Dissertations / Theses on the topic 'VLBI'
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Casey, Simon Paul. "The development of e-VLBI." Thesis, University of Manchester, 2008. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.489523.
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Since its introduction, Very Long Baseline Interferometry has enabled high resolution imaging of astronomical sources. The work presented in this thesis demonstrates the feasibility of transmitting VLBI data over the UDP protocol at constant bit rates. Previously TCP had been used for e-VLBI, but TCP is not a good choice of protocol for an application that requires constant bandwidth.
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Gontier, Graffan Anne-Marie. "Orientation de la Terre par mesure VLBI : contribution à la chaîne d'analyse de VLBI astrométrique GLORIA." Observatoire de Paris (1667-....), 1992. https://hal.science/tel-02095270.
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Ce travail de thèse se situe dans le cadre du développement d'une chaîne originale d'analyse de VLBI (Very Large Baseline Interferometry) astrométrique et géodésique GLORIA (GLObal Radio Interferometric Analysis) par des équipes de l'Observatoire de Paris et de l'Institut Géographique National. Nous avons dans un premier temps développé et testé une modélisation astronomique et géophysique originale dans le cadre de la relativité générale avec une précision de 10 picosecondes. Dans un deuxième temps nous avons utilisé cette modélisation originale pour analyser trois ans d'observations du programme VLBI dédié à l'étude de la rotation terrestre. Nous avons développé un protocole statistique, basé sur des résolutions en norme l1 et en norme l2, permettant de détecter les observations atypiques. Les solutions obtenues, 747 évaluations journalière de temps universel, UT1-TAI, pour 1988-1990, ont une précision de 0,001 qui est celle attendue théoriquement étant donné la stratégie d'observation de ce programme.
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Mignard, François, and Sergei Klioner. "Gaia DR1 compared to VLBI positions." Cambridge University Press, 2017. https://tud.qucosa.de/id/qucosa%3A70690.
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Tsioumis, Anastasios K. "VLBI studies of compact radio sources." Thesis, The University of Sydney, 1987. https://hdl.handle.net/2123/25997.
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Radio observations of a complete sample of 44 flat spectrum southern radio sources plus an additional 11 sources of particular interest are presented. Trends in the properties of the complete sample are discussed.
All sources of special interest and 19 of the sources in the complete sample were observed with the Southern Hemisphere VLBI Experiment (SHEVE). This was the first extensive VLBI array in the southern hemisphere and its characteristics are presented in some detail. The data reduction methods and, in particular, the calibration of the large SHEVE dataset are described as this is critical to the quality of the final images. The imaging techniques are illustrated by detailed imaging of the radio source 1934-638. Since few closure phase and closure amplitude relations are present self—calibration methods cannot be employed. It was found that the appropriate imaging method for the SHEVE data was modelfitting with Gaussian components. A detailed image of 1934—638 is presented and the source structure compared with previous VLBI observations. All sources observed in SHEVE were fitted with Gaussian component models. The models are presented in a table and source images shown to illustrate the source structure. The SHEVE data and the fits of the models are also shown.
All sources in the complete sample and six of the additional VLBI sources were mapped at low resolution using the Molonglo Observatory Synthesis Telescope (MOST) at 843 MHz and the Fleurs Synthesis Telescope at 1415 MHz. Special techniques were used at both instruments to gain direct access to the higher spatial frequency data and hence increased effective resolution. This is necessary since most of the sources in the sample are compact with little or no arcsecond-level structure. The results are summarised in tables and maps are presented at both frequencies for all sources. In addition, accurate radio positions were measured and were compared with other radio and optical positions. Significant pointing offsets were revealed for both instruments.
The present results and other source information were then assembled to construct an extensive data base of source properties. Four new optical identifications were made and all objects but one have been identified except in the region obscured by the galactic plane. The database information was used to search for trends in the properties of the complete sample of radio sources. A brief account is given of the sources not in the complete sample and detailed analysis of two special sources is presentedin two appendices.
The thesis concludes with an examination of and suggestions for future VLBI observationsin Australia.
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Doeleman, Sheperd S. (Sheperd Samuel). "Imagining active galactic nuclei with 3mm-VLBI." Thesis, Massachusetts Institute of Technology, 1995. http://hdl.handle.net/1721.1/32655.
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Xu, Wenge Readhead Anthony C. S. Readhead Anthony C. S. "The first Caltech-Jodrell Bank VLBI survey /." Diss., Pasadena, Calif. : California Institute of Technology, 1995. http://resolver.caltech.edu/CaltechETD:etd-10262007-132906.
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Kirsten, Franz [Verfasser]. "Pulsar astrometry with VLBI and beyond / Franz Kirsten." Bonn : Universitäts- und Landesbibliothek Bonn, 2014. http://d-nb.info/1060787172/34.
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Despringre, Vincent. "Observation et modélisation des jets extragalactiques observés en VLBI." Toulouse 3, 1996. http://www.theses.fr/1996TOU30212.
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Les jets extragalactiques sont une des manifestations de l'activite intense de certains noyaux actifs de galaxies. Ce sont de gigantesques ejections de plasma du cur de la galaxie, a des distances atteignant parfois plusieurs centaines de kiloparsecs. La physique qui les entoure reste encore relativement mysterieuse. Le developpement de la vlbi (very large base interferometry) ou interferometrie continentale, a permis d'atteindre une resolution de l'ordre du milliarcseconde. Son utilisation pour l'observation des jets extragalactiques, a abouti a la decouverte de l'existence de jets proche du cur et a l'echelle du parsec. On les appelle des jets vlbi. Le but de cette these est l'etude de ces objets. A l'aide du reseau d'interferometrie continentale europeen, l'evn (europeen vlbi network), et du reseau connecte merlin, j'ai cartographie la radiosource 3c66b a la frequence de 5 ghz. J'ai alors decouvert, au cur de cette source, un jet vlbi jusque la inconnu. Les premieres cartes de ce jet ainsi qu'une etude detaillee de sa structure et des flux emis sont presentees dans cette these. La structure des jets vlbi, a l'echelle du parsec, montre une architecture relativement complexe: ils presentent un certain nombre de nodosites qui se deplacent le long du jet sur des trajectoires non rectilignes avec, parfois, des vitesses superluminiques. J'ai developpe un modele permettant d'expliquer les observations vlbi: les nodosites seraient des nuages relativistes, formes de paires electron-positron, se deplacant sur une trajectoire helicoidale enroulee autour d'un jet, non ou faiblement relativiste. Les premiers resultats de cette simulation ainsi qu'une comparaison aux observations font l'objet de la seconde partie de cette these
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Deller, Adam Travis. "Precision VLBI astrometry instrumentation, algorithms and pulsar parallax determination /." Swinburne Research Bank, 2009. http://hdl.handle.net/1959.3/48485.
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Thesis (PhD) - Faculty of Information and Communication Technology, Faculty of Life and Social Sciences, Swinburne University of Technology, 2009.Submitted in total fulfilment of the requirements for the degree of Doctor of Philosophy, Faculty of Information and Communication Technology, Swinburne University of Technology, 2009. Typescript. Bibliography: p. 165-186.
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Blecher, Tariq. "MEQSILHOUETTE: a mm-VLBI observation and signal corruption simulator." Thesis, Rhodes University, 2017. http://hdl.handle.net/10962/40713.
