3,211 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&
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
Estudio geomorfológico de las terrazas fluviales cuaternarias del Valle del Tajo (sector Aranjuez-Toledo)
[Resumen] En esta comunicación se analiza la morfología de los distintos niveles de terraza que pertenecientes al sistema fluvial Tajo-Jarama existen en el tramo comprendido entre Aranjuez y Toledo; igualmente se estudian las características sedimentológicas de los aluviones asociados a estas terrazas, así como otras formaciones detríticas de diferentes épocas geológicas que se advierten en esta zona de la Submeseta Sur. De esta manera se continuan las observaciones realizadas por nosotros hace unos años en las terrazas del Tajo, aguas arriba de Aranjuez y hasta las inmediaciones del embalse de Almoguera, en la provincia de Guadalajara (GONZALEZ MARTIN y ASENSIO AMOR, 1979), así como las investigaciones dedicadas a las deformaciones pseudo-tectónicas que en ocasiones afectan a los conjuntos sedimentarios fluviales (GONZALEZ MARTIN y ASENSIO AMOR, 1980)[Resumé] On étude dans ce travail les caracteres géomorphologiques des différents niveaux de terrasses dans la vallée du Tajo-Jarama (Aranjuez-Toledo). En meme temps, on analyse les caractéristiques sédimentologiques des materiaux associés aux ces formes morphologiques et des autres formations détritiques des ages tertiaires et quaternaires. De cette façon nous poursuivons les observations faites, il y a quelque temps, pour nous meme. D'un cote, le systeme de terrasses dans l'amont de Aranjuez et jusqu'a les environs de Almoguera (GONZALEZ MARTIN y ASENSIO AMOR, 1979). De l'autre, les recherches consacrées aux déformations pseudo-tectoniques visibles quelques fois dans ces ensambles sédimentaires quaternaires (GONZALEZ MARTIN y ASENSIO AMOR, 1980)
Synthesis of periglacial processes in Galicia (Spain)
[Resumen] En esta comunicación, de carácter bibliográfico, los autores incluyen con un breve comentario las publicaciones que hacen referencia a fenómenos periglaciares en Galicia[Abstract] In this note, of bibliographical characteristic, the authors enclose with a brief commentary about the periglacial geomorphology publications in the country of the Galicia
Synthesis of geomorphological processes in the «Galaico-Astur» Bounds
[Resumen] En este trabajo se analizan los distintos procesos geomorfológicos que han intervenido en el modelado de formas en el sector Galaico-Astur. Actualmente, los procesos de índole dinámica (fluviales, marinos, etc.) y los edafogenéticos son los que muestran una mayor incidencia aunque condicionados por la influencia de factores estructurales y de orden climático; buena parte de su eficacia está favorecida, igualmente, por las acciones antrópicas. En lo que respecta a los procesos antiguos, éstos vuelven a revestir su mayor importancia dentro de la tipología dinámica; no obstante, ellos presentan una originalidad más contrastada al haber actuado bajo condiciones climáticas más rigurosas y extremas, y con niveles de base marinos muy diferentes.[Abstract] In this synthesis we analyse the several geomorphological processes that have contributed to model the landforms in the «Galaico-Astur» area. At present, the dynamic nature processes (fluvial, marine, etc.) show a greater incidence though they are conditioned by the inf1uén:~ of climatic and structural factors; most of their efficacy results also increased by antrhopactions. As regards the ancient processes, those of dynamic type are again the more important ones. Furthermore, they appear more stressed because their action was performed on different sea-marine levels and their effects were reinforced by extreme and severe climatic conditions
Temporal analysis of fresh leaf spectroscopy and chemical properties in Quercus ilex trees
Peer reviewe
Diferenciación de carnes de jabalí europeo (Sus scrofa scrofa) y cerdo doméstico (Sus scrofa domestica) mediante el análisis por PCR de la región mitocondrial D-loop y el gen nuclear MC1R
En este trabajo se describe la diferenciación de carnes procedentes de jabalí europeo (Sus scrofa scrofa) y cerdo doméstico (Sus scrofa domestica) mediante el análisis por PCR de la región mitocondrial D-loop y del gen nuclear que codifica para el receptor 1 de la melanocortina (MC1R). La discriminación por PCR-RFLP de jabalí y cerdo en la región D loop no fue posible. Sin embargo, la técnica de PCR-RFLP desarrollada en el gen MC1R determinó perfiles de bandas característicos que permitieron la diferenciación de jabalí y cerdo mediante el empleo de las endonucleasas BspHI y BstUI.This work describes the differentiation of European wild boar (Sus scrofa scrofa) and domestic swine (Sus scrofa domestica) meats by PCR targeting sequences from the mitochondrial displacement loop (D-loop) region and the nuclear melanocortin receptor 1 (MC1R) gene. Detailed analysis of every D-loop sequence obtained indicated that PCR RFLP differentiation between wild and domestic Sus scrofa meats was not possible. Nevertheless, the PCR-RFLP technique developed targeting the MC1R gene generated characteristic PCR–RFLP profiles that allowed discrimination among meats from wild and domestic swine specimens using BspHI and BstUI endonucleases
Conformational Control of Tetrahydropyran-based Hybrid Dipeptide Catalysts Improves Activity and Stereoselectivity
© 2019 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim Herein, we introduce and demonstrate how carbohydrates can be used as conformational control units of organocatalysts to tune their catalytic properties. New hybrid dipeptide-like organocatalysts based on ζ-sugar aminoacids and proline were prepared and tested for the asymmetric Michael addition of aldehydes to β-nitrostyrenes. Taking full advantage of the modular nature of the carbohydrate motif, both reactivity and stereoselectivity were significantly improved. By simple structural changes, such as the elimination of the methoxy group in the C4 position of the tetrahydropyran ring, we obtained two complementary catalysts that allow access to both enantiomers of the γ-nitroaldehydes with excellent yields, diastereoselectivity, and enantiomeric excesses between 97 and 99%, using a catalytic load even below 1 mol%
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