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Dissertations / Theses on the topic 'Astrophysics'
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1
Baring, Matthew Geoffrey. "Pair plasmas in astrophysics." Thesis, University of Cambridge, 1988. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.279144.
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Price, Daniel James. "Magnetic fields in astrophysics." Thesis, University of Cambridge, 2004. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.616258.
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Reid, Giles Adrian. "Neutrino Oscillations in Astrophysics." Thesis, University of Canterbury. Physics and Astronomy, 2010. http://hdl.handle.net/10092/4935.
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A survey of the theory of neutrino oscillations in dense matter and neutrino
backgrounds is presented. We discuss collective neutrino systems using the gyroscopic
pendulum analogy and describe the motion that results from self-induced
parametric resonances. The effects of dense matter on the
flavour oscillations
of neutrinos are also detailed. This theory is applied to the case of continuous
supernova neutrino spectra and explanations of the spectral swapping behaviour
seen in numerical studies are summarized.
The results of numerical simulations of supernova oscillations in turbulent
supernova backgrounds are presented and discussed. We study the motion of two
example supernova neutrino spectra and examine the differences in the dynamics
and
flavour evolution that results from adding turbulent
fluctuations to the
supernova matter background. We also investigate the effect that
fluctuations
in the neutrino density can have on the oscillation behaviour. We find that in
general the final neutrino spectra emerging from the inner supernova regions
are quite robust to
fluctuations in the backgrounds in our model, while the
intermediate dynamics can be very strongly altered. Some significant changes
in the final spectra are also found to occur when the neutrino background density
fluctuations are large.
We give a detailed review of the resonant matter effects that determine the
survival probabilities of atmospheric muon neutrinos. The differences between
various Earth density models are described, and these models are then used to
predict the flux of muon-type neutrino events in the Deep Core extension to the
IceCube detector. We use recent results from the detector collaboration and
build on previous work which considered the sensitivity of the detector to the
mass hierarchy, and show that uncertainties in the Earth's density can have a
significant influence on the event rates.
4
Li, Chao Thorne Kip S. Thorne Kip S. Chen Yanbei Cooray Asantha. "Topics in theoretical astrophysics /." Diss., Pasadena, Calif. : California Institute of Technology, 2009. http://resolver.caltech.edu/CaltechETD:etd-10142008-155140.
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Noble, Patrick. "Stochastic processes in Astrophysics." Thesis, The University of Sydney, 2013. http://hdl.handle.net/2123/10013.
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This thesis makes two contributions to the solar literature. The first is the development and application of a formal statistical framework for describing short-term (daily) variation in the level of magnetic activity on the Sun. Modelling changes on this time-scale is important because rapid developments of magnetic structures on the sun have important consequences for the space weather experienced on Earth (Committee On The Societal & Economic Impacts Of Severe Space Weather Events, 2008). The second concerns how energetic particles released from the Sun travel through the solar wind. The contribution from this thesis is to resolve a mathematical discrepancy in theoretical models for the transport of charged particles.
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Gao, Shu. "Energy transport mechanisms in astrophysics." Bachelor's thesis, Alma Mater Studiorum - Università di Bologna, 2021. http://amslaurea.unibo.it/22758/.
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To enable humans to observe, understand, and analyze celestial bodies, celestial bodies must emit energy from themselves and transmit them to the earth through free space, such as light, gravitational waves, or other things. Therefore, humans can analyze the content of celestial bodies based on the obtained information to understand the universe. This process can be called the energy transport process. He has exerted an important role in the communication between the earth and the universe, and we will discuss the main energy transport mechanisms in the stellar atmosphere in detail.
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Oppermann, Niels. "Signal inference in Galactic astrophysics." Diss., lmu, 2013. http://nbn-resolving.de/urn:nbn:de:bvb:19-154689.
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Haddock, D. J. "Stellar masses and laboratory astrophysics." Thesis, University of Oxford, 1987. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.379945.
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Johnson, Colin Terence. "Fine structure transitions in astrophysics." Thesis, Queen's University Belfast, 1987. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.317096.
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Nicholson, Kerry Louise. "The astrophysics of LINER galaxies." Thesis, University College London (University of London), 1998. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.298427.
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AN, JIN HYEOK. "Astrophysics from binary-lens microlensing." The Ohio State University, 2002. http://rave.ohiolink.edu/etdc/view?acc_num=osu1029254036.
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An, Jin Hyeong. "Astrophysics from binary-lens microlensing." Columbus, Ohio : Ohio State University, 2002. http://rave.ohiolink.edu/etdc/view?acc%5Fnum=osu1029254036.
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Thesis (Ph. D.)--Ohio State University, 2002.<br>Title from first page of PDF file. Document formatted into pages; contains xxix, 171 p., also contains graphics. Includes abstract and vita. Advisor: Andrew P. Gould. Includes bibliographical references (p. 153-157).
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An, Jin Hyeok. "Astrophysics from binary-lens microlensing." Columbus, Ohio : Ohio State University, 2002. http://rave.ohiolink.edu/etdc/view?acc%5Fnum=osu1029254036.
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Thesis (Ph. D.)--Ohio State University, 2002.<br>Title from first page of PDF file. Document formatted into pages; contains xxix, 171 p. Includes abstract and vita. Advisor: Andrew P. Gould, Dept. of Astronomy. Includes bibliographical references (p. 153-157).
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MASCARETTI, CARLO. "Open problems in neutrino astrophysics." Doctoral thesis, Gran Sasso Science Institute, 2020. http://hdl.handle.net/20.500.12571/15041.
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In my thesis I addressed a selection of the open problems in neutrino astrophysics. These open problems regard the fundamental properties of neutrinos, as well as their role as an astrophysical messenger, on which we focussed our efforts.
In the context of solar neutrinos, the main contribution of my work concerns the luminosity constraint, a strict relation between the solar luminosity in photons and in neutrinos. Such relation is based on few assumptions, among which the stationarity of the Sun, and that 4He is the only accumulating nuclear specie. We reformulated the derivation of the luminosity constraint in a simpler and clearer way, and generalised it by including the contribution of other accumulating elements and of the variation of the solar gravitational potential. The resulting updated luminosity constraint is more general, accurate, and powerful at connecting pp and CNO neutrinos, which are of paramount interest for solar neutrino detectors and for solving the solar metallicity problem.
Atmospheric neutrinos have been studied both as a product of cosmic rays and as a background to the cosmic neutrino analyses. Two primary cosmic-ray flux models have been defined by fitting the data by ARGO-YBJ and KASCADE-Grande, which measure the knee of p+He at, respectively, ~700 TeV and ~5 PeV. Two atmospheric neutrino fluxes have been computed using such primary models as input, and have been compared to the available data to discriminate the knee position. Unfortunately, the uncertainties on the data only allow to slightly favour the KASCADE-Grande knee. The potential of atmospheric neutrinos as a proxy for cosmic-ray physics was discussed.
The previous work pushed me to assess the possibility to detect prompt neutrinos as well as to investigate their role in cosmic neutrino analyses. A cosmic neutrino flux model was built by combining the expectation on the neutrino spectrum produced in pp collisions in starburst Galaxies with the experimental result of the through-going muons analysis. Comparing the components of the neutrino spectra, it was clear that prompt neutrinos cannot be extracted from samples rich in νμ-induced events. The cascades dataset resulted to be the most promising, and the yearly rates of cascade events in IceCube due to all components (and flavours) of the neutrino spectrum were computed for 1 TeV ≤ Eν ≤ 10 PeV. Prompt neutrinos resulted to contribute to less than 3% of the total rate of shower-like events. The detection of prompt neutrinos could be feasible by studying inclined cascades with a higher energy threshold, so as to reduce the conventional background. Finally, it was concluded that the contribution of prompt neutrinos could cause the spectral difference between the fluxes resulting from the HESE and through-going muons analyses.
