Pair-induced spectral changes and variability in compact X-ray sources

Inverse Compton scattering of ultraviolet photons by GeV electrons produces γ-rays which in turn create electron–positron pairs if the source is sufficiently compact. The pairs modify the emergent radiation spectrum through their own inverse Compton scattering and through thermal Comptonization after they have cooled to sub-relativistic temperatures. Recent calculations of spectral reprocessing under these conditions are extended to situations in which the Thomson optical depth of the pair plasma exceeds unity, and to demonstrate time-dependent behaviour explicitly. The relevance of our results to X-ray observations of active galactic nuclei, binary X-ray sources and γ-ray bursters is discussed briefly

Dynamic & Cyclic behaviour of ballast in the long term as determined in Cedex's track box

The 6 cylinder servo-hydraulic loading system of CEDEX's track box (250 kN, 50 Hz) has been recently implemented with a new piezoelectric loading system (±20 kN, 300 Hz) allowing the incorporation of low amplitude high frequency dynamic load time histories to the high amplitude low frequency quasi-static load time histories used so far in the CEDEX's track box to assess the inelastic long term behavior of ballast under mixed traffic in conventional and high- speed lines. This presentation will discuss the results obtained in the first long-duration test performed at CEDEX's track box using simultaneously both loading systems, to simulate the pass-by of 6000 freight vehicles (1M of 225 kN axle loads) travelling at a speed of 120 km/h over a line with vertical irregularities corresponding to a medium quality lin3e level. The superstructure of the track tested at full scale consisted of E 60 rails, stiff rail pads (mayor que 450 kN/mm), B90.2 sleepers with USP 0.10 N/mm and a 0.35 m thick ballast layer of ADIF first class. A shear wave velocity of 250 m/s can be assumed for the different layers of the track sub-base. The ballast long-term settlements will be compared with those obtained in a previous long-duration quasi- static test performed in the same track, for the RIVAS [EU co-funded] project, in which no dynamic loads where considered. Also, the results provided by a high diameter cyclic triaxial cell with ballast tested in full size will be commented. Finally, the progress made at CEDEX's Geotechnical Laboratory to reproduce numerically the long term behavior of ballast will be discussed

Persistent behavioral sensitization to chronic L-DOPA requires A2A adenosine receptors

To investigate the role of A2A adenosine receptors in adaptive responses to chronic intermittent dopamine receptor stimulation, we compared the behavioral sensitization elicited by repeated L-DOPA treatment in hemiparkinsonian wild-type (WT) and A2A adenosine receptor knock-out (A2A KO) mice. Although the unilateral nigrostriatal lesion produced by intrastriatal injection of 6-hydroxydopamine was indistinguishable between WT and A2A KO mice, they developed strikingly different patterns of behavioral sensitization after daily treatment with low doses of L-DOPA for 3 weeks. WT mice initially displayed modest contralateral rotational responses and then developed progressively greater responses that reached a maximum within 1 week and persisted for the duration of the treatment. In contrast, any rotational behavioral sensitization in A2A KO mice was transient and completely reversed within 2 weeks. Similarly, the time to reach the peak rotation was progressively shortened in WT mice but remained unchanged in A2A KO mice. Furthermore, daily L-DOPA treatment produced gradually sensitized grooming in WT mice but failed to induce any sensitized grooming in A2A KO mice. Finally, repeated L-DOPA treatment reversed the 6-OHDA-induced reduction of striatal dynorphin mRNA in WT but not A2A KO mice, raising the possibility that the A2A receptor may contribute to L-DOPA-induced behavioral sensitization by facilitating adaptations within the dynorphin-expressing striatonigral pathway. Together these results demonstrate that the A2A receptor plays a critical role in the development and particularly the persistence of behavioral sensitization to repeated L-DOPA treatment. Furthermore, they raise the possibility that the maladaptive dyskinetic responses to chronic L-DOPA treatment in Parkinson's disease may be attenuated by A2A receptor inactivation.Peer Reviewe

Resolución de problemas científicos escolares y promoción de competencias de pensamiento científico. ¿Qué piensan los docentes de química en ejercicio?