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The Event Horizon Telescope (EHT) aims to resolve the innermost emission of nearby supermassive black holes, Sgr A* and M87, on event horizon scales. This emission is predicted to be gravitationally lensed by the black hole which should produce a shadow (or silhouette) feature, a precise measurement of which is a test of gravity in the strong-field regime. This emission is also an ideal probe of the innermost accretion and jet-launch physics, offering the new insights into this data-limited observing regime. The EHT will use the technique of Very Long Baseline Interferometry (VLBI) at (sub)millimetre wavelengths, which has a diffraction limited angular resolution of order ~ 10 µ-arcsec. However, this technique suffers from unique challenges, including scattering and attenuation in the troposphere and interstellar medium; variable source structure; as well as antenna pointing errors comparable to the size of the primary beam. In this thesis, we present the meqsilhouette software package which is focused towards simulating realistic EHT data. It has the capability to simulate a time-variable source, and includes realistic descriptions of the effects of the troposphere, the interstellar medium as well as primary beams and associated antenna pointing errors. We have demonstrated through several examples simulations that these effects can limit the ability to measure the key science parameters. This simulator can be used to research calibration, parameter estimation and imaging strategies, as well as gain insight into possible systematic uncertainties.
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Morgan, John <1981>. "Very Long Baseline Interferometry in Italy Wide-field VLBI imaging and astrometry and prospects for an Italian VLBI network including the Sardinia Radio Telescope." Doctoral thesis, Alma Mater Studiorum - Università di Bologna, 2010. http://amsdottorato.unibo.it/2830/.
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In this thesis the use of widefield imaging techniques and VLBI observations with a limited number of antennas are explored.
I present techniques to efficiently and accurately image extremely large UV datasets. Very large VLBI datasets must be reduced into multiple, smaller datasets if today’s imaging algorithms are to be used to image them. I present a procedure for accurately shifting the phase centre of a visibility dataset. This procedure has been thoroughly tested and found to be almost two orders of magnitude more accurate than existing techniques. Errors have been found at the level of one part in 1.1 million. These are unlikely to be measurable except in the very largest UV datasets.
Results of a four-station VLBI observation of a field containing multiple sources are presented. A 13 gigapixel image was constructed to search for sources across the entire primary beam of the array by generating over 700 smaller UV datasets. The source 1320+299A was detected and its astrometric position with respect to the calibrator J1329+3154 is presented. Various techniques for phase calibration and imaging across this field are explored including using the detected source as an in-beam calibrator and peeling of distant confusing sources from VLBI visibility datasets.
A range of issues pertaining to wide-field VLBI have been explored including; parameterising the wide-field performance of VLBI arrays; estimating the sensitivity across the primary beam both for homogeneous and heterogeneous arrays; applying techniques such as mosaicing and primary beam correction to VLBI observations; quantifying the effects of time-average and bandwidth smearing; and calibration and imaging of wide-field VLBI datasets. The performance of a computer cluster at the Istituto di Radioastronomia in Bologna has been characterised with regard to its ability to correlate using the DiFX software correlator. Using existing software it was possible to characterise the network speed particularly for MPI applications. The capabilities of the DiFX software correlator, running on this cluster, were measured for a range of observation parameters and were shown to be commensurate with the generic performance parameters measured.
The feasibility of an Italian VLBI array has been explored, with discussion of the infrastructure required, the performance of such an array, possible collaborations, and science which could be achieved.
Results from a 22 GHz calibrator survey are also presented. 21 out of 33 sources were detected on a single baseline between two Italian antennas (Medicina to Noto).
The results and discussions presented in this thesis suggest that wide-field VLBI is a technique whose time has finally come. Prospects for exciting new science are discussed in the final chapter.
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Schnell, Dorothee. "Quality aspects of short duration VLBI observations for UT1 determinations." [S.l.] : [s.n.], 2006. http://deposit.ddb.de/cgi-bin/dokserv?idn=982380534.
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Zhang, Zhongkai [Verfasser]. "VLBI observations of the Chang'E-3 lunar lander / Zhongkai Zhang." Bonn : Universitäts- und Landesbibliothek Bonn, 2019. http://d-nb.info/1200098110/34.
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Junor, W. "MERLIN and VLBI observations of the radio galaxy Virgo A." Thesis, University of Manchester, 1987. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.233133.
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Halsig, Sebastian [Verfasser]. "Atmospheric refraction and turbulence in VLBI data analysis / Sebastian Halsig." Bonn : Universitäts- und Landesbibliothek Bonn, 2018. http://d-nb.info/116067339X/34.
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Kulkarni, Madhav Narayan. "A feasibility study of space VLBI for geodesy and geodynamics /." The Ohio State University, 1992. http://rave.ohiolink.edu/etdc/view?acc_num=osu1487776210796273.
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Fayard, Thierry. "Instrumentation en VLBI et application à l'observation des pulsars rapides." Observatoire de Paris, 1989. https://hal.archives-ouvertes.fr/tel-02149892.
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Première partie : pour relier le référentiel extragalactique lié aux quasars au référentiel dynamique lié au système solaire on peut utiliser conjointement le chronométrage et le positionnement par VLBI des pulsars millisecondes récemment découverts. Outre les intérêts scientifiques, une application directe de cette mesure serait de faciliter la localisation par radio-interférométrie donc dans le référentiel extragalactique d'une sonde spatiale dont on aurait calculé la position dans le référentiel dynamique. Malheureusement le plus puissant de ces pulsars (PSR 1937+214) ne fait que quelques milli jansky en moyenne. Pour permettre une observation en interférométrie à très grande base, il est nécessaire de fenêtrer les pulses et de les filtrer à cause de la dispersion interstellaire. Pour pouvoir faire ces traitements ainsi que la corrélation sur un ordinateur, il a été nécessaire de réaliser une chaine de transfert de données du magnétoscope mark II à l'ordinateur hôte. Les premiers résultats obtenus indiquent la validité du concept. Deuxième partie : les synthétiseurs numériques quoique de plus en plus souvent employés dans l'industrie, méritent une attention particulière quand on souhaite les utiliser en radio-astronomie. Si pour certaines applications comme les oscillateurs programmables agiles, ils sont difficilement remplaçables, pour des applications nécessitant des propriétés de phase exceptionnelles comme en interférométrie à très grande base, il est nécessaire d'optimiser à l'extrême la boucle à verrouillage de phase sans que le résultat soit pour autant pleinement satisfaisant.
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Akoto-Danso, Alexander. "A pilot wide-field VLBI survey of the GOODS-North field." Thesis, Rhodes University, 2019. http://hdl.handle.net/10962/72296.
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Very Long Baseline Interferometry (VLBI) has significant advantages in disentangling active galactic nuclei (AGN) from star formation, particularly at intermediate to high-redshift due to its high angular resolution and insensitivity to dust. Surveys using VLBI arrays are only just becoming practical over wide areas with numerous developments and innovations (such as multi-phase centre techniques) in observation and data analysis techniques. However, fully automated pipelines for VLBI data analysis are based on old software packages and are unable to incorporate new calibration and imaging algorithms. In this work, the researcher developed a pipeline for VLBI data analysis which integrates a recent wide-field imaging algorithm, RFI excision, and a purpose-built source finding algorithm specifically developed for the 64kx64k wide-field VLBI images. The researcher used this novel pipeline to process 6% (~ 9 arcmin2 of the total 160 arcmin2) of the data from the CANDELS GOODS- North extragalactic field at 1.6 GHz. The milli-arcsec scale images have an average rms of a ~ 10 uJy/beam. Forty four (44) candidate sources were detected, most of which are at sub-mJy flux densities, having brightness temperatures and luminosities of >5x105 K and >6x1021 W Hz-1 respectively. This work demonstrates that automated post-processing pipelines for wide-field, uniform sensitivity VLBI surveys are feasible and indeed made more efficient with new software, wide-field imaging algorithms and more purpose-built source- finders. This broadens the discovery space for future wide-field surveys with upcoming arrays such as the African VLBI Network (AVN), MeerKAT and the Square Kilometre Array (SKA).
