973,582 research outputs found
Temporal Evolution of Financial Market Correlations
We investigate financial market correlations using random matrix theory and
principal component analysis. We use random matrix theory to demonstrate that
correlation matrices of asset price changes contain structure that is
incompatible with uncorrelated random price changes. We then identify the
principal components of these correlation matrices and demonstrate that a small
number of components accounts for a large proportion of the variability of the
markets that we consider. We then characterize the time-evolving relationships
between the different assets by investigating the correlations between the
asset price time series and principal components. Using this approach, we
uncover notable changes that occurred in financial markets and identify the
assets that were significantly affected by these changes. We show in particular
that there was an increase in the strength of the relationships between several
different markets following the 2007--2008 credit and liquidity crisis.Comment: 15 pages, 10 figures, 1 table. Accepted for publication in Phys. Rev.
E. v2 includes additional section
Temporal evolution of oscillating coronal loops
Context. Transverse oscillations of coronal structures are currently
intensively studied to explore the associated magnetohydrodynamic wave physics
and perform seismology of the local medium. Aims. We make a first attempt to
measure the thermodynamic evolution of a sample of coronal loops that undergo
decaying kink oscillations in response to an eruption in the corresponding
active region. Methods. Using data from the six coronal wavelengths of SDO/AIA,
we performed a differential emission measure (DEM) analysis of 15 coronal loops
before, during, and after the eruption and oscillation. Results. We find that
the emission measure, temperature, and width of the DEM distribution undergo
significant variations on time scales relevant for the study of transverse
oscillations. There are no clear collective trends of increases or decreases
for the parameters we analysed. The strongest variations of the parameters
occur during the initial perturbation of the loops, and the influence of
background structures may also account for much of this variation. Conclusions.
The DEM analysis of oscillating coronal loops in erupting active regions shows
evidence of evolution on time scales important for the study of the
oscillations. Further work is needed to separate the various observational and
physical mechanisms that may be responsible for the variations in temperature,
DEM distribution width, and total emission measure.Comment: Accepted in A&
Evolution of network structure by temporal learning
We study the effect of learning dynamics on network topology. A network of
discrete dynamical systems is considered for this purpose and the coupling
strengths are made to evolve according to a temporal learning rule that is
based on the paradigm of spike-time-dependent plasticity. This incorporates
necessary competition between different edges. The final network we obtain is
robust and has a broad degree distribution.Comment: revised manuscript in communicatio
Propagation of hydrodynamic interactions between particles in a compressible fluid
Hydrodynamic interactions are transmitted by viscous diffusion and sound
propagation: the temporal evolution of hydrodynamic interactions by both
mechanisms is studied by direct numerical simulation in this paper. The
hydrodynamic interactions for a system of two particles in a fluid are
estimated by the velocity correlation of the particles. In an incompressible
fluid, hydrodynamic interactions propagate instantaneously at the infinite
speed of sound, followed by the temporal evolution of viscous diffusion. On the
other hand, in a compressible fluid, sound propagates at a finite speed, which
affects the temporal evolution of the hydrodynamic interactions by the order of
magnitude relation between the time scales of viscous diffusion and sound
propagation. The hydrodynamic interactions are characterized by introducing the
ratio of these time scales as an interactive compressibility factor.Comment: 12 pages, 8 figure
The very fast evolution of Sakurai's object
V4334 Sgr (a.k.a. Sakurai's object) is the central star of an old planetary
nebula that underwent a very late thermal pulse a few years before its
discovery in 1996. We have been monitoring the evolution of the optical
emission line spectrum since 2001. The goal is to improve the evolutionary
models by constraining them with the temporal evolution of the central star
temperature. In addition the high resolution spectral observations obtained by
X-shooter and ALMA show the temporal evolution of the different morphological
components.Comment: 2 pages, 2 figures to appear in the Proceedings of the IAU Symp. 323:
"Planetary nebulae: Multi-wavelength probes of stellar and galactic
evolution". Eds. X.-W. Liu, L. Stanghellini and A. Karaka
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