This spectral tension has been investigated also in my first paper, in which the results of the IceCube analyses have been discussed on the basis of the common set of assumptions used to interpret them. In fact, the spectra resulting from the HESE and through-going muons dataset would not be in tension if the astrophysical signal were anisotropic and distributed differently from an unbroken power law. An alternative two-component astrophysical neutrino spectrum proved to be compatible with all IceCube data, and was used to compute the expected rates of events due to tau neutrinos (double cascades) and Glashow resonances.
Double cascade events were the focus of my second paper, in which standard neutrino oscillations and the through-going muons spectrum were used to obtain the astrophysical flux of all neutrino flavours. These were convolved with the (analytically approximated) effective areas of IceCube, IceCube-gen2, and KM3NeT to compute the rates of double cascade events above 100 TeV. IceCube is predicted to be very close to the first detection of double cascade events, and the recent preliminary data by IceCube agree with such prediction. The non-observation of double cascade events would lead to new physics or to disproving the discovery of cosmic neutrinos.
15
Zemp, Marcel. "Collisionless and collisional dynamics in astrophysics /." Zürich : ETH, 2006. http://e-collection.ethbib.ethz.ch/show?type=diss&nr=16801.
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McGee, P. K. "Optical studies in high-energy astrophysics /." Title page, contents and abstract only, 2001. http://web4.library.adelaide.edu.au/theses/09PH/09phm14485.pdf.
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Mazur, Daniel Paul. "Nonperturbative quantum field theory in astrophysics." Thesis, University of British Columbia, 2012. http://hdl.handle.net/2429/43255.
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The extreme electromagnetic or gravitational fields associated with some astrophysical objects can give rise to macroscopic effects arising from the physics of the quantum vacuum. Therefore, these objects are incredible laboratories for exploring the physics of quantum field theories. In this dissertation, we explore this idea in three astrophysical scenarios.
In the early universe, quantum fluctuations of a scalar field result in the generation of particles, and of the density fluctuations which seed the large- scale structure of the universe. These fluctuations are generated through quantum processes, but are ultimately treated classically. We explore how a quantum-to-classical transition may occur due to non-linear self-interactions of the scalar field. This mechanism is found to be too inefficient to explain classicality, meaning fields which do not become classical because of other mechanisms may maintain some evidence of their quantum origins.
Magnetars are characterized by intense magnetic fields. In these fields, the quantum vacuum becomes a non-linear optical medium because of interactions between light and quantum fluctuations of electron-positron pairs. In addition, there is a plasma surrounding the magnetar which is a dissipative medium. We construct a numerical simulation of electromagnetic waves in this environment which is non-perturbative in the wave amplitudes and background field. This simulation reveals a new class of waves with highly non-linear structure that are stable against shock formation.
The dense nuclear material in a neutron star is expected to be in a type-II superconducting state. In that case, the star’s intense magnetic fields will penetrate the core and crust through a dense lattice of flux tubes. However,
depending on the details of the free energy associated with these flux tubes, the nuclear material may be in a type-I state which completely expels the field. We compute the quantum corrections to the classical energies of these flux tubes by creating a new, massively parallel Monte-Carlo simulation. The quantum contribution tends to make a small contribution which adds to the classical free energy. We also find a non-local interaction energy with a sign that depends on the field profile and spacing between flux tubes.
18
Doherty, Daniel Thomas. "Experimental studies for explosive nuclear astrophysics." Thesis, University of Edinburgh, 2014. http://hdl.handle.net/1842/18022.
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In the ejecta from ONe novae outbursts nuclei up to A~40 are observed. The 30P(p,γ)31S reaction is thought to be the bottleneck for the production of all elements heavier than sulphur. However, due to uncertainties in the properties of key proton-unbound resonances the reaction rate is not well determined. In this thesis work, excited states in 31S were populated via the 28Si(4He,n) light-ion fusion-evaporation reaction and the prompt electromagnetic radiation was then detected with the GAMMASPHERE detector array. This γ-ray spectroscopy study, and comparisons with the stable mirror nucleus 31P, allowed the determination of the 31S level structure below the proton-emission threshold and also of the key proton-unbound states for the 30P(p,γ)31S reaction. In particular, transitions from key, low-spin states were observed for the first time. This new information was then used for the re-evaluation of the 30P(p,γ)31S reaction in the temperature range relevant for ONe novae. The newly calculated rate is higher than previous estimates implying a greater flux of material processed to high-Z elements in novae. Astrophysical X-ray bursts are thought to be a result of thermonuclear explosions on the atmosphere of an accreting neutron star. Between these bursts, energy is thought to be generated by the hot CNO cycles. The 15O(α,γ)19Ne reaction is one reaction that allows breakout from these CNO cycle and into the rp-process to fuel outbursts. The reaction is expected to be dominated by a single 3/2+ resonance at 4.033 MeV in 19Ne, however, limited information is available on this key state. This thesis work reports on a pioneering study of the 20Ne(p,d)19Ne reaction in inverse kinematics performed at the Experimental Storage Ring (ESR) as a means of accessing the 4.033-MeV state in 19Ne. The unique background free, high luminosity conditions of the ESR were utilised for this, the first transfer reaction performed at the ESR. The results of this pioneering test experiment are presented along with suggestions for future measurements at storage ring facilities.
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Tunyasuvunakool, Saran. "Applications of numerical relativity beyond astrophysics." Thesis, University of Cambridge, 2017. https://www.repository.cam.ac.uk/handle/1810/262481.
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Numerical relativity has proven to be a successful and robust tool for non-perturbative studies of gravitational phenomena in the highly dynamical and/or non-linear regime. Perhaps the most prominent achievement in the field is the breakthrough success in simulating the merger of binary black hole systems. Gravitational waveforms resulting from these simulations serve as precise theoretical predictions of general relativity, which can be tested against observational data, such as those recently made by the LIGO experiment. This dissertation explores applications of numerical relativity which lie beyond the realm of astrophysics. One motivation for this comes from the AdS/CFT correspondence, which allows us to study strongly coupled quantum field theories by considering classical gravity with a negative cosmological constant. More concretely, we construct stationary asymptotically anti-de Sitter spacetimes by numerically solving the Einstein equations in a strongly elliptic form, subject to various boundary conditions corresponding to the physical setting of interest. Three applications of this technique are presented here. 1) A toroidal “black ring” in global AdS5, which provides a more complete phase diagram for AdS5 black holes. 2) A black hole on an AdS soliton background, which is dual to a localised ball of deconfined plasma surrounded by confined matter. 3) A rotating horizon extending to the AdS boundary, which allows us to the study the behaviour of the CFT in the presence of a rotating black hole. Outside of AdS/CFT, time-dependent numerical relativity in higher dimensions can also inform inquiries into the mathematical properties of general relativity as a theory of gravity. In particular, long, thin black hole horizons are known to be subject to the Gregory–Laflamme instability, and this is expected to result in an eventual violation of the weak cosmic censorship conjecture. A landmark simulation of the black string confirmed this in the Kaluza–Klein setting, however the generalisation of this setup to asymptotically flat black rings poses new challenges for numerical relativity. Even after a successful simulation, the resulting apparent horizons possess nontrivial geometries which are problematic for existing horizon finding methods. This dissertation also covers aspects of technical development in the GRChombo adaptive mesh refinement code which were necessary for the successful evolution and analysis of a black ring instability.
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Kulikovskiy, Vladimir. "Neutrino astrophysics with the ANTARES telescope." Paris 7, 2014. http://www.theses.fr/2014PA077050.
