3,123 research outputs found
X-ray spectral variability of seven LINER nuclei with XMM-Newton and Chandra data
One of the most important features in active galactic nuclei (AGN) is the
variability of their emission. Variability has been discovered at X-ray, UV,
and radio frequencies on time scales from hours to years. Among the AGN family
and according to theoretical studies, Low-Ionization Nuclear Emission Line
Region (LINER) nuclei would be variable objects on long time scales. Our
purpose is to investigate spectral X-ray variability in LINERs and to
understand the nature of these kinds of objects, as well as their accretion
mechanism. Chandra and XMM-Newton public archives were used to compile X-ray
spectra of seven LINER nuclei at different epochs with time scales of years. To
search for variability we fit all the spectra from the same object with a set
of models, in order to identify the parameters responsible for the variability
pattern. We also analyzed the light curves in order to search for short time
scale (from hours to days) variability. Whenever possible, UV variability was
also studied. We found spectral variability in four objects, with variations
mostly related to hard energies (2-10 keV). These variations are due to changes
in the soft excess, and/or changes in the absorber, and/or intrinsic variations
of the source. Another two galaxies seem not to vary. Short time scale
variations during individual observations were not found. Our analysis confirms
the previously reported anticorrelation between the X-ray spectral index and
the Eddington ratio, and also the correlation between the X-ray to UV flux
ratio and the Eddington ratio. These results support an Advection Dominated
Accretion Flow (ADAF) as the accretion mechanism in LINERs.Comment: 35 pages, 53 figures, recently accepted pape
X-ray spectral variability of Seyfert 2 galaxies
Variability across the electromagnetic spectrum is a property of AGN that can
help constraining the physical properties of these galaxies. This is the third
of a serie of papers with the aim of studying the X-ray variability of
different families of AGN. The main purpose of this work is to investigate the
variability pattern in a sample of optically selected type 2 Seyfert galaxies.
We use the 26 Seyferts in the Veron-Cetty and Veron catalogue with data
available from Chandra and/or XMM-Newton public archives at different epochs,
with timescales ranging from a few hours to years. All the spectra of the same
source are simultaneously fitted and we let different parameters to vary in the
model. Whenever possible, short-term variations and/or long-term UV flux
variations are studied. We divide the sample in Compton-thick, Compton-thin,
and changing-look candidates. Short-term variability at X-rays is not found.
From the 25 analyzed sources, 11 show long-term variations; eight (out of 11)
are Compton-thin, one (out of 12) is Compton-thick, and the two changing-look
candidates are also variable. The main driver for the X-ray changes is related
to the nuclear power (nine cases), while variations at soft energies or related
with absorbers at hard X-rays are less common, and in many cases these
variations are accompained with variations of the nuclear continuum. At UV
frequencies nuclear variations are nor found. We report for the first time two
changing-look candidates, MARK273 and NGC7319. A constant reflection component
located far away from the nucleus plus a variable nuclear continuum are able to
explain most of our results; the Compton-thick candidates are dominated by
reflection, which supresses their continuum making them seem fainter, and not
showing variations, while the Compton-thin and changing-look candidates show
variations.Comment: Accepted for publication in A&
Monumental entrances at iberian Neolithic and Chalcolithic ditches enclosures. Entrance 1 at Perdigoes (Portugal) as case study
Ditched enclosures have been found in many European regions during the Neolithic and Copper Age. This widespread, almost pan-European monumental tradition has also been documented in the Iberian Peninsula in the last 25 years, although the impact of their discovery in the continental debates has been, for the most part, rather small.
In these enclosed sites, ditches delimit spaces by establishing an inside and an outside. Crucially, however, in most enclosures known to date with enough detail, ditches show one or more interruptions interpreted as entrances or points of access that connect the outside with the inside. These entrances appear to be architectural elements of great importance and possibly special meanings.
During the Neolithic, European ditched enclosures showed great formal variability, from the simplest (a break or discontinuity in the ditch's layout) to more complex forms such as the pince de crabe (crab's claw). Many of them show astrological awareness and/or orientation towards certain prominent features of the surrounding landscape. Changes in form and transformations through time, both of the entrance itself and of the nearby elements, are also very common. All in all, they are an important component of almost every ditched enclosure, and often some of the most visibly monumentalised areas of these sites.
This presentation has two objectives. First, we shall examine the monumental character and the diversity of entrances at Southern Iberian Neolithic and Chalcolithic ditched enclosures (4th-3rd millennia cal BC). Second, we will focus on Entrance 1 at Perdigões (Portugal), its form, features and temporality, as a particular case to be contextualised in the wider Southern Iberian scene.Universidad de Málaga. Campus de Excelencia Internacional Andalucía Tech
X-ray spectral variability of LINERs selected from the Palomar sample
Variability is a general property of active galactic nuclei (AGN). At X-rays,
the way in which these changes occur is not yet clear. In the particular case
of low ionisation nuclear emission line region (LINER) nuclei, variations on
months/years timescales have been found for some objects, but the main driver
of these changes is still an open question. The main purpose of this work is to
investigate the X-ray variability in LINERs, including the main driver of such
variations, and to search for eventual differences between type 1 and 2
objects. We use the 18 LINERs in the Palomar sample with data retrieved from
Chandra and/or XMM-Newton archives corresponding to observations gathered at
different epochs. All the spectra for the same object are simultaneously fitted
in order to study long term variations. The nature of the variability patterns
are studied allowing different parameters to vary during the spectral fit.