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Weston, Stuart Duncan. "Development of Very Long Baseline Interferometry (VLBI) techniques in New Zealand array simulation, image synthesis and analysis : a thesis submitted to Auckland University of Technology in fulfilment of the requirements for the degree of Master of Philosophy (MPhil), 2008 /." Click here to access this resource online, 2008. http://hdl.handle.net/10292/449.
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This thesis presents the design and development of a process to model Very Long Base Line Interferometry (VLBI) aperture synthesis antenna arrays. In line with the Auckland University of Technology (AUT) Institute for Radiophysics and Space Research (IRSR) aims to develop the knowledge, skills and experience within New Zealand, extensive use of existing radio astronomical software has been incorporated into the process namely AIPS (Astronomical Imaging Processing System), MIRIAD (a radio interferometry data reduction package) and DIFMAP (a program for synthesis imaging of visibility data from interferometer arrays of radio telescopes). This process has been used to model various antenna array configurations for two proposed New Zealand sites for antenna in a VLBI array configuration with existing Australian facilities and a passable antenna at Scott Base in Antarctica; and the results are presented in an attempt to demonstrate the improvement to be gained by joint trans-Tasman VLBI observation. It is hoped these results and process will assist the planning and placement of proposed New Zealand radio telescopes for cooperation with groups such as the Australian Long Baseline Array (LBA), others in the Pacific Rim and possibly globally; also potential future involvement of New Zealand with the SKA. The developed process has also been used to model a phased building schedule for the SKA in Australia and the addition of two antennas in New Zealand. This has been presented to the wider astronomical community via the Royal Astronomical Society of New Zealand Journal, and is summarized in this thesis with some additional material. A new measure of quality (“figure of merit”) for comparing the original model image and final CLEAN images by utilizing normalized 2-D cross correlation is evaluated as an alternative to the existing subjective visual operator image comparison undertaken to date by other groups. This new unit of measure is then used in the presentation of the results to provide a quantative comparison of the different array configurations modelled. Included in the process is the development of a new antenna array visibility program which was based on a Perl code script written by Prof Steven Tingay to plot antenna visibilities for the Australian Square Kilometre Array (SKA) proposal. This has been expanded and improved removing the hard coded fixed assumptions for the SKA configuration, providing a new useful and flexible program for the wider astronomical community. A prototype user interface using html/cgi/perl was developed for the process so that the underlying software packages can be served over the web to a user via an internet browser. This was used to demonstrate how easy it is to provide a friendlier interface compared to the existing cumbersome and difficult command line driven interfaces (although the command line can be retained for more experienced users).
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de, Oliveira Souza Wanessa. "Modelagem do movimento da placa tectônica sul-americana por meio de dados VLBI e GNSS." Universidade Federal de Pernambuco, 2013. https://repositorio.ufpe.br/handle/123456789/10510.
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Reuni, CAPES, CNPQ
Há cerca de 220 milhões de anos os continentes vêm afastando-se uns dos outros. Esse fenômeno se explica a partir da teoria da deriva continental, complementada pela teoria da tectônica de placas, onde se acreditava que estruturas rígidas como as placas litosféricas, eram capazes de transmitir tensões em longas distâncias sem causar deformações internas. Desta forma, devido ao movimento das placas e como consequência a deriva dos continentes, as estações a eles pertencentes também vêm se movendo em relação a um referencial geocêntrico. Considerando que, com a evolução tecnológica e as facilidades geradas no tratamento de dados globais, a Geodésia ganhou um dinamismo sem precedentes, incorporando em seus métodos de posicionamento, atualizações periódicas dos referencias globais, como exemplo, as estruturas iniciadas pelos ITRF’s de 1988 até o de 2008. Do ponto de vista científico, não há problema nenhum quanto ao dinamismo das estruturas geodésicas, mas do ponto de vista prático e legal, tal dinamismo gera alguns inconvenientes, que podem ser resolvidos pelo conhecimento e modelagem adequada para as transformações a serem executadas. No tocante ao deslocamento da placa tectônica Sul-americana, alguns modelos de velocidade vêm sendo utilizados desde a virada do milênio, entre eles: NNR-NUVEL-1A; APKIM2000; APKIM8.8; ITRF2000; COSTA, SANTOS e GEMAEL (2003); PEREZ, MONICO e CHAVES (2003); e VEMOS2009. Como o Sistema Geodésico Brasileiro, SIRGAS2000, oficializado pela RPR 01 (IBGE), de 25 de fevereiro de 2005, passou a utilizar a época de referência 2000,4, então o posicionamento atual de medidas podem precisar de uma redução de 13 anos. Confrontando os modelos citados e gerando as discrepâncias, analisaram-se na ordem milimétrica e também na ordem centimétrica as equivalências dos modelos para o território brasileiro. O modelo VEMOS2009 atualmente utilizado pelo IBGE no serviço PPP online foi tomado como referência padrão, na análise comparativa. Além desses, com base no processamento semanal do SIRGAS-CON (http://www. sirgas.org/index.php?id=153) dos anos de 2008 a 2011 de algumas estações da RBMC, cuja geometria permitisse uma amostra significativa da placa sul-americana (SOAM) e também se aproveitando de dados de duas estações VLBI (Very Long Baseline System) sobre a placa SOAM; uma (Ft) localizada na cidade de Fortaleza – CE, Brasil e a outra (TIGO) localiza em Concepción no Chile, processadas pelo IVS (International VLBI service for Geodesy and astrometry), pôde-se gerar um modelo atual de velocidades e participar das análises comparativas. Os processamentos foram realizados com os módulos: modeloSOAM@ versão 2013.07.08 e ModelTectonica@2013.07.07 do software AstGeo- Top. Os resultados mostraram a possibilidade de utilizar vários dos modelos citados quando a discrepância puder ser até de 5 cm em 13 anos, mas ficou reduzida a algumas regiões do Brasil e a três dos modelos quando a discrepância não puder ultrapassar a 1cm em 13 anos.
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Moldón, Vara Francisco Javier. "Structure and nature of gamma-ray binaries by means of VLBI observations." Doctoral thesis, Universitat de Barcelona, 2012. http://hdl.handle.net/10803/96996.
