401 research outputs found
Investigating Light Curve Modulation via Kernel Smoothing. I. Application to 53 fundamental mode and first-overtone Cepheids in the LMC
Recent studies have revealed a hitherto unknown complexity of Cepheid
pulsation. We implement local kernel regression to search for both period and
amplitude modulations simultaneously in continuous time and to investigate
their detectability, and test this new method on 53 classical Cepheids from the
OGLE-III catalog. We determine confidence intervals using parametric and
non-parametric bootstrap sampling to estimate significance and investigate
multi-periodicity using a modified pre-whitening approach that relies on
time-dependent light curve parameters. We find a wide variety of period and
amplitude modulations and confirm that first overtone pulsators are less stable
than fundamental mode Cepheids. Significant temporal variations in period are
more frequently detected than those in amplitude. We find a range of modulation
intensities, suggesting that both amplitude and period modulations are
ubiquitous among Cepheids. Over the 12-year baseline offered by OGLE-III, we
find that period changes are often non-linear, sometimes cyclic, suggesting
physical origins beyond secular evolution. Our method more efficiently detects
modulations (period and amplitude) than conventional methods reliant on
pre-whitening with constant light curve parameters and more accurately
pre-whitens time series, removing spurious secondary peaks effectively.Comment: Re-submitted including revisions to Astronomy and Astrophysic
Model misspecification in peaks over threshold analysis
Classical peaks over threshold analysis is widely used for statistical
modeling of sample extremes, and can be supplemented by a model for the sizes
of clusters of exceedances. Under mild conditions a compound Poisson process
model allows the estimation of the marginal distribution of threshold
exceedances and of the mean cluster size, but requires the choice of a
threshold and of a run parameter, , that determines how exceedances are
declustered. We extend a class of estimators of the reciprocal mean cluster
size, known as the extremal index, establish consistency and asymptotic
normality, and use the compound Poisson process to derive misspecification
tests of model validity and of the choice of run parameter and threshold.
Simulated examples and real data on temperatures and rainfall illustrate the
ideas, both for estimating the extremal index in nonstandard situations and for
assessing the validity of extremal models.Comment: Published in at http://dx.doi.org/10.1214/09-AOAS292 the Annals of
Applied Statistics (http://www.imstat.org/aoas/) by the Institute of
Mathematical Statistics (http://www.imstat.org
Likelihood estimation of the extremal index
The article develops the approach of Ferro and Segers (J.R. Stat. Soc., Ser. B 65:545, 2003) to the estimation of the extremal index, and proposes the use of a new variable decreasing the bias of the likelihood based on the point process character of the exceedances. Two estimators are discussed: a maximum likelihood estimator and an iterative least squares estimator based on the normalized gaps between clusters. The first provides a flexible tool for use with smoothing methods. A diagnostic is given for condition , under which maximum likelihood is valid. The performance of the new estimators were tested by extensive simulations. An application to the Central England temperature series demonstrates the use of the maximum likelihood estimator together with smoothing method
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