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Le télescope à neutrinos sous-marin a été construit en 2008 au fond de la mer Méditerranée et est depuis exploité avec succès. Il a la capacité de détecter des particules chargées grâce à la lumière Tcherenkov émise, en utilisant un réseau de 875 photomultiplicateurs. Les neutrinos muoniques de haute énergie produisent des muons préservant globalement la direction et l'énergie des neutrinos d'origine fournissant des informations sur la source astrophysique d'origine. Le but principal du télescope est la recherche de sources cosmiques de neutrinos. Les restes de supernovae, les noyaux actifs de galaxie, les micro-quasars sont autant de candidats pour les sources ponctuelles de neutrinos. Le Centre Galactique, le plan galactique, et les bulles de Fermi sont des candidats pour les sources étendues de neutrinos. La thèse de doctorat présentée s'intéresse au flux de neutrinos potentiellement émis par les deux grandes structures (bulles) récemment détectées par le satellite Fermi. Une recherche de neutrinos issus de ces structures est présentée, qui repose sur les données recueillies à partir de 2008 par le télescope Antares<br>The ANTARES underwater neutrino telescope was successfully build and operating in the Mediterranean Sea. It has a capability to detect charged particles by the Cerenkov light using an array of the 875 photomultipliers. Ultra high energetic muon neutrinos produce muons with almost the same direction and energy as the origin neutrinos so their detection gives information about the astrophysical sources. The main goal of the telescope is the search for the cosmic source of the neutrinos. Supernovae remnants, active galactic nuclei and micro-quasars are candidates for point like neutrino sources, while the Galactic Center, the Galactic Plane and Fermi¬Bubbles are candidates for extended sources. The PhD thesis consists in studying the possible neutrino flux emitted by the two large Fermi Bubbles structures with the Antares telescope, on the basis of the data collected since 2008
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Todd, Elizabeth. "Particle Astrophysics at the Galactic Center." Diss., The University of Arizona, 2011. http://hdl.handle.net/10150/145121.
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The presence of turbulence in astrophysical magnetic fields can have a significant effect on the diffusion of particles and, therefore, should be taken into account when performing simulations involving particle propagation. After reviewing the constructionof the turbulent magnetic field component, we incorporate this feature in two separate projects. In the first, we consider the possible source(s) of hadronic cosmic rays thought to be responsible for the diffuse TeV gamma-ray emission in the vicinity ofthe Galactic center. Assuming a completely turbulent magnetic field with an average strength of 10-100microG, we find that relativistic protons do not travel far enough to produce gamma-rays spatially correlated with the giant molecular clouds, as seen by HESS,when injected into the interstellar medium by a single point source, such as the supermassive black hole Sagittarius A*. Increasing the number of point sources to five does improve the longitudinal extent of the emission but either shows only weak correlation with the molecular gas or highlights the source positions - both pictures areinconsistent with HESS observations. We conclude that protons must be accelerated throughout the Galactic center region via e.g. a second-order Fermi process in order to reproduce the HESS gamma-ray map if the magnetic field there is completely turbulent. Secondly, we examine the possible link between the asymmetric 511keV electron-positron annihilation emission from the inner Galactic disk and hard low mass X-ray binaries (LMXBs). Three different magnetic field configurations were considered: a completely turbulent field, a field in which the turbulent component has equal energy density as the mean component, and a strongly ordered field with little turbulence. Assuming the environment around each LMXB system is the same, we find that the LMXBs alone cannot account for all the positrons necessary to sufficiently fill the region regardless of the particular magnetic field structure chosen. Another transport mechanism (e.g. a galactic wind) in addition to the diffusive motion caused by the magnetic field fluctuations and/or allowing the LMXBs to be embedded in different phases of the interstellar medium is needed for the LMXB picture to remain a viable possibility.
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Almergren, Joachim. "Energy and Rotation in Relativistic Astrophysics." Doctoral thesis, SISSA, 2003. http://hdl.handle.net/20.500.11767/4245.
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de, Naurois Mathieu. "L'astronomie gamma de très haute énergie de H.E.S.S. à CTA. Ouverture d'une nouvelle fenêtre astronomique sur l'Univers non thermique." Habilitation à diriger des recherches, Université Pierre et Marie Curie - Paris VI, 2012. http://tel.archives-ouvertes.fr/tel-00687872.
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Les dix dernières années ont été marquées par l'arrivée à maturité de la technique d'imagerie Cherenkov atmosphérique, ce qui a permis, notamment grâce au réseau de télescopes HESS, l'ouverture d'une nouvelle fenêtre sur l'Univers non thermique. Ce mémoire d'habilitation retrace dix années de recherche en Astronomie Gamma de Très Haute énergie avec HESS puis CTA. Les aspects techniques tels que la conception de l'instrument, son calibrage, la reconstruction des événements et l'analyse de données sont présentées dans une première partie, tandis que la seconde brosse un panorama des grandes découvertes dans ce domaine.
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Zimmermann, Jens. "Statistical Learning in High Energy and Astrophysics." Diss., lmu, 2005. http://nbn-resolving.de/urn:nbn:de:bvb:19-43537.
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Jaroschek, Claus. "Critical Kinetic Plasma Processes In Relativistic Astrophysics." Diss., lmu, 2005. http://nbn-resolving.de/urn:nbn:de:bvb:19-46601.
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Khiar, Benjamin. "Laboratory astrophysics with magnetized laser-produced plasmas." Thesis, Paris 6, 2017. http://www.theses.fr/2017PA066310/document.
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Nous présentons dans ce travail différentes configurations utilisées pour étudier des éxperiences, pertinentes d'un point de vue astrophysique, mettant en jeu des plasma produits par laser ainsi que des champs magnétiques intenses. les outils théoriques et numériques sont d'abord présentés avec la dérivation complète du modèle de magnétohydrodynamique (mhd) résistive à deux températures. nous décrivons aussi les nouveaux modules de physique implémentés au cours de cette thèse. la configuration de base utilisée pour notre travail consiste en une ou plusieurs cibles solides sur lesquelles un laser intense est envoyé dans le but de générer un plasma se propageant dans le vide. on montre que l'ajout d'un champ magnétique de plusieurs dizaines de teslas influence fortement la dynamique de ce plasma et que selon l'orientation initiale du champ, il est possible de générer différentes structures telles que des jets supersonic/superalfvenic ou encore des «crêpes» de plasma. par exemple, les jets ainsi produits sont caractérisés par des régimes tels que des lois d'échelles entre le système du laboratoire et le système astrophysique (jeunes étoiles connues sous le nom de t tauri) sont applicables. un sujet important et inédit traité dans cette thèse concerne la génération de chocs d'accrétion magnétisés en utilisant les jets mentionnés ci-dessus comme flots accrétant sur des cibles solides. nous mettons notamment l'accent, contrairement à la plupart des travaux précédents, sur la structure 3d de ces chocs et els instabilités présentes. pour chaque cas étudié, nous présentons des nouveaux résultats expérimentaux obtenus par notre collaboration sur le laser elfie du luli<br>We present in this work different configurations used as a mean to study astrophysically-relevant (by scaling) experiments using laser-produced plasmas and strong magnetic fields. This work is a contribution to the relatively recent field known as high energy density laboratory astrophysics (hedla). The theoretical and numerical framework used in this this work is first introduced with a detailed derivation of the magnetohydrodynamic (mhd) model for bi-temperature and resistive plasmas. The three-dimensional mhd code gorgon and the new physical modules implemented during this thesis are presented. The basic setup studied here involve one or several solid slabs being used as targets for a joule-class laser. The expanding plasma thus produced is embedded in magnetic fields of strengths up to 40 t. Depending on the orientation of the field relative to the target surface, we show that the resulting plasma dynamic, relatively well described by ideal mhd, is strongly modified by the presence of the field. The first topic treated is related to the production, when the field is perpendicular to the target surface, of super-sonic/alfvenic jets relevant in the context of astrophysical jets observed around young star objects (t tauri stars). When the field is oriented parallel to the surface, we show that the configuration results in the formation of thin unstable plasma slabs. We also studied the possibility to generate magnetized accretion shocks in the laboratory and we detail the 3d structure obtained in this case. Alongise the numerical work, we present for each case mentioned previously, new experimental results obtained by the collaboration on the elfie laser facility (luli)
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Kennedy, Joan Loomis. "Representations of spectra in early British astrophysics." Thesis, University of Oxford, 2000. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.365627.
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Castell, Pedro Ruiz. "Astronomy and astrophysics in Spain (1850-1914)." Thesis, University of Oxford, 2006. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.432115.
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Mumby-Croft, Paul David. "Tactic : A New Detector for Nuclear Astrophysics." Thesis, University of York, 2009. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.507686.
