4,609 research outputs found
On Self-Organized Criticality and Synchronization in Lattice Models of Coupled Dynamical Systems
Lattice models of coupled dynamical systems lead to a variety of complex
behaviors. Between the individual motion of independent units and the
collective behavior of members of a population evolving synchronously, there
exist more complicated attractors. In some cases, these states are identified
with self-organized critical phenomena. In other situations, with
clusterization or phase-locking. The conditions leading to such different
behaviors in models of integrate-and-fire oscillators and stick-slip processes
are reviewed.Comment: 41 pages. Plain LaTeX. Style included in main file. To appear as an
invited review in Int. J. Modern Physics B. Needs eps
The XMM-Newton spectral-fit database
The XMM-Newton spectral-fit database is an ongoing ESA funded project aimed
to construct a catalogue of spectral-fitting results for all the sources within
the XMM-Newton serendipitous source catalogue for which spectral data products
have been pipeline-extracted (~ 120,000 X-ray source detections). The
fundamental goal of this project is to provide the astronomical community with
a tool to construct large and representative samples of X-ray sources by
allowing source selection according to spectral properties.Comment: Conference proceedings of IAU Symposium 304: Multiwavelength AGN
surveys and studie
Acoustic Emission Monitoring of the Syracuse Athena Temple: Scale Invariance in the Timing of Ruptures
We perform a comparative statistical analysis between the acoustic-emission time series from the ancient Greek Athena temple in Syracuse and the sequence of nearby earthquakes. We find an apparent association between acoustic-emission bursts and the earthquake occurrence. The waiting-time distributions for acoustic-emission and earthquake time series are described by a unique scaling law indicating self-similarity over a wide range of magnitude scales. This evidence suggests a correlation between the aging process of the temple and the local seismic activit
Universality of rain event size distributions
We compare rain event size distributions derived from measurements in
climatically different regions, which we find to be well approximated by power
laws of similar exponents over broad ranges. Differences can be seen in the
large-scale cutoffs of the distributions. Event duration distributions suggest
that the scale-free aspects are related to the absence of characteristic scales
in the meteorological mesoscale.Comment: 16 pages, 10 figure
Point-occurrence self-similarity in crackling-noise systems and in other complex systems
It has been recently found that a number of systems displaying crackling
noise also show a remarkable behavior regarding the temporal occurrence of
successive events versus their size: a scaling law for the probability
distributions of waiting times as a function of a minimum size is fulfilled,
signaling the existence on those systems of self-similarity in time-size. This
property is also present in some non-crackling systems. Here, the uncommon
character of the scaling law is illustrated with simple marked renewal
processes, built by definition with no correlations. Whereas processes with a
finite mean waiting time do not fulfill a scaling law in general and tend
towards a Poisson process in the limit of very high sizes, processes without a
finite mean tend to another class of distributions, characterized by double
power-law waiting-time densities. This is somehow reminiscent of the
generalized central limit theorem. A model with short-range correlations is not
able to escape from the attraction of those limit distributions. A discussion
on open problems in the modeling of these properties is provided.Comment: Submitted to J. Stat. Mech. for the proceedings of UPON 2008 (Lyon),
topic: crackling nois
A Markov chain model to investigate the spread of antibiotic-resistant bacteria in hospitals
Ordinary differential equation (ODE) models used in mathematical epidemiology
assume explicitly or implicitly large populations. For the study of infections
in a hospital this is an extremely restrictive assumption as typically a
hospital ward has a few dozen, or even fewer, patients. This work reframes a
well-known model used in the study of the spread of antibiotic-resistant
bacteria in hospitals, to consider the pathogen transmission dynamics in small
populations. In this vein, this paper proposes a Markov chain model to describe
the spread of a single bacterial species in a hospital ward where patients may
be free of bacteria or may carry bacterial strains that are either sensitive or
resistant to antimicrobial agents. We determine the probability law of the
\emph{exact} reproduction number , which is here defined as
the random number of secondary infections generated by those patients who are
accommodated in a predetermined bed before a patient who is free of bacteria is
accommodated in this bed for the first time. Specifically, we decompose the
exact reproduction number into two contributions allowing
us to distinguish between infections due to the sensitive and the resistant
bacterial strains. Our methodology is mainly based on structured Markov chains
and the use of related matrix-analytic methods. This guarantees the
compatibility of the new, finite-population model, with large population models
present in the literature and takes full advantage, in its mathematical
analysis, of the intrinsic stochasticity.Comment: 30 pages, 9 figure
Searching for highly obscured AGN in the XMM-Newton serendipitous source catalog
The majority of active galactic nuclei (AGN) are obscured by large amounts of
absorbing material that makes them invisible at many wavelengths. X-rays, given
their penetrating power, provide the most secure way for finding these AGN. The
XMM-Newton serendipitous source catalog is the largest catalog of X-ray sources
ever produced; it contains about half a million detections. These sources are
mostly AGN. We have derived X-ray spectral fits for very many 3XMM-DR4 sources
( 114 000 observations, corresponding to 77 000 unique
sources), which contain more than 50 source photons per detector. Here, we use
a subsample of 1000 AGN in the footprint of the SDSS area (covering
120 deg) with available spectroscopic redshifts. We searched for highly
obscured AGN by applying an automated selection technique based on X-ray
spectral analysis that is capable of efficiently selecting AGN. The selection
is based on the presence of either a) flat rest-frame spectra; b) flat observed
spectra; c) an absorption turnover, indicative of a high rest-frame column
density; or d) an Fe K line with an equivalent width > 500 eV. We found
81 highly obscured candidate sources. Subsequent detailed manual spectral fits
revealed that 28 of them are heavily absorbed by column densities higher than
10 cm. Of these 28 AGN, 15 are candidate Compton-thick AGN on the
basis of either a high column density, consistent within the 90% confidence
level with N 10 cm, or a large equivalent width
(>500 eV) of the Fe K line. Another six are associated with
near-Compton-thick AGN with column densities of 510
cm. A combination of selection criteria a) and c) for low-quality
spectra, and a) and d) for medium- to high-quality spectra, pinpoint highly
absorbed AGN with an efficiency of 80%.Comment: 18 pages, 10 figures, accepted for publication in A&
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