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Gamma-ray binaries are extreme systems that produce non-thermal emission from radio to very-high-energy (above TeV) gamma rays, with the energy output in the spectral energy distribution (SED) dominated by the MeV–GeV photons. Their broadband emission is usually modulated by the orbital cycle of the system, which suggests that the physical conditions are also periodic and reproducible. The diversity of systems, together with the reproducibility of the conditions within each system, makes gamma-ray binaries excellent physical laboratories in which high energy particle acceleration, diffusion, absorption, and radiation mechanisms can be explored. Nevertheless, the number of known gamma-ray binaries is still very limited, and only a six binary systems have been classified as gamma-ray binaries. These systems produce outflows of relativistic particles emitting synchrotron radio emission that extend up to several astronomical units, which correspond to projected angular scales of a few milliarcseconds (mas) at typical distances of 2-3 kpc. Very Long Baseline Interferometry (VLBI) provide mas resolution and therefore can be used to directly see this radio outflow. In this thesis we present VLBI observations of five of the six gamma-ray binaries known. We have revealed for the first time the radio structure of two gamma-ray binaries, and found periodic changes in the structure of other two. Based on these results, we have established the basic properties and behaviour of the radio emission of gamma-ray binaries on AU scales, and we have contributed to find characteristics that are common to all of them.En los últimos años se ha producido una mejora significativa de los instrumentos que permiten observar fenómenos astrofísicos en rayos gamma de alta y muy alta energía. Gracias a estos avances, se ha podido detectar emisión de rayos gamma en sistemas binarios. Tan sólo seis sistemas binarios han sido clasificados como estrellas binarias de rayos gamma (tres de ellos aún son candidatos). Estos sistemas producen chorros de material relativista que a su vez producen grandes cantidades de energía en todo el espectro electromagnético, desde ondas radio hasta emisión de rayos gamma hasta energías del teraelectronvolt (TeV). Estos chorros se desplazan a alta velocidad produciendo estructuras en escalas de varias unidades astronómicas (UA). Estas estructuras pueden ser observadas directamente mediante técnicas de interferometría radio de muy larga línea de base (VLBI). En esta tesis nos centramos en el estudio de las propiedades morfológicas y astrométricas de binarias de rayos gamma observadas mediante VLBI. De las seis binarias conocidas, se han observado cinco de ellas. Los resultados principales son los siguientes. Se ha detectado estructura extendida en escalas de 120 UA en el sistema binario PSR B1259-63. Esta ha sido la primera evidencia observacional de que púlsares jóvenes no acretantes interaccionando con estrellas jóvenes pueden producir emisión radio extendida. Se ha descubierto que la emisión del sistema LS 5039 muestra cambios periódicos en su morfología, que son estables en escalas de varios años. También se ha determinado el movimiento propio de este sistema y se ha obtenido su trayectoria galáctica en el pasado. Se ha confirmado que el sistema LS I +61 303 muestra variabilidad orbital periódica, aunque presenta cambios significativos en ciertas fases orbitales. Se ha encontrado un desplazamiento del pico de la emisión a varias frecuencias, así como un cambio en sus posiciones relativas. Se ha descubierto emisión extendida procedente de la fuente de rayos gamma HESS J0632+057, y se ha confirmado inequívocamente su asociación con el sistema binario MWC 148. Por último, no se ha encontrado contrapartida radio a la fuente de rayos gamma AGL 2241+4454, cuya contrapartida óptica ha sido propuesta en el sistema MWC 656. Estos resultados permiten sentar las bases de la estructura en escalas de varias UA de los sistemas binarios de rayos gamma, así como su comportamiento en función de la fase orbital. Las características comunes halladas en estos sistemas ha permitido encontrar enlaces observacionales entre estos sistemas, dando consistencia a este particular grupo de estrellas que presentan emisión en rayos gamma.
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Pagels, Anke. "Beobachtungen der unmittelbaren Umgebung von supermassiven Schwarzen Löchern Millimeter-VLBI von AGN." [S.l.] : [s.n.], 2006. http://deposit.ddb.de/cgi-bin/dokserv?idn=982461755.
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Richter, Laura. "A VLBI polarisation study of 43 GHZ SiO masers towards VY CMA /." Link to this resource, 2005. http://eprints.ru.ac.za/784/.
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Charlot, Patrick. "Structure des sources extragalactiques dans les observations VLBI d'astrométrie et de géodynamique." Observatoire de Paris, 1989. https://hal.archives-ouvertes.fr/tel-02095282.
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Les observations VLBI d'astrométrie et de géodynamique (par exemple, celles du programme Crustal Dynamics) peuvent être utilisées pour cartographier les radio sources extragalactiques. 38 cartes VLBI ayant une résolution angulaire de l'ordre de 0. 001 ont été réalisées avec ce type de donnée. Ces cartes ont permis, d'une part d'étudier la structure des sources cartographiées, notamment les mouvements superluminiques, d'autre part de modéliser les effets produits par ces structures dans les observations VLBI d'astrométrie et de géodynamique. Au niveau de précision actuel, ces effets sont significatifs ; ils doivent maintenant être pris en compte lors de l'analyse des mesures VLBI de haute précision pour l'astrométrie et la géodynamique.
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Richter, Laura. "A VLBI polarisation study of 43 GHZ SiO masers towards VY CMA." Thesis, Rhodes University, 2006. http://hdl.handle.net/10962/d1005284.
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This thesis reports the calibration, imaging and analysis of one epoch of VLBI observations of the v (italics) = J (italics) = 1-0 transition of SiO towards VY CMa. Full polarisation information was recorded, allowing high resolution synthesis maps of each of the four Stokes parameters to be produced. A total of 81 maser components were extracted from the total intensity map, each approximately 1 mas in size. The emission spans approximately 100 x 80 mas in right ascension and declination and is concentrated to the east. The maser component positions were fitted to a ring of radius ~ 3.2R₊ (italics), or 7.2 x 1O¹⁴ cm for a stellar distance of 1.5 kpc. If the stellar position is assumed to be the centre of this ring then almost all of the maser components fall within the inner dust shell radius, which is at ~ 5R (italics)ϰ All of the maser components fall between 1.5R (italics)ϰ and 6R (italics)ϰ. A velocity gradient with position angle was observed in the sparsely filled western region of the maser ring. If interpreted as evidence of shell rotation, this gradient implies a rotational velocity of v (italics) rot (subscirpt) sin i (italics) = 18 km.s⁻¹. The fractional circular and linear polarisations of the maser spots were derived from the Stokes parameter maps. The mean fractional circular polarisation of the masers components was ~ 2 percent and the median fractional linear polarisation was ~ 6 percent, with many spots displaying over ~ 30 percent linear polarisation. The mean circular polarisation implies a magnetic field of ~ 4 G in the SiO maser region if the polarisation is due to Zeeman splitting. Two maser components display a rotation of linear polarisation position angle with velocity, possibly implying a connection between the magnetic field and the velocity field variations in the region of these components.
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Vennebusch, Markus. "Singular value decomposition and cluster analysis as regression diagnostics tools in geodetic VLBI." [S.l.] : [s.n.], 2007. http://deposit.ddb.de/cgi-bin/dokserv?idn=984912878.
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Reynolds, Cormac. "Polarization VLBI monitoring of BL Lacertae : kinematics, Faraday effects and intermediate scale structure." Thesis, University of Central Lancashire, 2002. http://clok.uclan.ac.uk/19945/.
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This thesis presents the results of a series of polarization VLBI experiments on the wellknown variable radio source BL Lacertae (BL Lac). VLBI provides the only means to study radio sources on scales 1 mas, and the sensitivity to polarization allows the magnetic field structures in the source to be investigated, while also providing information about the environment of the source through the phenomenon of Faraday rotation. An attempt is also made to exploit the wealth of VLBI data on this source available in the literature to derive a kinematic model for superluminal components (knots) in its radio jet. Four epochs of multi-frequency monitoring of BL Lac presented here demonstrate the extreme variability of this source in both total intensity and polarized flux density. A number of superluminal knots are identified in the jet with speeds Sc, comparable to those found in previous studies of this source. It has been noted in past observations that the electric vectors of knots in the jets of BL Lac objects tend, in general, to be parallel to the jet direction (implying that the magnetic field is dominated by a transverse component), in contrast to quasars wherein the electric vectors are usually transverse to the jet direction. While such parallel electric vectors are observed in some knots, some evidence is also presented here for knots with a large component of longitudinal magnetic field, possibly as the result of a shear interaction with the ambient medium. These include compact knots which move with superluminal velocities close to the central axis of the jet as well as a more diffuse, slowly moving component, located away from the central axis of the jet and reminiscent of the "sheath" observed in 1055+018 (Attridge et al. 1999). These observations also provide the first examples of non-uniform Faraday rotation to be detected in a BL Lac object. This finding has important consequences for the interpretation of magnetic field alignments in this source, as the rotation measure (RM) applicable to parsec scale knots is shown to differ from the integrated RM of the source derived from VLA observations. The detection of significant quantities of thermal gas in the immediate vicinity of this object is somewhat unexpected given that BL Lac objects are distinguished from quasars by the small equivalent widths of their optical emission lines. The results demonstrate that, in future, observations of Faraday rotation will provide important information on the relative quantities of thermal gas in BL Lac objects and quasars. If BL Lacs are shown not to be depleted in thermal gas on scales of 1 pc, then it also seems likely that they are not depleted in gas on smaller scales and so some other explanation will be required to explain their low line luminosities. A kinematic model derived for the jet in BL Lac suggests that the flux density evolution of knots in the jet is not determined solely by changes in the Doppler factor resulting from changes in the direction of a constant speed jet. Other factors may include expansion losses and/or deceleration of the fluid in the jet. Deceleration from relativistic to nonrelativistic speeds on scales < 2 kpc has been inferred for many FRI radio galaxies (Laing et al. 1999). That significant deceleration on scales of several hundred parsecs in the jet of BL Lac is allowed by the kinematic models is therefore consistent with the FRI/BL Lac unification scheme. The high degree of polarization observed in component S2 in the region where the jet bends is difficult to reconcile with simple shock models, unless the fluid in the jet is moving more slowly than the observed pattern. It is interesting to note that in a similar study of the quasar 3C 345 by Wardle et al. (1994) the flux density evolution was successfully modelled without invoking the additional loss mechanisms required for BL Lac and the jet polarization indicated that for simple shock models the fluid in the jet should be moving at a greater speed than the observed pattern. Future studies will determine whether these represent real differences between the jets in quasars and BL Lac objects in general. This model constrains the jet angle to the line of sight in BL Lac to be less than 15°, and is therefore consistent with the extreme beaming model for this source. Finally, joint EVN/MERLIN observations presented here detect coherent structures on scales larger than those probed in conventional VLBI observations in 3 of 5 BL Lac objects observed. However, not all the flux density predicted to lie on these larger scales is detected. If the remaining "missing" flux is all associated with a diffuse, unbeamed component, then the extended power in all the sources is above the FRI/FRIT limit, providing a complication for currently favoured unification schemes. Having proven the feasibility of such a study with these observations, the experiment is to be repeated at lower frequency and with improved u, v coverage in order better to constrain the nature of these objects on intermediate scales.