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Ogley, Richard Neil. "The astrophysics of energetic X-ray binaries." Thesis, Open University, 1998. http://oro.open.ac.uk/57881/.
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This thesis is a study of the X-ray binary Cygnus X-3, and related objects, using primarily observations at radio, sub-rom and infrared wavelengths. I find the emission mechanism to be synchrotron in the radio and sub-mm, while the infrared emission is thermal from the hot wind. The upper limit to the synchrotron emission is interpreted as being due to spectral ageing and implies a magnetic field strength of 7 T at a distance of 700 R0 from the centre of the system. The nature of the companion star in Cyg X-3 was investigated by midinfrared observations using the ISO satellite. I find that the spectrum shows a steady decrease and flattening at longer wavelengths which is consist ant with a standard wind emission. Thus the complete quiescent spectrum from Cyg X-3 shows two different emission mechanisms and covers a full 5 decades of frequency. High sensitivity and high time resolution radio photometry, taken during a minor flare period, reveal rapid (10 minute) increases and decreases in intensity which severely constrain the size of plasmons. Brightness temperatures of typically 10 10 K are found. I also consider models for the superluminal expansion and contraction of the Cyg X-3 source, observed on a milli-arcsecond scale by LIewell et al. (1998). The elliptical shape and the superluminal contraction are particularly hard to explain. Models involving photon beams illuminating shells, or propagating photon patterns are the most plausible. A thorough survey for maser emission from these sources has produced strong upper limits and improved our understanding of the circumstellar environment ofCyg X-3. This thesis has probed the radio-jet X-ray binaries, and made significant advances, opening up more questions about the nature of these sources and new areas of research.
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Moreno-Gelos, Quentin. "Non-relativistic collisionless shocks in Laboratory Astrophysics." Thesis, Bordeaux, 2018. http://www.theses.fr/2018BORD0427/document.
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Les chocs sans collision sont omniprésents dans l'Univers, notamment dans les restes de supernova, et sont formés via diverses instabilités plasmas dépendant essentiellement de la vitesse et de la magnétisation des flux de plasmas. La description de tels chocs nécessite une approche cinétique, tant analytique que numérique.Dans cette thèse, nous avons étudié, au travers de simulations Particle-In-Cell (PIC), les processus sous-jacents par lesquels les instabilités rentrent en compétition les unes avec les autres. Nous avons montré que la diminution du rapport des masses entre ions et électrons, souvent utilisée en simulations numériques pour accélérer la dynamique des chocs, peut avoir de fortes conséquences sur le transfert d'énergie entre particules durant la phase non-linéaire des instabilités.Ces dernières, comme l'instabilité acoustique ionique (IAI) amènent sous certaines conditions à la formation de chocs électrostatiques, pouvant donner naissance à la formation de trous dans l'espace des phases, se propageant dans la région aval du choc, et accélérant ce dernier. L'ajout d'un champ magnétique externe conduit à un changement de médiation du choc, pouvant varier entre l'IAI et les ondes magnéto-soniques lente ou rapide en fonction de l'obliquité entre le champ magnétique et la normale au choc. De plus, nous avons montré que l'orientation du champ magnétique permet de choisir entre une dispersion convexe ou concave des ondes plasma conduisant à la création d'ondes précurseurs dans les régions amont ou aval du choc.Ces chocs magnétisés se trouvent être correctement représentés par le modèle magnétohydrodynamique (MHD) tant qu'ils restent laminaire et que leur potentiel dans la région aval n'est pas suffisamment grand pour réfléchir les particules du milieu amont.Nous avons montré que même pour des chocs sous critiques, une fraction d'ions réfléchis, ne pouvant pas être représentés par la MHD, est suffisante à la croissance d'ondes solitaires en amont du choc, conduisant à l’accélération de ce dernier, mais pas à un processus d'auto-reformation comme pour les chocs super critiques.Bien que les échelles spatio-temporelles soient très différentes, les lois d'échelle rendent possible l'étude de tels phénomènes en laboratoire. Nos études numériques ont été faites dans un cadre de type tube à choc pouvant être testé expérimentalement.A ce titre, nous proposons dans cette thèse une expérience sur la création d'îlots magnétiques, formés par l’interaction de plasmas générés par l'irradiation de cibles par laser baignant dans un champ magnétique externe, et conduisant à la formation de tels chocs.Enfin, nous avons démontré expérimentalement et numériquement la formation de chocs électromagnétiques sans collisions par le biais de l'instabilité de Weibel stimulée par l'instabilité de batterie Biermann, conduisant à l'accélération de particules par le mécanisme de Fermi. Ce nouveau type d'expérience pourrait expliquer l'origine du rayonnement cosmique provenant des restes de supernova<br>Collisionless shocks are ubiquitous in the Universe, especially in the supernova remnants, and are formed via various plasma instabilities mainly depending on the speed and magnetization of plasma flows. The description of such shocks requires a kinetic approach, both analytical and numerical.In this thesis, we have studied, through Particle-In-Cell (PIC) simulations, the underlying processes by which instabilities compete with each other.We have shown that the reduction of the ion-to-electron mass ratio, often used in numerical simulations to accelerate the dynamics of shocks, can have strong consequences on the energy transfer between particles during the non-linear phase of instabilities.These instabilities, like the ionic acoustic instability (IAI) lead under certain conditions to the formation of electrostatic shocks, which can give rise to phase space holes formation, propagating in the downstream shock region, and accelerating the shock.The addition of an external magnetic field leads to different shock mediation, which can vary between the IAI to the slow or fast magneto-sonic waves as a function of the obliquity between the magnetic field and the shock normal.Furthermore, we have shown that the orientation of the magnetic field makes it possible to choose between a convex or concave dispersion of the plasma waves leading to the creation of precursor waves in the upstream or downstream shock regions.These magnetized shocks are correctly represented by the magnetohydrodynamic (MHD) model as long as they remain laminar and their potential in the downstream region is not large enough to reflect the particles of the upstream medium.We have shown that even for sub-critical shocks, a fraction of reflected ions, which cannot be modeled by the MHD, is sufficient for the growth of solitary waves upstream of the shock, leading to the acceleration of the latter, but not to a process of 'self-reformation' as for super-critical shocks.Although spatio-temporal scales are very different, scaling laws make possible the study of such phenomena in the laboratory. Our numerical studies have been done in the context of shock tubes that can be experimentally tested.As such, we propose in this thesis an experiment on the creation of magnetic islands, formed by the interaction of plasmas generated by the irradiation of laser targets bathed in an external magnetic field, leading to the formation of such shocks.Finally, we experimentally and numerically demonstrated the formation of collisionless electromagnetic shocks through the Weibel instability stimulated by theBiermann Battery instability, and leading to particle acceleration by the Fermi mechanism.This new type of experiment could explain the origin of cosmic radiation from supernova remnants
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Higson, Edward John. "Bayesian methods and machine learning in astrophysics." Thesis, University of Cambridge, 2019. https://www.repository.cam.ac.uk/handle/1810/289728.
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This thesis is concerned with methods for Bayesian inference and their applications in astrophysics. We principally discuss two related themes: advances in nested sampling (Chapters 3 to 5), and Bayesian sparse reconstruction of signals from noisy data (Chapters 6 and 7). Nested sampling is a popular method for Bayesian computation which is widely used in astrophysics. Following the introduction and background material in Chapters 1 and 2, Chapter 3 analyses the sampling errors in nested sampling parameter estimation and presents a method for estimating them numerically for a single nested sampling calculation. Chapter 4 introduces diagnostic tests for detecting when software has not performed the nested sampling algorithm accurately, for example due to missing a mode in a multimodal posterior. The uncertainty estimates and diagnostics in Chapters 3 and 4 are implemented in the $\texttt{nestcheck}$ software package, and both chapters describe an astronomical application of the techniques introduced. Chapter 5 describes dynamic nested sampling: a generalisation of the nested sampling algorithm which can produce large improvements in computational efficiency compared to standard nested sampling. We have implemented dynamic nested sampling in the $\texttt{dyPolyChord}$ and $\texttt{perfectns}$ software packages. Chapter 6 presents a principled Bayesian framework for signal reconstruction, in which the signal is modelled by basis functions whose number (and form, if required) is determined by the data themselves. This approach is based on a Bayesian interpretation of conventional sparse reconstruction and regularisation techniques, in which sparsity is imposed through priors via Bayesian model selection. We demonstrate our method for noisy 1- and 2-dimensional signals, including examples of processing astronomical images. The numerical implementation uses dynamic nested sampling, and uncertainties are calculated using the methods introduced in Chapters 3 and 4. Chapter 7 applies our Bayesian sparse reconstruction framework to artificial neural networks, where it allows the optimum network architecture to be determined by treating the number of nodes and hidden layers as parameters. We conclude by suggesting possible areas of future research in Chapter 8.
