87,317 research outputs found
Regression analysis with missing data and unknown colored noise: application to the MICROSCOPE space mission
The analysis of physical measurements often copes with highly correlated
noises and interruptions caused by outliers, saturation events or transmission
losses. We assess the impact of missing data on the performance of linear
regression analysis involving the fit of modeled or measured time series. We
show that data gaps can significantly alter the precision of the regression
parameter estimation in the presence of colored noise, due to the frequency
leakage of the noise power. We present a regression method which cancels this
effect and estimates the parameters of interest with a precision comparable to
the complete data case, even if the noise power spectral density (PSD) is not
known a priori. The method is based on an autoregressive (AR) fit of the noise,
which allows us to build an approximate generalized least squares estimator
approaching the minimal variance bound. The method, which can be applied to any
similar data processing, is tested on simulated measurements of the MICROSCOPE
space mission, whose goal is to test the Weak Equivalence Principle (WEP) with
a precision of . In this particular context the signal of interest is
the WEP violation signal expected to be found around a well defined frequency.
We test our method with different gap patterns and noise of known PSD and find
that the results agree with the mission requirements, decreasing the
uncertainty by a factor 60 with respect to ordinary least squares methods. We
show that it also provides a test of significance to assess the uncertainty of
the measurement.Comment: 12 pages, 4 figures, to be published in Phys. Rev.
Potential of the next generation VHE instruments to probe the EBL (I): the low- and mid-VHE
The diffuse meta-galactic radiation field at ultraviolet to infrared
wavelengths - commonly labeled extragalactic background light (EBL) - contains
the integrated emission history of the universe. Difficult to access via direct
observations indirect constraints on its density can be derived through
observations of very-high energy (VHE; E>100 GeV) gamma-rays from distant
sources: the VHE photons are attenuated via pair-production with the low energy
photons from the EBL, leaving a distinct imprint in the VHE spectra measured on
earth. Discoveries made with current generation VHE observatories like H.E.S.S.
and MAGIC enabled strong constraints on the density of the EBL especially in
the near-infrared. In this article the prospect of future VHE observatories to
derive new constraints on the EBL density are discussed. To this end, results
from current generation instruments will be extrapolated to the future
experiment's sensitivity and investigated for their power to enable new methods
and improved constraints on the EBL density.Comment: Accepted for publication in Astroparticle Physics; v2: extended
discussion following referees comments, conclusions unchange
Improving likelihood-based inference in control rate regression
Control rate regression is a diffuse approach to account for heterogeneity
among studies in meta-analysis by including information about the outcome risk
of patients in the control condition. Correcting for the presence of
measurement error affecting risk information in the treated and in the control
group has been recognized as a necessary step to derive reliable inferential
conclusions. Within this framework, the paper considers the problem of small
sample size as an additional source of misleading inference about the slope of
the control rate regression. Likelihood procedures relying on first-order
approximations are shown to be substantially inaccurate, especially when
dealing with increasing heterogeneity and correlated measurement errors. We
suggest to address the problem by relying on higher-order asymptotics. In
particular, we derive Skovgaard's statistic as an instrument to improve the
accuracy of the approximation of the signed profile log-likelihood ratio
statistic to the standard normal distribution. The proposal is shown to provide
much more accurate results than standard likelihood solutions, with no
appreciable computational effort. The advantages of Skovgaard's statistic in
control rate regression are shown in a series of simulation experiments and
illustrated in a real data example. R code for applying first- and second-order
statistic for inference on the slope on the control rate regression is
provided
Systematic errors and combination of individual CRF solutions in the framework of the international pilot project for the next ICRF
A new international Pilot Project for the re-determination of the ICRF was
initiated by the International VLBI Service for Geodesy and Astrometry (IVS) in
January 2005. The purpose of this project is to compare the individual CRF
solutions and to analyze their systematic and random errors with focus on the
selection of the optimal strategy for the next ICRF realization. Eight radio
source catalogues provided by the IVS Analysis Centers GA, SHAO, DGFI,
GIUB-BKG, JPL, MAO NANU, GSFC, USNO were analyzed. In present study, four
analytical models were used to investigate the systematic differences between
solutions: solid rotation, rotation and deformation (IERS method), and
expansion in orthogonal functions: Legendre-Fourier polynomials and spherical
functions. It was found that expansions by orthogonal function describe the
differences between individual catalogues better than the two former models.
Finally, the combined CRF was generated. Using the radio source positions from
this combined catalogue for estimation of EOP has shown improvement of the
uncertainty of the celestial pole offset time series.Comment: 9 pages, 8 figures. Presented at the XXVIth IAU General Assembly,
JD16, Prague, Czech Republic, 14-25 August 200
High-cadence spectroscopy of M-dwarfs – II. Searching for stellar pulsations with HARPS
Stellar oscillations appear all across the Hertzsprung–Russell diagram. Recent theoretical studies support their existence also in the atmosphere of M dwarfs. These studies predict for them short periodicities ranging from 20 min to 3 h. Our Cool Tiny Beats (CTB) programme aims at finding these oscillations for the very first time. With this goal, CTB explores the short time domain of M dwarfs using radial velocity data from the High Accuracy Radial velocity Planet Searcher (HARPS)-European Southern Observatory and HARPS-N high-precision spectrographs. Here we present the results for the two most long-term stable targets observed to date with CTB, GJ 588 and GJ 699 (i.e. Barnard's star). In the first part of this work we detail the correction of several instrumental effects. These corrections are especially relevant when searching for subnight signals. Results show no significant signals in the range where M dwarfs pulsations were predicted. However, we estimate that stellar pulsations with amplitudes larger than ∼0.5 m s−1 can be detected with a 90 per cent completeness with our observations. This result, along with the excess of power regions detected in the periodograms, opens the possibility of non-resolved very low amplitude pulsation signals. Next generation more precise instrumentation would be required to detect such oscillations. However, the possibility of detecting pulsating M-dwarf stars with larger amplitudes is feasible due to the short size of the analysed sample. This motivates the need for completeness of the CTB survey
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