Whenever possible, short term variations from the analysis of the light curves
and UV variability are studied.Comment: 49 pages, accepted. arXiv admin note: text overlap with
arXiv:1305.222
Classical properties of algebras using a new graph association
We study the relation between algebraic structures and Graph Theory. We have
defined five different weighted digraphs associated to a finite dimensional
algebra over a field in order to tackle important properties of the associated
algebras, mainly the nilpotency and solvability in the case of Leibniz
algebras
Physical anthropology in Europe and beyond
Methodologies and legislative frameworks regarding the excavation, retrieval, analysis, curation and potential reburial of human skeletal remains different throughout Europe. As work forces within Europe and beyond have become increasingly mobile and international research collaborations are steadily increasing, the need for a more comprehensive understanding of different national research traditions, methodologies and legislative structures within the academic and commercial sector of physical anthropology has arisen. Establishing how human osteoarchaeology is practiced and dealt with throughout Europe and beyond will promote sharing knowledge between countries and form the basis for pan-European exchanges and discussions on the best practice. The current paper focuses on an ongoing project entitled ‘Physical Anthropology and Legislation: European Perspectives and Beyond’ and provides insight into some of the results obtained so far
Local versus regional processes and the control of community structure
The contribution of local (e.g., competition) and regional (e.g., dispersal) processes in the structure of communities remains an unresolved issue. In general, a tendency to assume local processes to be deterministic and regional to be stochastic dominates, although it is challenged. Fortunately, it can be cast as a testable proposition: if correct, the degree of determinism in the final community structure might indicate which process is more prominent in the control of community structure. However, recent findings have also suggested that stochastic patterns can arise from local processes and that dispersal can homogenize communities, which would make them appear deterministic irrespective of the mechanism involved. To evaluate these competing expectations we conducted an experiment where the initial communities had the same composition and species abundances. We hypothesized that if local processes dominate, then arrays of communities will show divergence of community structures whether connected by dispersal or not (i.e., being fully isolated). Alternatively, if regional processes dominate, the dispersal connected communities should converge while isolated ones should not. We found, however, that both groups of experimental communities showed similar patterns of change - a decline in similarity and a tendency to diverge. This suggests that biological interactions, demographic stochasticity, or both, exert noticeable control over community structure such that they reduce similarity among replicate communities and diversify their final states. We speculate that these mechanisms enhance potential for species additions, particularly in conjunction with factors such as dispersal and the size of the regional species pool
Epidemiological study of enteric infections in children by comparing different countries
Correlaciones entre muertes por diarrea en niños y parámetros tanto socioeconómicos como ambientales, son estudiadas aquí para 10 países, mostrando asociaciones entre casos y población, lluvia y producto interno bruto (PIB). Las variables correlacionadas incluyen: casos de muerte, población (total y densidad), área (tierra y agua), temperatura promedio, lluvia, productividad, desigualdad de ingreso (índice Gini), indicador de alta tecnología (tecnología de desarrollo de armas nucleares). Cuando se divide en 2 grupos la base de datos: países desarrollados y en desarrollo, algunas asociaciones crecen para los países en desarrollo (R más grandes, p-values <0.05). Las correlaciones encontradas aquí apoyan que el exceso de lluvias y los asentamientos atestados contribuyen a la transmisión de las infecciones; dando a entender que la densidad de población impone un límite práctico en el control de las infecciones entéricas en los países en desarrollo.Correlates among diarrhea-deaths in children, and social and environmental parameters, are studied here for 10 countries, showing significant associations among cases and population, rainfall, and gross domestic product (GDP). The variables correlated include: cases, population (total and density), area (land and water), average temperature, rainfall, productivity (GDP), income inequality (Gini index), and hi-technology indicator (by nuclear weapons technology). When the dataset is split in two sets: developed and developing countries, some of the associations for developing countries grow up (larger R, p-values < 0.05). Correlations found here support that excessive rainfall and crowded settlements contribute to the transmission of infections; hinting that population density imposes a practical boundary in controlling enteric infections in developing countries
Willingness to change car use to commute to the UPTC main campus, Colombia: A hybrid discrete choice modeling approach
This paper studies the willingness to change car use when commuting to a university campus. We estimated a hybrid discrete choice (HDC) model to test the hypothesis that, in addition to traditional tangible attributes, the willingness to change car use to more sustainable transportation modes also depends on the pro-environmental attitude and the perceived convenience of each transportation alternative. We found that teachers have better pro-environmental attitudes than students and administrative staff, but senior individuals and people who own an above-average priced car have negative effects on this attitude. We concluded that in addition to car ownership, the price of a car is also a decisive factor in the willingness to change car use. On-campus parking fees were identified as a key variable for reducing car use when commuting to campus and for financing more sustainable transportation modes. This paper contributes to the literature on sustainable mobility on university campuses and is the first based on an HDC modeling approach that integrates tangible attributes and latent variables into this context. © 2019 Luis Márquez, Luis F. Macea, & Jose J. Soto
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