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Mingarelli, Chiara Maria Francesca. "Gravitational wave astrophysics with pulsar timing arrays." Thesis, University of Birmingham, 2014. http://etheses.bham.ac.uk//id/eprint/5117/.
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This thesis focuses on gravitational wave (GW) astrophysics with Pulsar Timing Arrays (PTAs). Firstly it is shown that anisotropy in the GW background may be present, and that its characterization at different angular scales carries important information. The standard analysis for isotropic backgrounds is then generalized by decomposing the angular distribution of the GW energy density into multipole moments. Generalized overlap reduction functions (ORFs) are computed for a generic level of anisotropy and PTA configuration. A rigorous analysis is then done of the assumptions made when calculating ORFs. It is shown that correlated phase changes introduce previously unmodeled effects for pulsars pairs separated by less than a radiation wavelength. The research then turns to the study of continuous GW sources from supermassive black hole binaries (SMBHBs). Here it shown that the detection of GWs from SMBHB systems can yield direct information about the masses and spins of the black holes, provided that the GW-induced timing fluctuations both at the pulsar and at Earth are detected. This in turn provides a map of the nonlinear dynamics of the gravitational field and a new avenue to tackle open problems in astrophysics connected to the formation and evolution of SMBHs.
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Hernandez, David Michael. "Solving the N-body problem in astrophysics." Thesis, Massachusetts Institute of Technology, 2018. http://hdl.handle.net/1721.1/119107.
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Thesis: Ph. D., Massachusetts Institute of Technology, Department of Physics, 2018.<br>Cataloged from PDF version of thesis.<br>Includes bibliographical references (pages 299-307).<br>The astrophysical N-body problem describes N point masses interacting with each other through pairwise gravitational forces. A solution of this problem is frequently necessary in dynamical astronomy. In the collisional N-body problem, the relaxation time is small compared to the timescale we are interested in studying. Collisional N-body problems include open and globular clusters and protoplanetary disks during the stage, typically lasting hundreds of Myrs, when planetary embryos collide and merge. In the first part of this Thesis, I develop new symplectic integrators which provide a solution for the N-body problem. The integrators decompose the N-body problem into a superposition of two-body problems, which are integrable. Since they are symplectic, the integrators conserve all Poincaré invariants (the evolution is Hamiltonian). We used the integrators to compute the evolution of a globular cluster through core collapse up to 20 times faster than standard techniques. In the second part of this Thesis, I apply the results from the first part of the Thesis to planetary dynamics finding that for problems with hierarchical binaries (planets with moons, planetary systems with binary stars, etc.), the integrators are far more efficient than alternatives. I show numerically that a popular code is neither symplectic nor time-symmetric, and can yield incorrect three-body dynamics. I derive symplectic integrators in various coordinate systems with different Hamiltonian splittings and compare them through backward error analysis and tests of Pluto's orbital element evolution. The final part of this Thesis is concerned with time-symmetric and time-reversible integration in astrophysics, whether we are integrating the N-body problem or other ordinary differential equations. These integrators have been proposed as an alternative to symplectic integration. I show, again using backward error analysis, that such integrators are usually useful, but can behave worse than symplectic integrators. I find time-reversibility can be eliminated in some cases while good error behavior is still maintained.<br>by David Michael Hernandez.<br>Ph. D.
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Feroz, Farhan. "Bayesian methods for astrophysics and particle physics." Thesis, University of Cambridge, 2008. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.612370.
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Hu, Yiming. "Novel inference methods for gravitational wave astrophysics." Thesis, University of Glasgow, 2015. http://theses.gla.ac.uk/6441/.
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With the development of more and more elegant and sensitive interferometric gravitational wave detectors, we are expecting the first direct detection of gravitational waves in a short time. This triggers huge interest to develop more powerful tools to perform data analysis on these signals, and to develop a good understanding of the analysis so that confident conclu- sions can be made. A further step would be to view into the future, as the first detections will boost the scientific demands for more powerful future generation detectors, which identifies the task of optimising the site of such detectors. Bayesian Inference plays a vital role in data analysis, and one excellent example that demon- strates its usefulness is its ability to resolve the tension between multiple models using the methodology of Bayesian Model Selection. In this thesis we apply this methodology to the timing data of pulses from the pulsar 1E 2259+586. With a set of different choices for the prior range, a fair and quantitative comparison can be made between two competing models: that of so-called successive anti-glitches and an anti-/normal glitch pair. Our analysis of the data shows a consistent support for the successive anti-glitches model, with a Bayes Factor of ∼ 45, where the uncertainty has been estimated from nested sampling and from multiple runs that are slightly different, but still within a factor of two, showing a general consistency. Simplifying the timing model will only make the Bayes Factor even bigger, while the two event model is overwhelmingly supported over the one event model. In gravitational wave data analysis, posteriors are generally complicated structures contain- ing multiple modes. A novel algorithm to achieve efficient sampling for multi-modal pos- teriors, known as mixed MCMC, is proposed in this thesis. This enables communication between multiple regions within the parameter space by adopting a novel jump proposal. We present the mixed MCMC algorithm and first apply it to a toy model problem, where the likelihood may be determined theoretically. By comparing the theoretical and empiri- cally sampled values of 2 log(L) for credible regions that correspond to 68.27%, 95.45% and 99.73%, we conclude that for our illustrative model the sampling result of mixed MCMC is consistent with the theoretical prediction with small uncertainty. Since it does not re- quire multiple chains with different temperatures, mixed MCMC can boost the efficiency of sampling by design, compared with (for example) parallel tempering MCMC. The sampling strategy of mixed MCMC can be helpful for not only Bayesian Inference, but also more general problems like the global optimisation of future generations of Gravita- tional Wave Detectors. As we expect such problem to be intrinsically high dimensional and multi-modal, mixed MCMC is a suitable sampling method, and we develop and apply it in this thesis. Based on our analysis it is concluded that for both a 3-detector-network and a 5-detector-network, Australia hosts the “best” site, in the sense that such site is most flex- ible, i.e. it can be involved in the largest number of detector networks, involving different component sites, that have a high ‘Figure of Merit’. The work of gravitational wave data analysis leads to the ultimate goal of making a direct detection of gravitational waves, which in turn requires the ability of distinguish astronomi- cal signals from a noisy background, and assess the significance of each gravitational wave ‘trigger’ (i.e. candidate event) appropriately. There are two types of method for estimating significance and these differ by the key distinction of either removing the foreground events from the background estimation or keeping them in the analysis. This thesis presents the results of a Mock Data Challenge (MDC), carried out within the LIGO Scientific Collabo- ration using different data analysis pipelines, designed to investigate these two methods for estimating significance. It contains a variety of background complexity ranging from simple, realistic to complex, and foreground event rate ranging from zero, low, medium and high. Analysis of the MDC results illustrated that generally all methods for determining the sig- nificance agree well with each other, irrespective of the background complexity. However, a discrepancy became apparent between the results for removal or non-removal of foreground events, for events below a significance level of < 10−3. Our results demonstrated that the removal method is an unbiased estimator for the mean of the significance. However, as the most scientifically interesting events are likely to have a very small numerical value for their significance, such method would overestimate that significance for most of the realisations.
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Obrocka, Monika. "High-time resolution astrophysics using digital beamforming." Thesis, University of Manchester, 2015. https://www.research.manchester.ac.uk/portal/en/theses/hightime-resolution-astrophysics-using-digital-beamforming(5293d2b8-2054-4d67-9fca-435f7482be31).html.
