Entre los problemas más importantes de la astrofísica se destaca la falta de una clara identificación para las fuentes detectadas a muy altas energías (>100 MeV) por telescopios satelitales. En la actualidad, detecciones a energías más altas aún (>200 GeV) están siendo obtenidas por telescopios Cherenkov. En combinación con estudios multifrecuencia, estas observaciones plantean numerosos desafíos teóricos. Entre las posibles contrapartes de las fuentes gamma galácticas, los sistemas binarios con acreción brindan excelentes perspectivas, a las que se suma la reciente detección en el rango TeV de 4 binarios de rayos X. En la presente investigación doctoral nos hemos focalizado en la emisión gamma originada en binarias de gran masa con acreción, debido a la interacción de protones relativistas con el medio local. Dicho proceso genera radiación gamma a través del decaimiento de los piones neutros que se crean en las colisiones pp. Específicamente, hemos estudiado el caso de un objeto compacto de bajo campo magnético, capaz de efectuar eyecciones de masa confinadas en chorros de partículas relativistas (microcuásares). En este contexto, hemos explorado también la emisión leptónica secundaria. Dentro de los jets se generan pares relativistas al decaer piones cargados, que son otro subproducto de las colisiones pp. Se resolvió en forma consistente la evolución de esta población de pares, teniendo en cuenta las pérdidas de energía radiativas y adiabática, y se calculó su emisión sincrotrón y Compton inversa.

Marek’s disease (MD) is a lymphoproliferative disease of chickens caused by an oncogenic herpesvirus, Marek’s disease virus (MDV). Despite the availability of MD vaccines, little is known about the underlying immunological mechanisms that mediate vaccine-induced immunity. The objective of this research was to elucidate these mechanisms. To characterize host responses in the lungs, chickens were vaccinated with herpesvirus of turkeys (HVT) and infected with MDV-RB1B. Vaccinated MDV-infected chickens had a higher accumulation of viral genome in the lungs, associated with T cell infiltration in lung tissue and an up-regulation of interferon (IFN) - and interleukin (IL) -10. This finding led us to conclude that IFN-γ has a role in immunity; hence, we further investigated the role of this cytokine. The hypothesis tested was that the protective efficacy of HVT against MDV-RB1B would be enhanced when combined with recombinant chicken IFN-γ (rChIFN-). Chicken IFN-γ coding sequence was cloned into an expression plasmid, and the bioactivity of rChIFN- was confirmed. Administration of this plasmid led to a significant reduction in tumour occurrence in HVT vaccinated MDV-infected chickens, suggesting enhanced vaccine-induced immunity. To shed more light on the relevance of IFN-γ to immunity against MD, studies were designed to down-regulate the expression of IFN-γ in chicken tissues. Three small interfering (si)RNAs specific for chicken IFN- were selected which significantly inhibited expression of IFN-γ by up to 80% in cultured cells. These three siRNAs and a non-target control were cloned and expressed as short hairpin RNA (shRNA) using an avian adeno-associated virus (rAAAV) vector system. An MDV challenge trial was conducted once shRNA expression by the rAAAV was confirmed in vitro. It was reasoned that down-regulation of IFN- would lead to abrogation of immunity conferred by HVT. There was an increase in the number of chickens with tumours that received HVT and rAAAV + IFN-γ shRNA compared to the control group, though not statistically significant. However, no difference in MDV genome load in feathers was detected between vaccinated MDV-infected birds with or without rAAAV. In summary, we have demonstrated here that cytokines are induced in the course of vaccination against Marek’s disease and that IFN-γ plays a role in vaccine-induced immunity against MD.

No description available.

No description available.

Hyperflares from Soft Gamma Repeaters have revealed quasi-periodic oscillations, suggested to be torsional modes of the neutron star crust produced during starquakes. We study how a magnetic field affects these modes. We make a plane-parallel model of the neutron star crust with a vertical and non-vertical field and solve for the eigenvalues and eigenfunctions. In the vertical field case we find a discrete set of modes comparable to the observed frequencies giving evidence for this theory. We find that the lower order modes do not depend on B while the higher order modes do when the field strength is low or extremely high. We make a simple analytic model of a non-vertical field in a closed box representing the neutron star crust. The mode spectrum in this model is continuous rather than discrete, raising the puzzle of why only specific modes are excited during the hyperflare.

No description available.

No description available.

This thesis investigates the feasibility of designing a simple nuclear medicine data processing system based on an inexpensive microcomputer system, which is affordable to small hospitals and to developing countries where resources are limited. Since the main need for a computer is to allow dynamic studies to be carried out, the relevant criteria for choosing the computer are its speed and memory capacity. The benchmark chosen for these criteria is renography, one of the commonest nuclear medicine procedures. The Acorn Archimedes model 310 microcomputer was found to meet these requirements, and a suitable camera-computer interface has been designed. Because of the need for ensuring that the gain and offset controls of the interface are set optimally before connecting to the camera, it was necessary to design a circuit which produces a test pattern on the screen for use during this operation. Having also developed and tested the data acquisition and image display software successfully, atttention was concentrated on finding ways of characterising and measuring the performance of the computer interface and the display device, two important areas which have been largely neglected in the quality control of camera-computer systems. One of the characteristics of the interface is its deadtime. A procedure has been outlined for measuring this by means of a variable frequency pulse generator and also for interpreting the data correctly. A theoretical analysis of the way in which the interface deadtime affects the overall count rate performance of the system has also been provided. The spatial linearity, resolution and uniformity characteristics of the interface are measured using a special dual staircase generator circuit designed to simulate the camera position and energy signals. The test pattern set up on the screen consists of an orthogonal grid of points which can be used for a visual assessment of linearity, while analysis of the data in memory enables performance indices for resolution, linearity and uniformity to be computed. The thesis investigates the performance characteristics of display devices by means of radiometric measurements of screen luminance. These reveal that the relationship between screen luminance and display grey level value can be taken as quadratic. Characterisation of the display device in this way enables software techniques to be employed to ensure that screen luminance is a linear function of display grey level value; screen luminance measurements, coupled with film density measurements, are also used to optimise the settings of the display controls for using the film in the linear range of its optical densities. This in turn ensures that film density is a linear function of grey level value. An alternative approach for correcting for display nonlinearity is by means of an electronic circuit described in this thesis. Intensity coding schemes for improving the quality of grey scale images can be effective only if distortion due to the display device is corrected for. The thesis also draws attention to significant variations in film density which may have their origins in nonuniformities in the display screen, the recording film, or in the performance of the film processor. The work on display devices has been published in two papers.

No description available.

No description available.