1,110 research outputs found
Quasi-Periodic Oscillations in Short Recurring Bursts of the magnetars SGR 1806-20 and SGR 1900+14 Observed With RXTE
Quasi-periodic oscillations (QPOs) observed in the giant flares of magnetars
are of particular interest due to their potential to open up a window into the
neutron star interior via neutron star asteroseismology. However, only three
giant flares have been observed. We therefore make use of the much larger data
set of shorter, less energetic recurrent bursts. Here, we report on a search
for QPOs in a large data set of bursts from the two most burst-active
magnetars, SGR 1806-20 and SGR 1900+14, observed with the Rossi X-ray Timing
Explorer (RXTE). We find a single detection in an averaged periodogram
comprising 30 bursts from SGR 1806-20, with a frequency of 57 Hz and a width of
5 Hz, remarkably similar to a giant flare QPO observed from SGR 1900+14. This
QPO fits naturally within the framework of global magneto-elastic torsional
oscillations employed to explain the giant flare QPOs. Additionally, we uncover
a limit on the applicability of Fourier analysis for light curves with low
background count rates and strong variability on short timescales. In this
regime, standard Fourier methodology and more sophisticated Fourier analyses
fail in equal parts by yielding an unacceptably large number of false positive
detections. This problem is not straightforward to solve in the Fourier domain.
Instead, we show how simulations of light curves can offer a viable solution
for QPO searches in these light curves.Comment: accepted for publication in ApJ; 12 pages, 7 figures; code +
instructions at https://github.com/dhuppenkothen/MagnetarQPOSearchPaper ;
associated data products at
http://figshare.com/articles/SGR_1900_14_RXTE_Data/1184101 (SGR 1900+14) and
http://figshare.com/articles/SGR_1806_20_Bursts_RXTE_data_set/1184427 (SGR
1806-20
Characterization of random stress fields obtained from polycrystalline aggregate calculations using multi-scale stochastic finite elements
The spatial variability of stress fields resulting from polycrystalline
aggregate calculations involving random grain geometry and crystal orientations
is investigated. A periodogram-based method is proposed to identify the
properties of homogeneous Gaussian random fields (power spectral density and
related covariance structure). Based on a set of finite element polycrystalline
aggregate calculations the properties of the maximal principal stress field are
identified. Two cases are considered, using either a fixed or random grain
geometry. The stability of the method w.r.t the number of samples and the load
level (up to 3.5 % macroscopic deformation) is investigated
Single-trial multiwavelet coherence in application to neurophysiological time series
A method of single-trial coherence analysis is presented, through the application of continuous muldwavelets. Multiwavelets allow the construction of spectra and bivariate statistics such as coherence within single trials. Spectral estimates are made consistent through optimal time-frequency localization and smoothing. The use of multiwavelets is considered along with an alternative single-trial method prevalent in the literature, with the focus being on statistical, interpretive and computational aspects. The multiwavelet approach is shown to possess many desirable properties, including optimal conditioning, statistical descriptions and computational efficiency. The methods. are then applied to bivariate surrogate and neurophysiological data for calibration and comparative study. Neurophysiological data were recorded intracellularly from two spinal motoneurones innervating the posterior,biceps muscle during fictive locomotion in the decerebrated cat
Punctuated vortex coalescence and discrete scale invariance in two-dimensional turbulence
We present experimental evidence and theoretical arguments showing that the
time-evolution of freely decaying 2-d turbulence is governed by a {\it
discrete} time scale invariance rather than a continuous time scale invariance.
Physically, this reflects that the time-evolution of the merging of vortices is
not smooth but punctuated, leading to a prefered scale factor and as a
consequence to log-periodic oscillations. From a thorough analysis of freely
decaying 2-d turbulence experiments, we show that the number of vortices, their
radius and separation display log-periodic oscillations as a function of time
with an average log-frequency of ~ 4-5 corresponding to a prefered scaling
ratio of ~ 1.2-1.3Comment: 22 pages and 38 figures. Submitted to Physica
Non-Stationary Noise Power Spectral Density Estimation Based on Regional Statistics
International audienceEstimating the noise power spectral density (PSD) is essential for single channel speech enhancement algorithms. In this paper, we propose a noise PSD estimation approach based on regional statistics. The proposed regional statistics consist of four features representing the statistics of the past and present periodograms in a short-time period. We show that these features are efficient in characterizing the statistical difference between noise PSD and noisy speech PSD. We therefore propose to use these features for estimating the speech presence probability (SPP). The noise PSD is recursively estimated by averaging past spectral power values with a time-varying smoothing parameter controlled by the SPP. The proposed method exhibits good tracking capability for non-stationary noise, even for abruptly increasing noise level
Kalman-filter control schemes for fringe tracking. Development and application to VLTI/GRAVITY
The implementation of fringe tracking for optical interferometers is
inevitable when optimal exploitation of the instrumental capacities is desired.
Fringe tracking allows continuous fringe observation, considerably increasing
the sensitivity of the interferometric system. In addition to the correction of
atmospheric path-length differences, a decent control algorithm should correct
for disturbances introduced by instrumental vibrations, and deal with other
errors propagating in the optical trains. We attempt to construct control
schemes based on Kalman filters. Kalman filtering is an optimal data processing
algorithm for tracking and correcting a system on which observations are
performed. As a direct application, control schemes are designed for GRAVITY, a
future four-telescope near-infrared beam combiner for the Very Large Telescope
Interferometer (VLTI). We base our study on recent work in adaptive-optics
control. The technique is to describe perturbations of fringe phases in terms
of an a priori model. The model allows us to optimize the tracking of fringes,
in that it is adapted to the prevailing perturbations. Since the model is of a
parametric nature, a parameter identification needs to be included. Different
possibilities exist to generalize to the four-telescope fringe tracking that is
useful for GRAVITY. On the basis of a two-telescope Kalman-filtering control
algorithm, a set of two properly working control algorithms for four-telescope
fringe tracking is constructed. The control schemes are designed to take into
account flux problems and low-signal baselines. First simulations of the
fringe-tracking process indicate that the defined schemes meet the requirements
for GRAVITY and allow us to distinguish in performance. In a future paper, we
will compare the performances of classical fringe tracking to our Kalman-filter
control.Comment: 17 pages, 8 figures, accepted for publication in A&
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