In the article the results of an investigation conducted within the framework of the project FONDECYT 1070795 are exposed. The aims of the research were to identify and characterize the representations and knowledge that teachers in service have, from an average level of education, with respect to the competences of scientific thought and problem solution, and the way in which both should be approached. With the use of an intentional sample of 133, teachers selected from a total of 117 who were incorporated to other phases of the research trough the administration of a questionnaire especially designed, was show the predominance of a very fragmented image, sometimes contradictory, about the solution of problems and about relation between competences of scientific thought; this representation incorporates aspects of great value for the formation of the competent students, and at the same time it includes other aspects that in no way favor this formation, according to the present assessments. It is remarkable that both competences for scientific thought and solution of problems are not constituted as a totality and coherent system in the thinking of the teachers who participated in the research. It should be pointed out that we found data that show an oscillating valuation of the subjective character of the problematic situations and the problem itself; these data show, as well, that teachers give a very important role to language. It is stated that teachers give an oscillating value to the fact that students must deal with theories. It should be noted too that their approaches to the algorithmic processes are very vague, and sometimes contradictory.En el artículo se exponen los resultados de una investigación realizada en el marco del proyecto FONDECYT 1070795, cuyo objetivo consistió en identificar y caracterizar las nociones que tienen los profesores de nivel medio respecto a las competencias de pensamiento científico y solución de problemas, y la manera en que ambos deben trabajarse en la enseñanza de la química. Con el empleo de una muestra intencional de 33 profesores, seleccionados de un total de 117 que se incorporaron a otras fases de la investigación y mediante la administración de un cuestionario especialmente diseñado, se logró poner de manifiesto que en ellos predomina una imagen fragmentada y a veces contradictoria de la solución de problemas y de las competencias de pensamiento científico; dicha representación incorpora aspectos de inestimable valor para la formación del estudiante competente, a la vez que incluye otros que en manera alguna favorecen dicha formación en correspondencia con las exigencias actuales. Es notable que competencias de pensamiento científico y solución de problemas no se constituyan como sistema totalmente coherente en la conceptualización de los profesores investigados. Como dato relevante, se constató la oscilante valoración del carácter subjetivo de las situaciones problémicas y los problemas; junto al papel importante que, en opinión de gran parte de los profesores, desempeña el lenguaje, se le otorga un valor oscilante al trabajo con la teoría por parte de los estudiantes; igualmente resulta difusa, y a veces contradictoria, su aproximación a los procesos de algoritmización

CARACTERIZACION VOZ Y VIDEO

La motivación para caracterizar el tráfico de voz y video está en la necesidad de las empresas proveedoras de servicio en mantener redes de transporte de información con capacidades acordes a los requerimientos de los usuarios.  Poder determinar en forma oportuna como los elementos técnicos que hacen parte de las redes afectan su desempeño, teniendo en cuenta que cada tipo de servicio es afectado en mayor o menor medida por dichos elementos dentro de los que tenemos el jitter, las demoras y las pérdidas de paquetes entre otros. El presente trabajo muestra varios casos de caracterización de tráfico tanto de voz como de video en las que se utilizan una diversidad de técnicas para diferentes tipos de servicio

Emergent Spin-Filter at the interface between Ferromagnetic and Insulating Layered Oxides

We report a strong effect of interface-induced magnetization on the transport properties of magnetic tunnel junctions consisting of ferromagnetic manganite La$_{0.7}$Ca$_{0.3}$MnO$_{3}$ and insulating cuprate PrBa$_{2}$Cu$_{3}$O$_{7}$. Contrary to the typically observed steady increase of the tunnel magnetoresistance with decreasing temperature, this system exhibits a sudden anomalous decrease at low temperatures. Interestingly, this anomalous behavior can be attributed to the competition between the positive spin polarization of the manganite contacts and the negative spin-filter effect from the interface-induced Cu magnetization.Comment: 5 pages, 4 figures, with supplemental materials (2 figures). Physical Review Letters, in pres