
Data series subtraction with unknown and unmodeled background noise
LISA Pathfinder (LPF), ESA's precursor mission to a gravitational wave
observatory, will measure the degree to which two test-masses can be put into
free-fall, aiming to demonstrate a residual relative acceleration with a power
spectral density (PSD) below 30 fm/s/Hz around 1 mHz. In LPF data
analysis, the measured relative acceleration data series must be fit to other
various measured time series data. This fitting is required in different
experiments, from system identification of the test mass and satellite dynamics
to the subtraction of noise contributions from measured known disturbances. In
all cases, the background noise, described by the PSD of the fit residuals, is
expected to be coloured, requiring that we perform such fits in the frequency
domain. This PSD is unknown {\it a priori}, and a high accuracy estimate of
this residual acceleration noise is an essential output of our analysis. In
this paper we present a fitting method based on Bayesian parameter estimation
with an unknown frequency-dependent background noise. The method uses noise
marginalisation in connection with averaged Welch's periodograms to achieve
unbiased parameter estimation, together with a consistent, non-parametric
estimate of the residual PSD. Additionally, we find that the method is
equivalent to some implementations of iteratively re-weighted least-squares
fitting. We have tested the method both on simulated data of known PSD, and to
analyze differential acceleration from several experiments with the LISA
Pathfinder end-to-end mission simulator.Comment: To appear Phys. Rev. D90 August 201
Photon pressure induced test mass deformation in gravitational-wave detectors
A widely used assumption within the gravitational-wave community has so far
been that a test mass acts like a rigid body for frequencies in the detection
band, i.e. for frequencies far below the first internal resonance. In this
article we demonstrate that localized forces, applied for example by a photon
pressure actuator, can result in a non-negligible elastic deformation of the
test masses. For a photon pressure actuator setup used in the gravitational
wave detector GEO600 we measured that this effect modifies the standard
response function by 10% at 1 kHz and about 100% at 2.5 kHz
Instrument data simulations for GRACE Follow-on: Observation and noise models
The Gravity Recovery and Climate Experiment (GRACE) mission has yielded data on the Earth's gravity field to monitor temporal changes for more than 15 years. The GRACE twin satellites use microwave ranging with micrometre precision to measure the distance variations between two satellites caused by the Earth's global gravitational field. GRACE Follow-on (GRACE-FO) will be the first satellite mission to use inter-satellite laser interferometry in space. The laser ranging instrument (LRI) will provide two additional measurements compared to the GRACE mission: Interferometric inter-satellite ranging with nanometre precision and inter-satellite pointing information. We have designed a set of simulated GRACE-FO data, which include LRI measurements, apart from all other GRACE instrument data needed for the Earth's gravity field recovery. The simulated data files are publicly available via https://doi.org/10.22027/AMDC2 and can be used to derive gravity field solutions like from GRACE data. This paper describes the scientific basis and technical approaches used to simulate the GRACE-FO instrument data.data. This paper describes the scientific basis and technical approaches used to simulate the GRACE-FO instrument data
Understanding of a negative bowel screening result and potential impact on future symptom appraisal and help-seeking behaviour: a focus group study.
This is the final version of the article. It first appeared from Wiley via http://dx.doi.org/10.1111/hex.12484BACKGROUND: Colorectal cancer (CRC) screening using a faecal occult blood test (FOBt) has the potential to reduce cancer-related mortality. Symptom vigilance remains crucial as a proportion of cancers will be diagnosed between screening rounds. A negative FOBt has the potential to influence how participants respond to future symptoms of CRC. OBJECTIVE: To explore (i) understanding of a negative FOBt and (ii) the potential impact of a negative FOBt upon future symptom appraisal and help-seeking behaviour. DESIGN: Qualitative methodology utilizing focus groups with participants who received a negative FOBt within the National Bowel Cancer Screening Programme in Coventry and Lothian. Topics explored included: experience of screening participation, interpretation and understanding of a negative result, symptom awareness and attitudes towards help-seeking. RESULTS: Four broad themes were identified: (i) emotional response to a negative FOBt, (ii) understanding the limitations of FOBt screening, (iii) symptom knowledge and interpretation and (iv) over-reassurance from a negative FOBt. Participants were reassured by a negative FOBt, but there was variability in the extent to which the result was interpreted as an "all clear". Some participants acknowledged the residual risk of cancer and the temporal characteristic of the result, while others were surprised that the result was not a guarantee that they did not have cancer. DISCUSSION AND CONCLUSIONS: Participants recognized that reassurance from a negative FOBt could lead to a short-term delay in help-seeking if symptoms developed. Screening programmes should seek to emphasize the importance of the temporal nature of FOBt results with key messages about symptom recognition and prompt help-seeking behaviour.This study was funded by the National Awareness and Early Diagnosis Initiative led by Cancer Research UK, the Department of Health, NHS England and Public Health England. Award number C12357/A12240
Quantitative Analysis of LISA Pathfinder Test Mass Noise
In this paper we discuss two main problems associated with the analysis of
the data from LISA Pathfinder (LPF): i) Excess noise detection and ii) Noise
parameter identification. The mission is focused on the low frequency region
([0.1; 10] mHz) of the available signal spectrum. In such a region the signal
is dominated by the force noise acting on test masses. Noise analysis is
expected to deal with a limited amount of non-Gaussian data, since the spectrum
statistics will be far from Gaussian and the lowest available frequency is
limited by the data length. In this paper we analyze the details of the
expected statistics for spectral data and develop two suitable excess noise
estimators. One is based on the statistical properties of the integrated
spectrum, the other is based on Kolmogorov-Smirnov test. The sensitivity of the
estimators is discussed theoretically for independent data, then the algorithms
are tested on LPF synthetic data. The test on realistic LPF data allows the
effect of spectral data correlations on the efficiency of the different noise
excess estimators to be highlighted. It also reveals the versatility of the
Kolmogorov-Smirnov approach, which can be adapted to provide reasonable results
on correlated data from a modified version of the standard equations for the
inversion of the test statistic. Closely related to excess noise detection, the
problem of noise parameter identification in non-Gaussian data is approached in
two ways. One procedure is based on maximum likelihood estimator and another is
based on the Kolmogorov-Smirnov goodness of fit estimator. Both approaches
provide unbiased and accurate results for noise parameter estimation and
demonstrate superior performance with respect to standard weighted
least-squares and Huber's norm.Comment: Accepted for publication in Phys Rev
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