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Gattano, César. "Nutation de la Terre et stabilité du repère céleste : apport des observations VLBI." Thesis, Paris Sciences et Lettres (ComUE), 2016. http://www.theses.fr/2016PSLEO017/document.
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Le modèle de nutation de référence (MHB2000) et le repère céleste international (ICRF2) ont été établis en 2000 et en 2009 respectivement. Les observations VLBI s’étant accumulées depuis, ce sont autant de nouvelles données permettant d’affiner la nutation et de mieux caractériser l’instabilité des radiosources constituant le repère céleste. L’allongement des données disponibles révèle des défauts dans la nutation de référence comme des fluctuations dans la position des radiosources. Il importe donc de réajuster les termes de nutation, tout en évaluant l’impact de l’instabilité du repère de référence céleste sur celle-ci. Dans un premier temps, nous déterminons des corrections significatives des termes de nutation du modèle MHB2000 jusqu'à 50 micro-seconde d'arc sur la base des séries opérationnelles de l'IVS. Nous en évaluons aussi l'incertitude. Notre intérêt s’est focalisé sur la nutation libre du noyau et un terme spectralement proche, le terme annuel rétrograde. Nous tentons de caractériser leur variabilité multi-annuelle, en amplitude, phase et fréquence, malgré la corrélation qui les affecte.Dans un second temps, nous avons étudié la stabilité du repère céleste par une étude approfondie des sources susceptibles de définir ce repère. De telles sources sont sélectionnées par analyse de variance d'Allan de leurs déplacements astrométriques. Cette analyse donne la « couleur » du bruit aléatoire qui affecte leurs positions en fonction des différentes échelles de temps correspondant à 30 ans d’observations VLBI. Plus précisément, nous sélectionnons les radiosources présentant un bruit blanc. Il en résulte que sur 123 sources parmi celles choisies pour l'ICRF2 en 2009, seules 40 respectent ce critère de stabilité sur la période 1985-2016. Ce nombre est insuffisant pour construire un repère céleste stable et des compromis sont nécessaires. Enfin, la propagation des erreurs des positions des radiosources sur la nutation a été caractérisée, expliquant le bruit anormalement grands de certaines séries de nutation diffusées par l'IVSThe reference nutation model (MHB2000) and the International Celestial Reference Frame (ICRF2) was established in 2000 and 2009 respectively. VLBI observations are being accumulated since and these are all new data to refine the nutation and better characterize the instability of radio sources constituting the celestial frame. The lengthening of the available data reveals defects in the reference wobble and fluctuations in the position of radio sources.It is important to adjust the nutation terms while assessing the impact of the instability of the celestial reference frame on it. Initially, we obtain significant corrections in terms of nutation model MHB2000 up to 50 micro-seconds of degree on the basis of IVS operational nutation time series. We also evaluate their uncertainty. Our interest is focused on the the free core nutation and a term closed in frequency, the retrograde annual term. We try to characterize the multi-annual variability, on amplitude, phase and period, despite correlation that affects both terms.Secondly, we study the stability of the celestial reference frame by a comprehensive study of the potential sources to define the frame. Such defining sources are selected by analysis using Allan variance of their astrometric displacements. This analysis gives the "color" of the random noise that affects positions based on different time scales corresponding to 30 years of VLBI observations. Specifically, we select radio sources with white noise. It follows that on 123 sources from those chosen for ICRF2 in 2009, only 40 meet this criterion of stability over the period 1985-2016. This number is too small to build a new celestial reference frame and compromises are necessary. Finally, errors propagation of radio position on nutation has been characterized, explaining the noise unusually large of some nutation time series diffused by the IVS
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Yao, Kunliang. "Estimation de la nutation de la Terre par les techniques VLBI et GPS." Paris 6, 2013. http://www.theses.fr/2013PA066206.
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Le but de cette thèse est d’utiliser indépendamment les observations obtenues par les techniques VLBI et GPS, pour déterminer les nutations de l’axe de la Terre à toutes les fréquences avec la meilleure précision possible. Après avoir présenté les systèmes de référence, les échelles de temps, les paramètres et les modèles, le manuscrit décrit la méthodologie du travail de cette thèse, l’analyse des observations et les résultats obtenus. Dans le cas du VLBI, on a étudié les effets du système de référence et du modèle de précession-nutation afin d’optimiser la détermination de la précession-nutation. On a analysé 5069 observations couvrant une période de plus de trente ans, puis estimé des corrections aux amplitudes des termes de nutation. Dans le cas du GPS, on a d’abord estimé le mouvement du pôle et les corrections de nutation en utilisant comme a priori la solution C04 de l’IERS. Puis, un nouveau logiciel a été développé en environnement Matlab pour estimer les dérivées temporelles de la nutation. L’intégration numérique d’orbites des satellites, a été effectuée dans un système de référence défini par la position du pôle et de l’origine sur l’équateur au début de l’arc, ce qui permet de s’affranchir de l’influence des valeurs a priori de précession-nutation. En analysant 315 360 observations effectuées toutes les 300 s par environ 110 stations, les dérivées temporelles des coordonnées du pôle céleste sont déterminées toute les 6 h pendant 3 ans à partir du 1er janvier 2009. Puis, les corrections de nutation de périodes inférieures à 20 jours sont déduites, pour la première fois par la technique GPS seule, avec une précision de l’ordre de 10 µasThe purpose of this thesis is to use independently observations obtained by VLBI and GPS techniques, to determine the nutation of the Earth’s rotation axis at all frequencies with the best possible accuracy. After presenting the reference systems, timescales, parameters and models, the manuscript describes the methodology of this thesis, the analysis of the observations and the results. In the case of VLBI, we have studied the effects of the reference system and the precession-nutation model in order to optimize the precession-nutation determination; we have analyzed 5069 observations covering a period of more than 30 years and then estimated corrections to the amplitudes of the nutation terms. In the case of GPS, we have firstly estimated the polar motion and nutation corrections using the IERS C04 solution as a priori. Then, a new software has been developed in Matlab to estimate the time derivative of the nutation. The numerical integration of satellite orbit is computed in a reference system defined by the position of the pole and the origin on the equator at the beginning of the arc, which helps to minimize the influence of precession-nutation a priori values. Analyzing 315 360 observations obtained by about 110 stations every 300 s, we have determined the time derivatives of the coordinates of the celestial pole every 6 h for 3 years from 1 January 2009. Then, we have derived the corrections to nutation terms with periods shorter than 20 days, for the first time by the GPS technique only, with a precision of the order of 10 μas
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Nurul, Huda Ibnu. "Etude des propriétés rhéologiques globales de la Terre à l'aune des observations VLBI." Thesis, Paris Sciences et Lettres (ComUE), 2019. http://www.theses.fr/2019PSLEO007.