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In the past few years, a possible new population of sources emitting fast and bright transient radio bursts have been discovered. To explore this new transient phase space, the next generation of radio telescopes, typically interferometers, provide wide observing bandwidths in order to achieve high sensitivity for the detection of weak sources and multi-beaming capabilities to increase the field of view. In comparison to a traditional single beam radio dish, these telescopes collect vast volumes of data and still rely on the traditional observing techniques. Localising transient events during their discovery is essential as these events might not be repeatable by nature. I describe the development and construction of a prototype phased array, the Manchester University Student Telescope (MUST), capable of multi-beam operation. I present results of the Yagi antenna beam measurements and the optimum antenna separations; the investigation of the radio frequency interference spectrum around the preferred observing band and setting the subsequent bandpass filter specifications; simulations of the optimum configurations of the MUST tiles; and finally I describe implementation of the digital back-end. For the joint task of discovery and simultaneous localisation, the advantages of single dishes and interferometers are combined in a beam forming approach. I present an investigation into the wide-bandwidth time-domain signal processing techniques for time-domain beamforming that can be used in transient and pulsar observations. I discuss the efficient polyphase decomposition for interpolation digital filters and multiplication-free fractional delay filters that can be used to reduce the complexity of the beamformer implementation and avoid high sampling rates. This reduced complexity allows more simultaneous beams to be formed using time-domain techniques. This analysis was performed for the MUST array, but is applicable to a wide range of interferometers. I have developed and analysed a new proof-of-concept non-imaging method to localise transient sources observed with interferometers or phased array feeds. It utilises the additional spectral and comparative spatial information obtained from multiple overlapping tied array beams. This allows us to estimatea transient source location with up to arc second accuracy in almost real time and allows the required high time resolution to be preserved. We demonstrate that this method can work for a variety of interferometric configurations, including for LOFAR and MeerKAT, and that the estimated angular position may be sufficient to identify a host galaxy, or other related object, without reference to other simultaneous or follow-up observations.
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Cullen, Jason. "Inverse compton scattering in high energy astrophysics." Connect to full text, 2001. http://hdl.handle.net/2123/849.
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Thesis (Ph. D.)--University of Sydney, 2002.<br>Title from title screen (viewed Apr. 23, 2008). Submitted in fulfilment of the requirements for the degree of Doctor of Philosophy to the Research Centre for Theoretical Astrophysics & Theoretical Physics Group, School of Physics. Degree awarded 2002; thesis submitted 2001. Includes bibliography. Also available in print form.
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Cullen, Jason Graham. "Inverse compton scattering in high energy astrophysics." Thesis, The University of Sydney, 2001. http://hdl.handle.net/2123/849.
Full textAbstract:
This thesis investigates some aspects of the inverse Compton scattering process within various physical contexts in high energy astrophysics. Initially an introduction to the key results of Comptonization theory for the case of scattering in optically thick plasmas is given, using a diffusion approach, since these results are required for the interpreta- tion of Comptonized spectra. Since Comptonization in astrophysical systems is frequently treated using numerical techniques, an introduction to these is then presented. Such linear Monte Carlo photon transport codes are typically applied to scattering in plasmas without temperature and density gradients. Additionally, treating bulk motion can be difficult even for simple cases. It is demonstrated that these problems can be made tractable numerically with the use of algorithms associated with non-linear Monte Carlo codes. Such codes can already treat scattering within arbitrary velocity structures in a plasma, and an extension of the algo- rithm is proposed that enables the easy calculation of photon transport in plasmas with non-constant density as well as non-constant temperature and/or bulk motion. This algorithm and code has been developed to treat scattering in astrophysical situations where bulk motion, temperature gradients and density gradients are simultaneously present in a plasma. Both a semi-analytic approach and the numerical approach are then used to treat Comp- tonization problems of current interest. Firstly, the standard two-phase disk-corona model for the high-energy spectra of Active Galactic Nuclei is modified to include an an outflow or wind which may provide an additional source of disk cooling. Earlier slab disk-corona models predict a spectral index which is consistent with observations only if all the accretion power is dissipated in the corona. For the models investigated here, energy spectral indices that are consistent with observations can be obtained with less accretion power being dissipated in the corona, as a result of an outflow/wind. However, it is required that the wind extract large amounts of power from the disk, and it it yet to be seen if this is a plausible scenario.
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Cullen, Jason Graham. "Inverse compton scattering in high energy astrophysics." University of Sydney. Physics, 2001. http://hdl.handle.net/2123/849.
Full textAbstract:
This thesis investigates some aspects of the inverse Compton scattering process within various physical contexts in high energy astrophysics. Initially an introduction to the key results of Comptonization theory for the case of scattering in optically thick plasmas is given, using a diffusion approach, since these results are required for the interpreta- tion of Comptonized spectra. Since Comptonization in astrophysical systems is frequently treated using numerical techniques, an introduction to these is then presented. Such linear Monte Carlo photon transport codes are typically applied to scattering in plasmas without temperature and density gradients. Additionally, treating bulk motion can be difficult even for simple cases. It is demonstrated that these problems can be made tractable numerically with the use of algorithms associated with non-linear Monte Carlo codes. Such codes can already treat scattering within arbitrary velocity structures in a plasma, and an extension of the algo- rithm is proposed that enables the easy calculation of photon transport in plasmas with non-constant density as well as non-constant temperature and/or bulk motion. This algorithm and code has been developed to treat scattering in astrophysical situations where bulk motion, temperature gradients and density gradients are simultaneously present in a plasma. Both a semi-analytic approach and the numerical approach are then used to treat Comp- tonization problems of current interest. Firstly, the standard two-phase disk-corona model for the high-energy spectra of Active Galactic Nuclei is modified to include an an outflow or wind which may provide an additional source of disk cooling. Earlier slab disk-corona models predict a spectral index which is consistent with observations only if all the accretion power is dissipated in the corona. For the models investigated here, energy spectral indices that are consistent with observations can be obtained with less accretion power being dissipated in the corona, as a result of an outflow/wind. However, it is required that the wind extract large amounts of power from the disk, and it it yet to be seen if this is a plausible scenario.
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Addison, Eric. "Gravitational Wave Astrophysics with Compact Binary Systems." DigitalCommons@USU, 2014. https://digitalcommons.usu.edu/etd/2166.
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In this dissertation, I present two studies in the field of gravitational wave astrophysics applied to compact binary systems. In the first project, I investigate simulated encounters between a binary system comprised of two stellar mass black holes with a galactic supermassive black hole. It is found that binaries disrupted by the supermassive black hole form extreme mass ratio inspirals (EMRIs), which would begin with very high eccentricity, e ≈ 1 − O(10−2), but circularize dramatically by the emission of gravitational wave radiation. At the time when the stable orbit turns over to a plunge orbit, the EMRIs still have some small residual eccentricity, e ≈ 0.05 on average, which is slightly larger than previous estimates. The surviving binaries are classified based on their final relation with the supermassive black hole. When inspecting the merger lifetime of the surviving binaries, a mean new merger lifetime of ˜ T = 0.8T0 is found. Factoring in this new lifetime with other relevant data, I calculate the merger rate of these systems in the range of the advanced Laser Interferometer Gravitational Wave Observatory to be about 0.25 yr−1, which represents a small percentage of the current predicted CBC rates. In the second project I propose and explore a new method of estimating the radius of the accretion disc in cataclysmic variable binary systems though the use of coupled electromagnetic and gravitational wave observations. By identifying the angle of the hot spot formed by the impact of the accretion stream with the disc, φHS, the radius of the disc can be recovered. I test the proposed method against fully simulated lightcurve output, as well as the true observed AM CVn lightcurve. In both cases, I find our method capable of estimating the disc radius to high precision. I calculate a disc radius of ˆRD/a ≈ 0.476°”0.025 for the fully simulated data and ˆRD/a ≈ 0.481 °” 0.05 for the true lightcurve data. These estimates agree with the accepted value of RD = 0.478a within the uncertainties, and differ from the accepted value by 0.4% and 0.6%, respectively. Because this method does not rely on eclipses, it will be applicable to a much broader population of binaries.
