5,416 research outputs found
Recurrence intervals between earthquakes strongly depend on history
We study the statistics of the recurrence times between earthquakes above a
certain magnitude M\tau_0\hat \tau(\tau_0)\tau_0\tau_0\ov{\tau}, \hat\tau(\tau_0)\ov{\tau}\tau_0>\ov{\tau}\hat\tau(\tau_0)\ov{\tau}\tau_0\tau_0$ is, the larger is the mean residual time. The above features should be
taken into account in any earthquake prognosis.Comment: 5 pages, 3 figures, submitted to Physica
Self-organized critical earthquake model with moving boundary
A globally driven self-organized critical model of earthquakes with
conservative dynamics has been studied. An open but moving boundary condition
has been used so that the origin (epicenter) of every avalanche (earthquake) is
at the center of the boundary. As a result, all avalanches grow in equivalent
conditions and the avalanche size distribution obeys finite size scaling
excellent. Though the recurrence time distribution of the time series of
avalanche sizes obeys well both the scaling forms recently observed in analysis
of the real data of earthquakes, it is found that the scaling function decays
only exponentially in contrast to a generalized gamma distribution observed in
the real data analysis. The non-conservative version of the model shows
periodicity even with open boundary.Comment: 5 pages, 4 figures, accepted version in EPJ
Relativistic reflection in the average X-ray spectrum of AGN in the V\'eron-Cetty & V\'eron catalogue
The X-ray spectra of active galactic nuclei (AGN) unveil properties of matter
around the super massive black hole (SMBH). We investigate the X-ray spectra of
AGN focusing on Compton reflection and fluorescence, important processes of
interaction between primary radiation and circum-nuclear material. Unresolved
emission lines (most notably the Fe line) in the X-ray spectra of AGN indicate
that this material is located far away from the SMBH. Contributions from the
inner accretion disk, affected by relativistic effects, have also been detected
in several cases.
We studied the average X-ray spectrum of a sample of 263 X-ray unabsorbed AGN
that yield 419023 counts in the 2-12 keV rest-frame band distributed among 388
XMM-Newton spectra. We fitted the average spectrum using a (basically)
unabsorbed power law (primary radiation). From second model that represents the
interaction of the primary radiation with matter located far away from the
SMBH, we found that it was very significantly detected. Finally, we added a
contribution from interaction with neutral material in the accretion disk close
to the central SMBH, which is therefore smeared by relativistic effects, which
improved the fit at 6 sigma. The reflection factors are 0.65 for the accretion
disk and 0.25 for the torus. Replacing the neutral disk-reflection with
low-ionisation disk reflection, also relativistically smeared, fits the data
equally well, suggesting that we do not find evidence for a significant
ionisation of the accretion disk.
We detect distant neutral reflection in the average spectrum of unabsorbed
AGN with z=0.8. Adding the disk-reflection component associated with a
relativistic Fe line improves the data description at 6 sigma confidence level,
suggesting that both reflection components are present. The disk-reflection
component accounts for about 70 % of the total reflected flux.Comment: Accepted by A&A. 10 pages, 7 figure
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
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
Anomalous Transport in Conical Granular Piles
Experiments on 2+1-dimensional piles of elongated particles are performed.
Comparison with previous experiments in 1+1 dimensions shows that the addition
of one extra dimension to the dynamics changes completely the avalanche
properties, appearing a characteristic avalanche size. Nevertheless, the time
single grains need to cross the whole pile varies smoothly between several
orders of magnitude, from a few seconds to more than 100 hours. This behavior
is described by a power-law distribution, signaling the existence of scale
invariance in the transport process.Comment: Accepted in PR
Estimating Photometric Redshifts for X-ray sources in the X-ATLAS field, using machine-learning techniques
We present photometric redshifts for 1,031 X-ray sources in the X-ATLAS
field, using the machine learning technique TPZ (Carrasco Kind & Brunner 2013).
X-ATLAS covers 7.1 deg2 observed with the XMM-Newton within the Science
Demonstration Phase (SDP) of the H-ATLAS field, making it one of the largest
contiguous areas of the sky with both XMMNewton and Herschel coverage. All of
the sources have available SDSS photometry while 810 have additionally mid-IR
and/or near-IR photometry. A spectroscopic sample of 5,157 sources primarily in
the XMM/XXL field, but also from several X-ray surveys and the SDSS DR13
redshift catalogue, is used for the training of the algorithm. Our analysis
reveals that the algorithm performs best when the sources are split, based on
their optical morphology, into point-like and extended sources. Optical
photometry alone is not enough for the estimation of accurate photometric
redshifts, but the results greatly improve when, at least, mid-IR photometry is
added in the training process. In particular, our measurements show that the
estimated photometric redshifts for the X-ray sources of the training sample,
have a normalized absolute median deviation, n_mad=0.06, and the percentage of
outliers, eta=10-14 percent, depending on whether the sources are extended or
point-like. Our final catalogue contains photometric redshifts for 933 out of
the 1,031 X-ray sources with a median redshift of 0.9.Comment: 10 pages, 13 figures, A&A accepte
Theoretical investigation of a novel xylene-based light-driven unidirectional molecular motor
In this study, the working mechanism of the first light-driven rotary molecular motors used to control an eight-base-pair DNA hairpin has been investigated. In particular, this linker was reported to have promising photophysical properties under physiological conditions, which motivated our work at the quantum mechanical level. Cis-trans isomerization is triggered by photon absorption at wavelengths ranging 300 nm-400 nm, promoting the rotor to the first excited state, and it is mediated by an energy-accessible conical intersection from which the ground state is reached back. The interconversion between the resulting unstable isomer and its stable form occurs at physiological conditions in the ground state and is thermally activated. Here, we compare three theoretical frameworks, generally used in the quantum description of medium-size chemical systems: Linear-Response Time-Dependent Density Functional Theory (LR-TDDFT), Spin-Flip TDDFT (SF-TDDFT), and multistate complete active space second-order perturbation theory on state-averaged complete active space self consistent field wavefunctions (MS-CASPT2//SA-CASSCF). In particular, we show the importance of resorting to a multireference approach to study the rotational cycle of light-driven molecular motors due to the occurrence of geometries described by several configurations. We also assess the accuracy and computational cost of the SF-TDDFT method when compared to MS-CASPT2 and LR-TDDFT
Towards a Soft Evaluation and Refinement of Tagging in Digital Humanities
In this paper we estimate the soundness of tagging in digital repositories
within the field of Digital Humanities by studying the (semantic) conceptual structure
behind the folksnonomy. The use of association rules associated to this conceptual
structure (Stem and Luxenburger basis) allows to faithfully (from a semantic
point of view) complete the tagging (or suggest such a completion).Ministerio de Economía y Competitividad TIN2013-41086-PJunta de Andalucía TIC-606
Yacimientos de estaño del Oeste de España
[Resumen] En este trabajo se realiza una clasificación de los. yacimientos de estaño del oeste español basada, fundamentalmente, en consideraciones económicas. Tras una introducción explicativa de los objetivos y metodología empleados, así como una breve exposición del significado de las explotaciones de estaño españolas en el contexto mundial, se describen y clasifican los indicios y yacimientos más importantes, aportándose además un listado de la bibliografía específica más destacable[Abstract] This work presents a classification of the tin deposits in the westof Spain, based, mainly, on economic reasons. After an explaining introduction of the objetives and the method used, and a short exposition of the importance of spanish tin exploitations in the world context, the most important mines and occurrences are described and classified; in addition a list of the most relevant specific bibliography is given
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