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Depuis le début du vingtième siècle, l’observation des variations de rotation de la Terre par les techniques astro-géodésiques permet de dévoiler les propriétés rhéologiques globales de la Terre. En particulier, les paramètres des modes d'oscillation libre de l'axe de rotation, qui sont aussi des paramètres de résonance - reflètent les défauts d'élasticité de la Terre solide, la réponse de l’océan au forçage externe et les propriétés de la graine fluide, éventuellement du noyau solide. On peut obtenir de meilleures contraintes sur ces paramètres de résonance en confrontant le mouvement terrestre du pôle de rotation observé (mouvement du pôle) – et aussi la nutation en tant que mouvement du pôle en rétrograde diurne - à l’excitation modélisée qui le produit. Les propriétés rhéologiques de la Terre sont d'autant mieux déterminées que le mouvement du pôle (y compris dans la bande diurne) est observé précisément et l’excitation modélisée correctement. A l'heure actuelle, si la précision du mouvement du pôle est légèrement meilleure au delà de 2 jours, la meilleure reconstitution du forçage concerne la bande de nutation (la bande rétrograde diurne), car il est procède alors des marées gravimétriques luni-solaires, lesquelles sont modélisées avec une grande précision). C'est pourquoi notre travail est d'abord consacré à l'ajustement direct des composantes luni-solaires périodiques de la nutation à partir des retards du VLBI, puis à l'estimation des paramètres de résonances dans la fonction de transfert entre ces termes de nutation observés et ceux modélisés pour une terre rigide, lesquels reflètent le forçage luni-solaire. Les résultats obtenus confirme notamment le raccourcissement d’environ 40-50 jours de la période de résonance du mouvement du pôle dans la bande rétrograde diurne. Nous montrons comment cette diminution s'explique par la réponse dynamique des océans à la marée du pôle, laquelle est étudiée à l'aune des modèles de marées océaniques diurnes . Nous prédisons également un changement des paramètres de résonance du mouvement du pôle à proximité de la fréquence de résonance de la nutation libre du noyau; cette modification semble confirmée par la détermination de ces paramètres à partir des termes de nutation. Notre étude des paramètres de résonance s'étend au delà de la bande de nutation. D'une part, nous réexaminons l’estimation des paramètres de résonance du mouvement du pôle dans la bande saisonnière, dominée par l’oscillation de Chandler, en utilisant les derniers modèles hydro-atmosphériques. D'autre part, nous étudions la résonance du mouvement du pôle à la bande prograde diurne, où l’excitation est dominée par la marée océanique. Nous obtenons une période de résonance de l’ordre de 393 jours, corroboré par notre modélisation fondée sur les modèles de marée océanique. L'ensemble des résultats obtenus permet d'apporter des contraintes sur la dépendance en fréquence du nombre de Love et du nombre de Love océanique caractérisant respectivement la réponse de la Terre solide et des océans à un potentiel externe de degré 2Since the beginning of the 20th century, the observation of the Earth rotation variations through astro-geodetic techniques enables to investigate the global rheological properties of the Earth, in particular, the resonance parameters of the free rotation modes reflect the solid Earth anelasticity, the ocean response to an external forcing, and the properties of the fluid inner core, eventually of the solid inner core. Better constraints on these resonance parameters can be obtained by confronting the observed terrestrial motion of the rotation pole (the so-called polar motion) - including nutation as a retrograde diurnal polar motion - to the modelled excitation producing it. The more precise the modelled excitation and the observed polar motion are, the better the Earth rheological properties will be determined. For now, the best precision are reached in the nutation band. So, our work is first dedicated to a direct adjustment of the nutation components from VLBI delays, then the adjustment of the resonance parameters in the transfer function between the observed nutation terms and the corresponding rigid nutation terms that reflects the luni-solar forcing. The obtained resonance parameters confirms in particular the shortening of the polar motion resonance period of about 40 - 50 day in the retrograde diurnal band. Then, we show that the dynamical behaviour of the oceans in the diurnal band is mostly responsible for that. We also predicted a supplementary change of the resonance parameters in the vicinity of the free core nutation resonance, as expected from the solid Earth response, and confirmed by the adjustment of these parameters through the nutation terms. In addition to the nutation band, we revisit the estimation of the polar motion resonance parameters in the seasonal band, dominated by the Chandler wobble, in light of the most recent global circulation models of the hydro-atmospheric layers. Finally, we extend the investigation of polar motion resonance to the prograde diurnal polar motion, where the excitations mostly result from the ocean tides. We obtain a resonance period of about 393 days, and confirmed by our prediction based on the ocean tidal models. These results allow us to impose constraints on the frequency dependence of the body Love number and the Love number oceanic, characterizing respectively the response of the solid Earth and the oceans to an external potential of degree 2
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Bouffet, Romuald. "Evolution de la structure VLBI des sources de l'ICRF : lien entre astrométrie et astrophysique." Thesis, Bordeaux, 2015. http://www.theses.fr/2015BORD0083/document.
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Les Noyaux Actifs de Galaxies (AGN) se situent au centre de galaxies extrêmement lointainesdont la luminosité provient de l’interaction d’un trou noir central supermassif et d’undisque d’accrétion. Il en résulte l’éjection à des vitesses relativistes de jets de matière collimatés.L’interférométrie à très longue base (VLBI) permet, grâce aux très grandes résolutionsatteintes, d’observer finement la structure de ces jets et de déterminer très précisément laposition astrométrique des objets. En raison de leur distance, les AGN ne présentent pas demouvements propres, ce qui les rend idéaux pour la construction de systèmes de référenceultra-précis et très stables.Des instabilités en position de quelques centaines de microsecondes d’arc, généralementimputées aux variations de la structure des jets, sont toutefois souvent observées sur des échellesde temps de quelques mois à quelques années. Le travail présenté ici étudie le lien entre les deuxphénomènes de façon statistique. Sur la base d’observations VLBI régulières conduites entre1994 et 2003, nous comparons l’évolution de la position astrométrique et de la structure des jetspour un échantillon de 68 AGN sur une période de 10 ans. Les résultats de l’étude indiquent quela corrélation entre les deux phénomènes existe mais n’est pas aussi forte qu’attendue. Le travailest complété par une simulation des effets causés sur la trajectoire des jets par la précessiondu disque d’accrétion ainsi que par la présence d’un système binaire de trous noirs. Appliquéeau cas de la source 1308+326, l’étude montre que l’amplitude de ces effets est compatible avecles oscillations de la trajectoire observées en VLBIActive Galactic Nuclei (AGN) are located in the center of extremely distant and bright galaxies. Their luminosity comes from the interaction between a super-massive central blackhole and an accretion disk, producing a relativistic collimated jet of matter. Thanks to the extremely high resolution achieved by Very Long Base line Interferometry (VLBI), the jet structure may be studied in detail, while the astrometric position of the AGN is determined with ahigh accuracy. Because of their location at cosmological distances, no proper motions are detected for those objects, making them ideal fiducial points for building highly-precise celestial reference frames.Instabilities up to a few hundreds of micro arc seconds are yet often observed in astrometricpositions on time scales from months to years. This is generally thought to be caused by theevolution of source structure. The study presented here investigates the correlation between the two phenomena on a statistical basis. Based on regular VLBI observations conducted between1994 and 2003, astrometric position variations and source structure evolution are compared fora sample of 68 AGN over a period of 10 years. The results indicate that a correlation between the two phenomena does exist but it is not as strong as expected. Additionally, a simulation of the effects caused by the precession of the accretion disc and the potential presence of abinary black hole in the center of the AGN is presented. Applied to the source 1308+326, the simulation shows that the magnitude of the effects is consistent with the oscillations of the jet trajectory observed on VLBI scale
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Middelberg, Enno. "Gas around active galactic nuclei and new phase calibration strategies for high-frequency VLBI." [S.l.] : [s.n.], 2004. http://deposit.ddb.de/cgi-bin/dokserv?idn=971786852.