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TABASSAM, UZMA. "A Pair Spectrometer for Nuclear Astrophysics Applications." Doctoral thesis, Università degli Studi di Camerino, 2012. http://hdl.handle.net/11581/401785.
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A spectrometer using two fully depleted silicon detectors (in a configuration) has been realized with the goal of directly measuring the production rate of the e+e− pairs emitted in E0 transition of the 12C 16O reaction. This is a key reaction in nuclear astrophysics, which takes place during the He burning stage of red giant stars and thus regulates the carbon/oxygen abundance in the Universe. In particular, we are interested to determine the e+e− pair cross section at energies below 2 MeV, where theoretical estimate is possible by using the R- matrix extrapolation. Experimental e+e− pair emission data at this energy thus provides a valuable tool to validate such analytical approximate scheme. Resolution and efficiency measurements have been carried out using 241Am +239 Pu source, the α source, 32P,207 Bi β sources and the 19F(p, α)16O fusion evaporation reaction below 1 MeV on beam reaction at CIRCE tandem accelerator (Caserta, Italy). The results obtained approve to be in good agreement with our GEANT4 simulation.
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Pety, Jérôme. "Caractériser le milieu interstellaire : un clé pour comprendre l'Univers." Habilitation à diriger des recherches, Université Pierre et Marie Curie - Paris VI, 2012. http://tel.archives-ouvertes.fr/tel-00726959.
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Qu'ont en commun la détection de carbone atomique à un redshift de 4, la cartographie à 1" de résolution de l'émission 12CO(1-0) de la galaxie du tourbillon (M51), l'étude des avant-plans galactiques de Planck, et l'étude de la cinématique du disque et du flot moléculaire de la proto-étoile HH30 ? Au-delà du fait qu'elles sont réalisées dans le domaine (sub-)millimétrique, ces observations sont liées aux processus physiques et chimiques du milieu interstellaire. Caractériser ces processus permet de comprendre les objets les plus divers de l'univers, des plus proches au plus lointains, des plus petits au plus grands. Je décris ici une décennie de travail consacrée à la compréhension du milieu interstellaire. Je commence par présenter deux des approches scientifiques que j'ai prises. La première concerne la caractérisition d'une des transitions les moins bien comprises du gaz dans son chemin vers la formation des étoiles, à savoir la transition HI vers H2.Je montre comment l'interprétation de l'émission 12CO(1-0) pointe tout autant vers le milieu dense et froid que vers le milieu diffus et tiède. Dans un 2ème temps, je décris la nécessité et la mise en place d'une référence observationnelle (la chevelure de la nébuleuse de la Tête de Cheval) pour les modéles photochimiques, eux-mêmes utilisés dans tous les contextes évoqués ci-dessus. La décennie qui vient sera aussi féconde grâce à plusieurs événements. Tout d'abord, la communauté qui étudie le milieu interstellaire se structure rapidement autour de grands projets. A mon niveau, je suis porteur du contrat ANR << Structure and CHemistry of the Inter-Stellar Medium >> (SCHISM) qui réunit observateurs, numériciens et théoréticiens de l'IRAM et de l'Observatoire de Paris. Par ailleurs, l'instrumentation radio fait des progrès spectaculaires qui vont déboucher sur la spectro-imagerie grand champ à haute résolution angulaire et spectrale. L'IRAM a un rôle prépondérant dans cette aventure et j'y contribue au niveau logiciel et algorithmique. Enfin, je participe à la maturation des nouveaux instruments comme les caméras grand-champs pour les antennes uniques et les projets ALMA et NOEMA en interférométrie (sub-)millimétrique. La conjonction de ces facteurs contribuera à percer à jour l'origine des galaxies, des étoiles, des systèmes planétaires et des molécules prébiotiques.
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Chaulagain, Uddhab Prasad. "Radiative shocks : experiments, modelling and links to astrophysics." Thesis, Paris 6, 2015. http://www.theses.fr/2015PA066734.
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Les chocs radiatifs sont des chocs très violents qui sont caractérisés par des températures très élevées. Dans ce type de structure, une grande partie de l’énergie est convertie en rayonnement. Ces chocs sont présents dans de nombreux plasmas astrophysiques, notamment dans le cadre des jets et de l’accrétion stellaires, des restes de supernova etc. Ils peuvent être désormais générés sur terre en utilisant des lasers de grande puissance ce qui permet leur étude à l’interface entre l’astrophysique et la physique des plasmas.Cette thèse présente et discute les résultats d’une expérience réalisées sur l’installation Prague Asterix Laser System. Le choc est généré en focalisant le laser Infrarouge sur une cible de quelques millimètres de long, remplie de xénon à basse pression. Le choc ainsi généré se propage dans le gaz à une vitesse élevée, permettant d’atteindre le régime des chocs dom- inés par le flux radiatif. Nous avons utilisé différents diagnostics pour caractériser le choc, notamment une radiographie éclair, à l’aide d’un laser (Zinc) à 21.2 nm, capable de pénétrer les parties denses du plasma. Un autre important diagnostique consiste à analyser l’émission propre du plasma à l’aide d’une diode rapide.Les résultats expérimentaux montrent pour la première fois, et sans ambiguïté, une structure de choc complète, comprenant le post-choc et le précurseur. Nous avons aussi réalisé différentes mesures de la vitesse des chocs. Les résultats ont été comparés à ceux de simulations numériques, montrant un bon accord avec ces dernières<br>Radiative shocks are strong shocks which are characterized by a plasma at high temperatures emitting an important fraction of its energy as radiation. Radiative shocks are found in many astrophysical systems, including stellar accretion shocks, supernovae remnants, jet driven shocks, etc. Recently, radiative shocks have also been produced experimentally using high energy lasers. Thus opening the way to laboratory astrophysics studies of these universal phenomena.In this thesis we discuss the results of an experiment performed on the Prague Asterix Laser System facility. Shocks are generated by focusing the PALS Infrared laser beam on millimetre-scale targets filled with xenon gas at low pressure. The shock that is generated then propagates in the gas with a sufficiently high velocity such that the shock is in a radiative flux dominated regime. We used different diagnostics to characterize these shocks. The two main ones include a radiography of the whole shock structure using sub-nanosecond Zn X-ray laser at 21.2 nm, which is able to penetrate the dense post-shock layer, and a space-and-time resolved plasma self-emission using high speed diodes.The experimental results show, for the first time, an unambiguous shock structure which includes both the post-shock and the precursor, and we also obtained multiple shock velocity measurements from the different diagnostics. The experimental results are compared to simulations, and show good agreement with the numerical results
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Grassi, Anna. "Collisionless shocks in the context of Laboratory Astrophysics." Thesis, Paris 6, 2017. http://www.theses.fr/2017PA066483/document.
