454 research outputs found
Network of Earthquakes and Recurrences Therein
We quantify the correlation between earthquakes and use the same to
distinguish between relevant causally connected earthquakes. Our correlation
metric is a variation on the one introduced by Baiesi and Paczuski (2004). A
network of earthquakes is constructed, which is time ordered and with links
between the more correlated ones. Data pertaining to the California region has
been used in the study. Recurrences to earthquakes are identified employing
correlation thresholds to demarcate the most meaningful ones in each cluster.
The distribution of recurrence lengths and recurrence times are analyzed
subsequently to extract information about the complex dynamics. We find that
the unimodal feature of recurrence lengths helps to associate typical rupture
lengths with different magnitude earthquakes. The out-degree of the network
shows a hub structure rooted on the large magnitude earthquakes. In-degree
distribution is seen to be dependent on the density of events in the
neighborhood. Power laws are also obtained with recurrence time distribution
agreeing with the Omori law.Comment: 17 pages, 5 figure
Evaluation of Specific Symptoms of Bacterial Vaginosis Among Pregnant Women
Objective: Identification of the symptoms of bacterial vaginosis (BV) in pregnancy might be rational in order to identify a possible BV-associated group at risk of preterm delivery
ASCA Observations of the Composite Warm Absorber in NGC 3516
We obtained X-ray spectra of the Seyfert 1 galaxy NGC~3516 in March 1995
using ASCA. Simultaneous far-UV observations were obtained with HUT on the
Astro-2 shuttle mission. The ASCA spectrum shows a lightly absorbed power law
of energy index 0.78. The low energy absorbing column is significantly less
than previously seen. Prominent O~vii and O~viii absorption edges are visible,
but, consistent with the much lower total absorbing column, no Fe K absorption
edge is detectable. A weak, narrow Fe~K emission line from cold
material is present as well as a broad Fe~K line. These features are
similar to those reported in other Seyfert 1 galaxies. A single warm absorber
model provides only an imperfect description of the low energy absorption. In
addition to a highly ionized absorber with ionization parameter and
a total column density of , adding a lower
ionization absorber with and a total column of significantly improves the fit. The contribution of
resonant line scattering to our warm absorber models limits the Doppler
parameter to at 90\% confidence. Turbulence at the sound
speed of the photoionized gas provides the best fit. None of the warm absorber
models fit to the X-ray spectrum can match the observed equivalent widths of
all the UV absorption lines. Accounting for the X-ray and UV absorption
simultaneously requires an absorbing region with a broad range of ionization
parameters and column densities.Comment: 14 pages, 4 Postscript figures, uses aaspp4.sty To appear in the
August 20, 1996, issue of The Astrophysical Journa
Topological constraints on spiral wave dynamics in spherical geometries with inhomogeneous excitability
We analyze the way topological constraints and inhomogeneity in the
excitability influence the dynamics of spiral waves on spheres and punctured
spheres of excitable media. We generalize the definition of an index such that
it characterizes not only each spiral but also each hole in punctured,
oriented, compact, two-dimensional differentiable manifolds and show that the
sum of the indices is conserved and zero. We also show that heterogeneity and
geometry are responsible for the formation of various spiral wave attractors,
in particular, pairs of spirals in which one spiral acts as a source and a
second as a sink -- the latter similar to an antispiral. The results provide a
basis for the analysis of the propagation of waves in heterogeneous excitable
media in physical and biological systems.Comment: 5 pages, 6 Figures, major revisions, accepted for publication in
Phys. Rev.
Gegen den Brain Drain des wissenschaftlichen Nachwuchses: Reformvorschläge zur Hochschulpolitik in Deutschland
Seit langem beklagt der wissenschaftliche Nachwuchs, dass das deutsche Hochschulsystem kaum langfristige Karriereperspektiven und wenig individuelle Freiheiten bietet. Auf viele Jungwissenschaftler wirkt die Realität des deutschen Hochschulbetriebs abschreckend. Was müsste sich ändern? Vorschläge der German Scholars Organization (GSO)
1/f noise from correlations between avalanches in self-organized criticality
We show that large, slowly driven systems can evolve to a self-organized
critical state where long range temporal correlations between bursts or
avalanches produce low frequency noise. The avalanches can occur
instantaneously in the external time scale of the slow drive, and their event
statistics are described by power law distributions. A specific example of this
behavior is provided by numerical simulations of a deterministic ``sandpile''
model.Comment: Completely revised version: 4 pages (revtex), 3 eps figure
Infrared Candidates for the Intense Galactic X-ray Source GX 17+2
We present new astrometric solutions and infrared Hubble Space Telescope
observations of GX 17+2 (X1813-140), one of the brightest X-ray sources on the
celestial sphere. Despite 30 years of intensive study, and the existence of a
strong radio counterpart with a sub-arcsecond position, the object remains
optically unidentified. The observed X-ray characteristics strongly suggest
that it is a so-called "Z-source," the rare but important category that
includes Sco X-1 and Cyg X-2. Use of the USNO-A2.0 catalog enables us to
measure the position of optical and infrared objects near the radio source to
sub-arcsecond precision within the International Celestial Reference Frame, for
direct comparison with the radio position, which we also recompute using modern
calibrators. With high confidence we eliminate the V~17.5 star NP Ser, often
listed as the probable optical counterpart of the X-ray source, as a candidate.
Our HST NICMOS observations show two faint objects within our 0.5" radius 90%
confidence error circle. Even the brighter of the two, Star A, is far fainter
than expected (H~19.8), given multiple estimates of the extinction in this
field and our previous understanding of Z sources, but it becomes the best
candidate for the counterpart of GX 17+2. The probability of a chance
coincidence of an unrelated faint object on the radio position is high.
However, if the true counterpart is not Star A, it is fainter still, and our
conclusion that the optical counterpart is surprisingly underluminous is but
strengthened.Comment: 15 pages including 3 figures and 3 tables. Accepted for publication
in The Astrophysical Journa
Simple model for 1/f noise
We present a simple stochastic mechanism which generates pulse trains
exhibiting a power law distribution of the pulse intervals and a
power spectrum over several decades at low frequencies with close to
one. The essential ingredient of our model is a fluctuating threshold which
performs a Brownian motion. Whenever an increasing potential hits the
threshold, is reset to the origin and a pulse is emitted. We show that
if increases linearly in time, the pulse intervals can be approximated
by a random walk with multiplicative noise. Our model agrees with recent
experiments in neurobiology and explains the high interpulse interval
variability and the occurrence of noise observed in cortical
neurons and earthquake data.Comment: 4 pages, 4 figure
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