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McAlpine, Kim. "A VLBI study of OH masers in a proto-planetary nebula OH 0.9+1.3." Thesis, Rhodes University, 2008. http://hdl.handle.net/10962/d1005285.
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This thesis reports the calibration, imaging and analysis of one epoch of VLBA observations of the 1612 MHz OH maser emission from the protoplanetary nebula OH 0.9+1.3. These are the first polarisation VLBI observations of this source and the spatial morphology of the OH emission is resolved on this scale. Proto-planetary nebulae represent the transition phase in the evolution of stars between the asymptotic giant branch (AGB) phase and their emergence as planetary nebulae. A long-standing astronomical question is how the predominantly spherical circumstellar envelopes of AGB stars evolve into the bipolar and axisymmetric structures that are commonly observed in planetary nebula. Proto-planetary nebulae offer a unique opportunity to study this transformation process. The high-resolution VLBI maps produced in this thesis were used to investigate the morphology and kinematics of OH 0.9+1.3 with a view to gaining insight into the development of asymmetries in the circumstellar material. The OH maser emission of OH 0.9+1.3 has a double-peaked profile with one peak blue-shifted and the other red-shifted with respect to the stellar velocity. The total intensity maser maps demonstrate a considerable degree of asymmetry with the blue- and red-shifted emission located in spatially distinct regions of the envelope. The blue-shifted emission is distributed preferentially along an axis at a projected position angle of » 135± ( North through East). The morphology of this source is not consistent with the standard symmetric thin-shell model and an attempt to fit the traditional OH/IR kinematic model of a simple expanding shell to the maser components was found to be unsatisfactory. No definitive evidence of a bipolar outflow was observed either. The high degree of asymmetry observed in the source is consistent with its status as a proto-planetary nebula. The source was imaged in all four Stokes parameters and the fractional linear and circular polarisations of the maser components were derived from the Stokes parameter maps. In all except one of the components the total fractional polarisation was found to be low ( < 15%). The mean fractional linear and circular polarisation were calculated to be 5.54% and 7.11% respectively. The absence of an identifiable Zeeman pair in the Stokes V map prohibited the estimation of the magnetic field in the circumstellar envelope of this source.
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Rauch, Christoph [Verfasser], and Andreas [Gutachter] Eckart. "High frequency VLBI observations of galactic and extragalactic sources / Christoph Rauch. Gutachter: Andreas Eckart." Köln : Universitäts- und Stadtbibliothek Köln, 2016. http://d-nb.info/1110012446/34.
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Artz, Thomas [Verfasser]. "Determination of Sub-daily Earth Rotation Parameters from VLBI Observations / Thomas Artz. Landwirtschaftliche Fakultät." Bonn : Universitäts- und Landesbibliothek Bonn, 2011. http://d-nb.info/1017217548/34.
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Pradel, Nicolas. "Etude des objets compacts symétriques en VLBI : imagerie et astrométrie par référence de phase." Toulouse 3, 2005. http://www.theses.fr/2005TOU30242.
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Ce travail de thèse présente l'étude des objets compacts symétriques (CSO), une catégorie bien précise de noyaux actifs de galaxies présentant à très haute résolution angulaire (mas) une structure symétrique atypique, réalisée en interférométrie à très longue base (VLBI). L'étude de la précision astrométrique de la technique d'astrométrie par référence de phase montre que la précision de cette technique est d'environ 50 µas. L'ensemble des CSO sont ensuite décrits. L'étude des CSO a porté sur deux voix : une étude VLBI classique sur une longue base de temps et une étude de 8 CSO par référence de phase. L'étude classique a porté sur le CSO OQ208 et a permis de mettre en évidence des mouvements apparents avec une précision de 10 µas/an. L'étude par référence de phase a permis d'imager avec une résolution angulaire proche du mas les 8 CSO observés et de permettre d'effectuer l'astrométrie de chaque composante des CSO avec une précision de 20 à 30 µasThis study is about compact symetric objects (CSO). These objects are a specific type of active galactic nuclei only observed at a very accurate angular resolution (milliarsecond). This resolution is only obtained with the very long baseline interferometry (VLBI). CSO present a specific geometry with symetric jets on this scale. A presentation of the forty known CSO is included in this study. A preliminary study of an astrometric VLBI technique (phase-referencing) used in this study showed that this technique is accurate at a 50 microarcseconds level. In the first part of the study, classical VLBI observations of the CSO OQ208 showed apparent motions of the components with an accuracy of 10 µas/yr. In the second part of the study, phase referencing observation of 8 CSO produced maps with an 1 mas angular resolution and the accuracy of the phase-referencing astrometry of all components of the 8 CSO was about 20 to 30 µas
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Vennebusch, Markus [Verfasser]. "Singular Value Decomposition and Cluster Analysis as Regression Diagnostics Tools in Geodetic VLBI / Markus Vennebusch." Bonn : Universitäts- und Landesbibliothek Bonn, 2019. http://d-nb.info/1197798692/34.
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Roy, Alessandra [Verfasser]. "Effects on the Geodetic-VLBI Observables Due to Polarization Leakage in the Receivers / Alessandra Roy." Bonn : Universitäts- und Landesbibliothek Bonn, 2019. http://d-nb.info/1201819008/34.
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Leek, Judith [Verfasser]. "The application of impact factors to scheduling VLBI Intensive sessions with twin telescopes / Judith Leek." Bonn : Universitäts- und Landesbibliothek Bonn, 2015. http://d-nb.info/1077269110/34.
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Soja, Benedikt [Verfasser]. "Anwendung eines Kalman-Filters in der Auswertung von VLBI-Daten / Benedikt Soja ; Deutsches GeoForschungsZentrum GFZ." Potsdam : Deutsches GeoForschungsZentrum GFZ, 2016. http://d-nb.info/1107808464/34.
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Böckmann, Sarah [Verfasser]. "Robust determination of station positions and Earth orientation parameters by VLBI intra-technique combination / Sarah Böckmann." Bonn : Universitäts- und Landesbibliothek Bonn, 2020. http://d-nb.info/1202921108/34.
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Mittal, Rupal. "A VLBI investigation of the discrepant image flux density ratio in the gravitational lens JVAS B0218+357." [S.l.] : [s.n.], 2006. http://deposit.ddb.de/cgi-bin/dokserv?idn=980749530.
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Porter, David Gareth Jon. "Blazar jets imaged with Very Long Baseline Interferometry (VLBI) - Kinematics of helical trajectories in 3C273 and 3C345." Thesis, Cardiff University, 2009. http://orca.cf.ac.uk/54940/.
