48,602 research outputs found
Characterizing Intermittency of 4-Hz Quasi-periodic Oscillation in XTE J1550-564 using Hilbert-Huang Transform
We present the time-frequency analysis results based on the Hilbert-Huang
transform (HHT) for the evolution of a 4-Hz low-frequency quasi-periodic
oscillation (LFQPO) around the black hole X-ray binary XTE J1550-564. The
origin of LFQPOs is still debated. To understand the cause of the peak
broadening, we utilized a recently developed time-frequency analysis, HHT, for
tracking the evolution of the 4-Hz LFQPO from XTE J1550 564. By adaptively
decomposing the ~4-Hz oscillatory component from the light curve and acquiring
its instantaneous frequency, the Hilbert spectrum illustrates that the LFQPO is
composed of a series of intermittent oscillations appearing occasionally
between 3 Hz and 5 Hz. We further characterized this intermittency by computing
the confidence limits of the instantaneous amplitudes of the intermittent
oscillations, and constructed both the distributions of the QPO's high and low
amplitude durations, which are the time intervals with and without significant
~4-Hz oscillations, respectively. The mean high amplitude duration is 1.45 s
and 90% of the oscillation segments have lifetimes below 3.1 s. The mean low
amplitude duration is 0.42 s and 90% of these segments are shorter than 0.73 s.
In addition, these intermittent oscillations exhibit a correlation between the
oscillation's rms amplitude and mean count rate. This correlation could be
analogous to the linear rms-flux relation found in the 4-Hz LFQPO through
Fourier analysis. We conclude that the LFQPO peak in the power spectrum is
broadened owing to intermittent oscillations with varying frequencies, which
could be explained by using the Lense-Thirring precession model.Comment: 27 pages, 9 figures, accepted for publication in The Astrophysical
Journa
Tracking granules at the Sun's surface and reconstructing velocity fields. II. Error analysis
The determination of horizontal velocity fields at the solar surface is
crucial to understanding the dynamics and magnetism of the convection zone of
the sun. These measurements can be done by tracking granules.
Tracking granules from ground-based observations, however, suffers from the
Earth's atmospheric turbulence, which induces image distortion. The focus of
this paper is to evaluate the influence of this noise on the maps of velocity
fields.
We use the coherent structure tracking algorithm developed recently and apply
it to two independent series of images that contain the same solar signal.
We first show that a k-\omega filtering of the times series of images is
highly recommended as a pre-processing to decrease the noise, while, in
contrast, using destretching should be avoided. We also demonstrate that the
lifetime of granules has a strong influence on the error bars of velocities and
that a threshold on the lifetime should be imposed to minimize errors. Finally,
although solar flow patterns are easily recognizable and image quality is very
good, it turns out that a time sampling of two images every 21 s is not
frequent enough, since image distortion still pollutes velocity fields at a 30%
level on the 2500 km scale, i.e. the scale on which granules start to behave
like passive scalars.
The coherent structure tracking algorithm is a useful tool for noise control
on the measurement of surface horizontal solar velocity fields when at least
two independent series are available.Comment: in press in Astronomy and Astrophysics, 9 page
Efficient Decomposition of Image and Mesh Graphs by Lifted Multicuts
Formulations of the Image Decomposition Problem as a Multicut Problem (MP)
w.r.t. a superpixel graph have received considerable attention. In contrast,
instances of the MP w.r.t. a pixel grid graph have received little attention,
firstly, because the MP is NP-hard and instances w.r.t. a pixel grid graph are
hard to solve in practice, and, secondly, due to the lack of long-range terms
in the objective function of the MP. We propose a generalization of the MP with
long-range terms (LMP). We design and implement two efficient algorithms
(primal feasible heuristics) for the MP and LMP which allow us to study
instances of both problems w.r.t. the pixel grid graphs of the images in the
BSDS-500 benchmark. The decompositions we obtain do not differ significantly
from the state of the art, suggesting that the LMP is a competitive formulation
of the Image Decomposition Problem. To demonstrate the generality of the LMP,
we apply it also to the Mesh Decomposition Problem posed by the Princeton
benchmark, obtaining state-of-the-art decompositions
Tracking granules on the Sun's surface and reconstructing horizontal velocity fields: I. the CST algorithm
Determination of horizontal velocity fields on the solar surface is crucial
for understanding the dynamics of structures like mesogranulation or
supergranulation or simply the distribution of magnetic fields.
We pursue here the development of a method called CST for coherent structure
tracking, which determines the horizontal motion of granules in the field of
view.
We first devise a generalization of Strous method for the segmentation of
images and show that when segmentation follows the shape of granules more
closely, granule tracking is less effective for large granules because of
increased sensitivity to granule fragmentation. We then introduce the
multi-resolution analysis on the velocity field, based on Daubechies wavelets,
which provides a view of this field on different scales. An algorithm for
computing the field derivatives, like the horizontal divergence and the
vertical vorticity, is also devised. The effects from the lack of data or from
terrestrial atmospheric distortion of the images are also briefly discussed.Comment: in press in Astronomy and Astrophysics, 9 page
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