4,517 research outputs found
Linear Prediction of Long-Memory Processes: Asymptotic Results on Mean-squared Errors
We present two approaches for linear prediction of long-memory time series.
The first approach consists in truncating the Wiener-Kolmogorov predictor by
restricting the observations to the last terms, which are the only
available values in practice. We derive the asymptotic behaviour of the
mean-squared error as tends to . By contrast, the second
approach is non-parametric. An AR() model is fitted to the long-memory time
series and we study the error that arises in this misspecified model
Linear Prediction of Long-Range Dependent Time Series
We present two approaches for next step linear prediction of long memory time
series. The first is based on the truncation of the Wiener-Kolmogorov predictor
by restricting the observations to the last terms, which are the only
available values in practice. Part of the mean squared prediction error comes
from the truncation, and another part comes from the parametric estimation of
the parameters of the predictor. By contrast, the second approach is
non-parametric. An AR() model is fitted to the long memory time series and
we study the error made with this misspecified model
Afterglows after Swift
Since their discovery by the Beppo-SAX satellite in 1997, gamma-ray burst
afterglows have attracted an ever-growing interest. They have allowed redshift
measurements that have confirmed that gamma-ray bursts are located at
cosmological distances. Their study covers a huge range both in time (from one
minute to several months after the trigger) and energy (from the GeV to radio
domains). The purpose of this review is first to give a short historical
account of afterglow research and describe the main observational results with
a special attention to the early afterglow revealed by Swift. We then present
the standard afterglow model as it has been developed in the pre-Swift era and
show how it is challenged by the recent Swift and Fermi results. We finally
discuss different options (within the standard framework or implying a change
of paradigm) that have been proposed to solve the current problems.Comment: 16 page
Effect of branch position and light availability on shoot growth of understory sugar maple and yellow birch saplings
Phenotypic plasticity enables tree saplings to change their morphology according to their environment to grow toward a better light micro-habitat. Therefore, processes of crown development could be expected to vary as a function of light. The objectives of this study were to (i) evaluate the effects of position and light availability on shoot growth within the crowns of understory saplings of sugar maple (Acer saccharum Marsh.) and yellow birch (Betula alleghaniensis Britton.); (ii) develop a new vigour index for shoots; and (iii) evaluate the possible factors relating to branch mortality in the crown of sugar maple saplings. The results revealed that there is a clear branch position effect on shoot growth in the crown for yellow birch saplings and that it is partly related to the presence of two types of shoots. Dead branches were located at the bottom of the crown of sugar maple saplings; they were smaller in size, had wider angles and had lower indexes of vigour than live branches found nearby. Preliminary results obtained on the vigour index indicate that it is a potentially useful tool for predicting the growth and vigour status of a branch
The Chinese-French SVOM mission for GRBs studies
We present the SVOM (Space-based multi-band astronomical Variable Objects
Monitor) mission that the Chinese National Space Agency and the French Space
Agency have decided to jointly implement. SVOM has been designed to detect all
known types of gamma-ray bursts (GRBs), to provide fast and reliable GRB
positions, to measure the broadband spectral shape and temporal properties of
the GRB prompt emission, and to quickly identify the optical/near-infrared
afterglows of detected GRBs, including high-redshift ones. Scheduled to be in
orbit in the second half of the present decade, the SVOM satellite will carry a
very innovative scientific payload combining for the first time a wide field X-
and gamma-ray coded mask imager for GRB real-time localizations to few arcmin,
a non-imaging gamma-ray monitor, and two narrow-field instruments for the study
of the GRB early afterglow emission in the X-ray and visible bands. The SVOM
payload is complemented by ground-based instruments including a wide-field
camera to catch the GRB prompt emission in the visible band and two robotic
telescopes to measure the photometric properties of the early afterglow. A
particular attention is paid to the GRB follow-up in facilitating the
observation of the SVOM detected GRB by the largest ground based telescopes.Comment: 13 pages, 3 figures. To appear in a special issue of Comptes Rendus
Physique "GRB studies in the SVOM era", Eds. F. Daigne, G. Dubu
A new type of photoionized code required for the new era of X-ray spectroscopy
With the advent of the present and future spatial X-ray missions, it becomes
crucial to model correctly the line spectrum of X-ray emitting media. We have
built a photoionization code, Titan, solving the transfer of a thousand lines
and of the continuum with the "Accelerated Lambda Iteration" method, which is
most reliable for line transfer. In all other photoionization codes the line
intensities are computed with the so-called "escape probability formalism",
used in its simplest approximation. In a previous paper (Dumont et al. 2003),
it was shown that this approximation leads to errors which can exceed one order
of magnitude in the case a Thomson thickness of the order of unity. We show
here that it also happens in the case of a Thomson thickness of 0.001 to 0.1.
The errors on the line fluxes and line ratios are of the order of 30% for a
column density of 10^{20} cm^{-2}, and a factor five for a column density of
10^{23} cm^{-2}, in conditions appropriate for Seyfert 2 and for the Warm
Absorber of Seyfert 1.Comment: 11 pages, 7 figures. accepted by A&
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