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In this work a large collection of VLBI (Very Long Baseline Interferometry) maps are presented based upon data from the VLBA (Very Long Baseline Array) for 29 sources. The maps are the result of three epochs of imaging. A kinematic study was undertaken with a view to understanding some interesting structure seen in the quasars 3C273 and 3C345. Fortunately, these two sources present a large number of components and it was possible to perform model fitting to pinpoint their positions. In addition, model fitting was performed upon all of the sources that were successfully imaged. The aim of this was to eventually derive component velocities for as many blazars studied as possible. The outcomes of the work are presented in tables in the main thesis. In order to further understand the 3C273 and 3C345 jet structures seen a physical model was developed with A. Papageorgiou. With this kinematic model it was possible to trace out a variety of helical jet structures. The free parameters are the injection velocity for new components, the period of jet precession, the viewing angle and the jet half angle. Using the Levenberg- Marquardt algorithm (references given below) it was possible to fit jet trajectories to my model fitted component data. The algorithm produced quantitative output for some of the free parameters in my physical model. Therefore I report average jet half angles for 3C273 and 3C345 of 2.968 0.153 degrees and 2.519 0.573 degrees respectively. In ad dition, I find the precessional periods of 3C273 and 3C345 to be 71.161 19.066 years and 48.478 3.385 years respectively.
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Cooprider, Kirstin Marie. "OH Megamasers in Merging Galaxies: A Multi-Frequency Study of IIZw096." BYU ScholarsArchive, 2010. https://scholarsarchive.byu.edu/etd/2149.
Full textAbstract:
OH Megamasers (OHMs) generally appear in luminous infrared regions i.e. merging galaxies. In this study we assume that OHMs may not be represented by their association with star formation solely, because of the possibility of a compact AGN source in the merging galaxies. In fact, previously classified starburst galaxies where OHMs are found are now optically observed as AGN. OHMs may also serve as a reasonable criterion for the evolutionary stage of the merger. This project focuses on observations from a multi-frequency analysis of merging regions with known OHMs. Optically, Hα and BVRI filters provided an environmental perspective of the "masing" components. In the radio, 18-cm data was used to determine the structure and position of the OHM. We studied the source IIZw096 and compared our results with two familiar OHM sources. We were able to look at each source at high radio resolutions and compare the structure and classification of each.
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Boyce, Edward R. "The Extragalactic Lens VLBI Imaging Survey (ELVIS) investigating galaxy cores and black holes with gravitational lens central images /." Thesis, [Boston, Mass.] : Massachusetts Institute of Technology, 2006. http://69.63.217.22/elibsql44N_10017D_ocuments/Boycet_hesis-627.pdf.
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Rampadarath, Hayden. "Applications of high-frequency resolution, wide-field VLBI: observations of nearby star-forming galaxies & habitable exoplanetary candidates." Thesis, Curtin University, 2014. http://hdl.handle.net/20.500.11937/475.
Full textAbstract:
In this thesis, I investigate the radio source populations and properties of two nearby star-forming galaxies. In addition, I present an independent method to estimate the supernova and massive star-formation rates of star-forming galaxies. This study makes use of a new technique that allows multiple astronomical objects to be simultaneously observed via very high resolution radio interferometry. Furthermore, using this technique I develop a new method to search for evidence of extraterrestrial intelligence from habitable planetary candidates.
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Montaguti, Simonetta <1975>. "Studio delle deformazioni gravitazionali delle antenne VLBI di Medicina e di Noto con metodologiche Laser Scanning, Topografiche e FEM." Doctoral thesis, Alma Mater Studiorum - Università di Bologna, 2008. http://amsdottorato.unibo.it/1176/.
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Baczko, Anne-Kathrin [Verfasser], Andreas [Gutachter] Eckart, and J. Anton [Gutachter] Zensus. "Multi-Frequency VLBI Observations of the Active Galaxy NGC 1052 / Anne-Kathrin Baczko ; Gutachter: Andreas Eckart, J. Anton Zensus." Köln : Universitäts- und Stadtbibliothek Köln, 2019. http://d-nb.info/1216241082/34.
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West, Craig James, and cwest@astro swin edu au. "Development of disk-based baseband recorders and software correlators for radio astronomy." Swinburne University of Technology. Centre for Astrophysics and Supercomputing, 2004. http://adt.lib.swin.edu.au./public/adt-VSWT20050804.143155.
Full textAbstract:
This thesis details work undertaken in the field of radio astronomy instrumentation. Specific components of the data collection and processing systems used by radio astronomers have been implemented using non-traditional approaches. Traditionally, the correlation of radio astronomy data has taken place on dedicated,
specific hardware. This thesis deals with the implementation of equivalent correlators
using software running on generic clusters of personal computers - the software approach to radio astronomy. Toward this end a system has been developed that records the raw telescope output onto computer hard drives, allowing
easy access to the data on cluster supercomputers. Part of this thesis describes the design, construction, testing and utilisation of these data recording systems. The correlator software used to process these data
on supercomputers is also fully described, including extensive tests of the software
and a detailed comparison between its output and the output of an existing hardware correlator. The software correlator is shown to produce output that agrees extremely well with the hardware correlator, verifying its accuracy and performance. Finally, results of on-going scientific investigations that use the software correlators described in this thesis are outlined, illustrating the flexibility and usefulness of the software approach to radio astronomy.
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Munghemezulu, Cilence. "Determination of geodetic velocity field parameters for the African tectonic plate using the technique of Global Navigation Satellite Systems." Diss., University of Pretoria, 2013. http://hdl.handle.net/2263/40360.
Full textAbstract:
Space geodesy is one of the disciplines that contributes uniquely to the global society; its applications have grown to such an extent that system Earth is better understood today. The current accuracy of the Global Navigation Satellite Systems (GNSS) technique is below centimetre level and this allows very accurate determination of velocity field parameters. This study focused on utilizing GNSS to determine the inter-continental plate velocity field for Africa in support of the African Geodetic Reference Frame (AFREF). Data spanning 12.4 years were processed in the International Terrestrial Reference Frame (ITRF2008) using GAMIT/GLOBK 10.4 (developed at the Massachusetts Institute of Technology). Primarily, processing of data focused on International GNSS Service (IGS) stations with a few non-IGS stations (which are of geodetic quality) included, such as Hamburg (HAMB) and Matjiesfontein (MATJ). The same data set was analysed using the Combination and Analyses of Terrestrial Reference Frame (CATREF) software developed at Institut National de l’Information Géographique et Forestière (IGN). Validation of the results was achieved through comparison of the velocity solution from this study with a solution obtained from a core of IGS GNSS stations processed by the Jet Propulsion Laboratory (JPL). No significant differences were evident between the GAMIT/GLOBK 10.4, CATREF and JPL solutions. The results from the Matjiesfontein station indicated that the proposed Matjiesfontein Observatory site shows no significant vertical or horizontal local motion; this information is valuable in that there is no obvious local site instability. The velocity field as derived by GNSS displays no unexpected deviations and supports current understanding of the motion of the Nubian, Somalian and Arabian plates. Furthermore, the comparison of the velocity vectors derived from the IGS station HRAO, Satellite Laser Ranging (SLR) MOBLAS-6 station and 26 m Very Long Baseline Interferometry (VLBI) telescope, which are collocated at the Hartebeesthoek Radio Astronomy Observatory (HartRAO) indicated good agreement and both techniques exhibit no significant vertical motion. This study also contributed to the first computation of the AFREF solution. It is envisaged that as more stations are added to the sparsely distributed current network, more accurate results and better tectonic models can be derived. The availability of station velocities will facilitate adjustments within the AFREF.Dissertation (MSc)--University of Pretoria, 2013.
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Geography, Geoinformatics and Meteorology
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