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Cette thèse s'inscrit dans le cadre de l'astrophysique de laboratoire. Nous abordons divers aspects de la physique des chocs non-collisionels en présence de flots de plasma relativistes dans des configurations d'intérêt pour les communautés astrophysique et de l’interaction laser-plasma (ILP). Notre approche repose sur la modélisation analytique et la simulation cinétique haute-performance, outil central pour décrire les processus d'ILP et la physique non linéaire à l'origine des chocs étudiés. Le code Particle-in-Cell SMILEI a été largement utilisé et développé au cours ce travail. Trois configurations physiques sont étudiées. L’instabilité Weibel en présence de faisceaux d'électrons contre-propagatifs alignés avec un champ magnétique externe est décrite. Les phases linéaires et non linéaires sont expliquées à l’aide de modèles théoriques confirmés par des simulations. La génération de chocs non-collisionels lors de l’interaction de deux plasmas relativistes de paires est étudiée en présence d’un champ magnétique perpendiculaire. L’accent est mis sur la comparaison des prédictions théoriques sur les grandeurs macroscopiques avec les simulations, ainsi que sur la définition du temps de formation du choc, l’ensemble de ces grandeurs étant d’une grande importance pour de futures expériences. Enfin, nous proposons un schéma permettant de recréer, en laboratoire, l’instabilité Weibel ionique par l'utilisation d'un laser intense. Les flots de plasmas produits ici sont plus rapides et denses que dans les expériences actuelles, conduisant à un taux de croissance et des champs magnétiques plus élevés. Ces résultats sont également important pour l’ILP à très haute intensité<br>The work presented in this thesis belongs to the general framework of Laboratory Astrophysics. We address various aspects of the physics of collisionless shocks developing in the presence of relativistic plasma flows, in configurations of interest for the astrophysical and the laser-plasma interaction (LPI) communities. The approach used throughout this thesis relied on both analytical modeling and high-performance kinetic simulations, a central tool to describe LPI processes as well as the non-linear physics behind shock formation. The PIC code SMILEI has been widely used and developed during this work. Three physical configurations are studied. First we consider the Weibel instability driven by two counter-streaming electron beams aligned with an external magnetic field. The linear and non-linear phases are explained using theoretical models confirmed by simulations.Then the generation of non-collisional shocks during the interaction of two relativistic plasma pairs is studied in the presence of a perpendicular magnetic field. We focus on the comparison of theoretical predictions for macroscopic variables with the simulation results, as well as on the definition and measurement of the shock formation time, all of which are of great importance for future experiments.Finally, we proposed a scheme to produce, in the laboratory, the ion-Weibel-instability with the use of an ultra-high-intensity laser. The produced flows are faster and denser than in current experiments, leading to a larger growth rate and stronger magnetic fields. These results are important for the LPI at very high intensity
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Nilsson, Ulf. "Dynamical systems in cosmology and astrophysics /Ulf Nilsson." Stockholm : Stockholms Universitet, 1998. http://www.gbv.de/dms/goettingen/254604390.pdf.
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Margueron, J. "Modélisation microscopique pour l'astrophysique Microscopic modeling for astrophysics." Habilitation à diriger des recherches, Université Paris Sud - Paris XI, 2012. http://tel.archives-ouvertes.fr/tel-00940921.
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In this manuscript, some relations between theoretical nuclear physics and compact stars, which are known to be excellent tools to test matter under extreme conditions, are studied. Most of these links are performed within theoretical modelling which are used to describe both nuclei and nuclear systems in astrophysics. Self-consistent approaches unifying the description of isolated nuclei, dilute nuclei in a gas of light particles, and uniform matter, are presented in this manuscript and employed to understand the physics of compact stars. The manuscript is organized as follow: The first chapter is a general introduction to impact stars and supernovae physics, as well as to nuclear physics. In the second chapter, various aspects of the modeling of the in-medium nucleon-nucleon interaction are presented, such as, extension of the Skyrme interaction in the spin and spin-isospin channels, the properties of the V(low k) interaction and the low-density properties of nuclear matter. In the third chapter, the superfluid properties of dense matter are investigated, either through an iso-vector pairing interaction design to reproduce microscopic calculations in nuclear matter, or through the impact of superfluidity on derivatives of the EOS, such as the incompressibility and the symmetry energy, and finally, is presented the use of pairing vibration to study the properties of pairing. The questions of the in-medium effective mass and its impact for the dynamics of core-collapse supernovae, and the level density in nuclei are presented in the chapter 4. The microscopic modelling of the crust of neutron stars, as derived from band theory, and including superfluidity, is shown in chapter 5. In chapter 6, some links between the properties of nuclei, and neutron stars are presented: the properties of overflowing nuclei at the neutron drip, the correlation between the curst-core properties and nuclear empirical quantities, and the stiffness of the nuclear EOS, are shown. Finally, conclusions are presented in chapter 7.
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Powell, Andrew James. "The cosmology and astrophysics of axion-like particles." Thesis, University of Oxford, 2016. https://ora.ox.ac.uk/objects/uuid:bbbb3cbc-a0ba-4024-86b0-c720d8104270.
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In this thesis I study astrophysical and cosmological effects of axion-like particles (ALPs). ALPs are pseudo-scalar particles, which are generally very weakly-interacting, with a coupling α/M E · B to electromagnetism. They are predicted by many theories which extend the standard model (SM) of particle physics, most notably string theory. String theory compactifications also predict many scalar fields called moduli which describe the size and shape of the extra, compact dimensions. In string theory models generically the moduli fields are responsible for reheating the universe after inflation. Being gravitationally-coupled, they will also decay to any other particles or sectors of the theory, including any light ALPs, of which there are usually many. The ALPs produced by moduli decay will contribute to dark radiation, additional relativistic energy density. The amount of dark radiation is tightly constrained by observations, this bounds the branching fraction of moduli decays into ALPs, which constrains the string theory model itself. I calculate the amount of dark radiation produced in a model with one light modulus, solely responsible for reheating, called the Large Volume Scenario. I study a minimal version of this model with one ALP and a visible sector comprised of the minimal supersymmetric SM. The dominant visible sector decay mode is to two Higgses, I include radiative corrections to this decay and find that ALP dark radiation is over-produced in this minimal version of the model, effectively ruling it out. The production of ALPs from moduli decay at reheating seems to be a generic feature of string theory models. These ALPs would exist today as a homogeneous cosmic ALP background (CAB). The coupling of ALPs to electromagnetism allows ALPs to convert to photons and vice versa in a magnetic field, leading to potential observable astrophysical signals of this CAB. Observations have shown an excess in soft X-ray emission from many galaxy clusters. I use detailed simulations of galaxy cluster magnetic fields to show that a CAB can explain these observations by conversion of ALPs into X-ray photons. I simulate ALP-photon conversion in four galaxy clusters and compare to soft X-ray observations. I show the excesses (or lack thereof) can be fit consistently across the clusters for a CAB with ALP-photon inverse coupling of M = 6 - 12 x 10¹² GeV, if the CAB spectrum has energy ~ 200 eV. I also study the possibility of using galaxy clusters to search for and constrain the ALP coupling to photons using cluster X-ray emission. Conversion of X-ray photons into ALPs will cause spectral distortions to the thermal X-ray spectrum emitted by galaxy clusters. I show that the non-observation of these distortions is able to produce the strongest constraints to date on the ALP-photon inverse coupling, M ≳ 7 x 10¹¹ GeV.
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Darg, Daniel W. "Pattern recognition in astrophysics and the anthropic principle." Thesis, University of Oxford, 2012. http://ora.ox.ac.uk/objects/uuid:4cb9e1d5-d9d9-4993-8991-f43882d70016.
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The role of the Anthropic Principle in astrophysics and cosmology is examined in two principal parts. The first (minor) part takes a chiefly philosophical perspective and examines the manner in which human cognition features into discussions on cosmic origins. It is shown that the philosophical questions raised by the Anthropic Principle and ‘fine-tuning of life’ bear resemblances to problems within the philosophy of mind and we seek a common origin for this surprising parallel. A form of ‘epistemic structural realism’ is defended and used to critique the physicalist identity thesis. It is argued that equating ‘reality’ with mathematical structures, which is the basis of the identity thesis, leads to incoherent conclusions. Similar reasoning is used to critique infinite Multiverse theories. In the second (major) part, we gradually transition into mainstream astrophysics, first presenting a new line of research to explore counterfactual universes using semi-analytic models (SAMs) and offering a preliminary study wherein the cosmological constant is varied and the effects on ‘advanced civilisations’ are examined. The importance of galaxy mergers is highlighted and leads to their study. We first try solving the pattern-recognition problem of locating mergers using the Galaxy Zoo database and produce the largest homogenous merger catalogue to date. We examine their properties and compare them with the SAMs of the Millennium Simulation finding good general agreement. We develop the Galaxy Zoo approach with a new visual-interface design and double the size of the merger catalogue of SDSS mergers in the local Universe.
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Banerjee, Oindree. "Studies in particle astrophysics with the ANITA experiment." The Ohio State University, 2018. http://rave.ohiolink.edu/etdc/view?acc_num=osu1532097132391735.
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