3,098 research outputs found
The impact of celestial pole offset modelling on VLBI UT1 Intensive results
Very Long Baseline Interferometry (VLBI) Intensive sessions are scheduled to
provide operational Universal Time (UT1) determinations with low latency. UT1
estimates obtained from these observations heavily depend on the model of the
celestial pole motion used during data processing. However, even the most
accurate precession-nutation model, IAU 2000/2006, is not accurate enough to
realize the full potential of VLBI observations. To achieve the highest
possible accuracy in UT1 estimates, a celestial pole offset (CPO), which is the
difference between the actual and modelled precession-nutation angles, should
be applied. Three CPO models are currently available for users. In this paper,
these models have been tested and the differences between UT1 estimates
obtained with those models are investigated. It has been shown that neglecting
CPO modelling during VLBI UT1 Intensive processing causes systematic errors in
UT1 series of up to 20 microarcseconds. It has been also found that using
different CPO models causes the differences in UT1 estimates reaching 10
microarcseconds. Obtained results are applicable to the satellite data
processing as well.Comment: 8 pp., accepted for publication in Journal of Geodes
Analysis of the Accuracy of Prediction of the Celestial Pole Motion
VLBI observations carried out by global networks provide the most accurate
values of the precession-nutation angles determining the position of the
celestial pole; as a rule, these results become available two to four weeks
after the observations. Therefore, numerous applications, such as satellite
navigation systems, operational determination of Universal Time, and space
navigation, use predictions of the coordinates of the celestial pole. In
connection with this, the accuracy of predictions of the precession- nutation
angles based on observational data obtained over the last three years is
analyzed for the first time, using three empiric nutation models---namely,
those developed at the US Naval Observatory, the Paris Observatory, and the
Pulkovo Observatory. This analysis shows that the last model has the best of
accuracy in predicting the coordinates of the celestial pole. The rms error for
a one-month prediction proposed by this model is below 100 microarcsecond.Comment: 13 p
The influence of Galactic aberration on precession parameters determined from VLBI observations
The influence of proper motions of sources due to Galactic aberration on
precession models based on VLBI data is determined. Comparisons of the linear
trends in the coordinates of the celestial pole obtained with and without
taking into account Galactic aberration indicate that this effect can reach 20
as per century, which is important for modern precession models. It is
also shown that correcting for Galactic aberration influences the derived
parameters of low-frequency nutation terms. It is therefore necessary to
correct for Galactic aberration in the reduction of modern astrometric
observations
Higher order approximation of isochrons
Phase reduction is a commonly used techinque for analyzing stable
oscillators, particularly in studies concerning synchronization and phase lock
of a network of oscillators. In a widely used numerical approach for obtaining
phase reduction of a single oscillator, one needs to obtain the gradient of the
phase function, which essentially provides a linear approximation of isochrons.
In this paper, we extend the method for obtaining partial derivatives of the
phase function to arbitrary order, providing higher order approximations of
isochrons. In particular, our method in order 2 can be applied to the study of
dynamics of a stable oscillator subjected to stochastic perturbations, a topic
that will be discussed in a future paper. We use the Stuart-Landau oscillator
to illustrate the method in order 2
Evaluation of superalloy heavy-duty grinding based on multivariate tests
The quality and economy of grinding depend on proper selection of grinding conditions for the materials to be ground. In order to evaluate the effect of heavy-duty grinding, a new performance index, which includes specific material removal rate, size accuracy, and grinding forces, was proposed. Robust design of experiment, including orthogonal arrays, the signal-to-noise ratio (SNR) method, and analysis of variance (ANOVA) for multivariate data, was employed to estimate the effect of uniform experimental design and to optimize grinding parameters. Empirical models of grinding force were investigated for finite element analysis of new fixture design. These empirical models, based on robust design of experiments and multiple regression methodology, have been confirmed through further verification experiments. Correlation coefficients from 0.87 to 0.96 were achieved
Spin-Orbit Qubits of Rare-Earth-Metal Ions in Axially Symmetric Crystal Fields
Contrary to the well known spin qubits, rare-earth qubits are characterized
by a strong influence of crystal field due to large spin-orbit coupling. At low
temperature and in the presence of resonance microwaves, it is the magnetic
moment of the crystal-field ground-state which nutates (for several s) and
the Rabi frequency is anisotropic. Here, we present a study of the
variations of with the magnitude and direction of the
static magnetic field for the odd Er isotope in a single
crystal CaWO:Er. The hyperfine interactions split the
curve into eight different curves which are fitted
numerically and described analytically. These "spin-orbit qubits" should allow
detailed studies of decoherence mechanisms which become relevant at high
temperature and open new ways for qubit addressing using properly oriented
magnetic fields
Parity doubling in particle physics
Parity doubling in excited hadrons is reviewed. Parity degeneracy in hadrons
was first experimentally observed 40 years ago. Recently new experimental data
on light mesons caused much excitement and renewed interest to the phenomenon,
which still remains to be enigmatic. The present retrospective review is an
attempt to trace the history of parity doubling phenomenon, thus providing a
kind of introduction to the subject. We begin with early approaches of 1960s
(Regge theory and dynamical symmetries) and end up with the latest trends
(manifestations of broader degeneracies and AdS/QCD). We show the evolution of
various ideas about parity doubling. The experimental evidence for this
phenomenon is scrutinized in the non-strange sector. Some experiments of 1960s
devoted to the search for missing non-strange bosons are re-examined and it is
argued that results of these experiments are encouraging from the modern
perspective.Comment: Version to appear in Int. J. Mod. Phys. A, 63 pages, 9 figure
Cross-relaxation and phonon bottleneck effects on magnetization dynamics in LiYF4:Ho3+
Frequency and dc magnetic field dependences of dynamic susceptibility in
diluted paramagnets LiYF:Ho have been measured at liquid helium
temperatures in the ac and dc magnetic fields parallel to the symmetry axis of
a tetragonal crystal lattice. Experimental data are analyzed in the framework
of microscopic theory of relaxation rates in the manifold of 24
electron-nuclear sublevels of the lowest non-Kramers doublet and the first
excited singlet in the Ho ground multiplet split by the crystal
field of S symmetry. The one-phonon transition probabilities were computed
using electron-phonon coupling constants calculated in the framework of
exchange charge model and were checked by optical piezospectroscopic
measurements. The specific features observed in field dependences of the in-
and out-of-phase susceptibilities (humps and dips, respectively) at the
crossings (anti-crossings) of the electron-nuclear sublevels are well
reproduced by simulations when the phonon bottleneck effect and the cross-spin
relaxation are taken into account
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