1,720,981 research outputs found
Gravitational lensing in stationary and axisymmetric black hole spacetimes: acceleration and gravitomagnetic charge
No full textIn this thesis we investigate gravitational lensing in two different families of black hole spacetimes that are exact solutions of Einstein’s electrovacuum field equations with a cosmological constant. The first family of spacetimes are the charged C-de Sitter metrics. They are axisymmetric and static. The charged C-de Sitter metric is usually interpreted as an accelerating black hole with an electric charge and a cosmological constant. The second family of spacetimes are the charged NUT-de Sitter metrics. They are axisymmetric and stationary. The charged NUT-de Sitter metric is usually interpreted as a black hole with electric charge, cosmological constant and a gravitomagnetic charge. Both families of spacetimes are generalisations of the Reissner-Nordström-de Sitter metrics, however, they are of rather exotic nature. In Boyer-Lindquist-like coordinates both spacetimes contain conical singularities on the axes and, in the case of the charged NUT-de Sitter metrics, closed timelike curves, which violate causality. Due to these mathematical peculiarities the physical relevance of both families of spacetimes is rather unclear. Although we can be rather certain that conical singularities and closed timelike curves are unlikely to exist in nature both spacetimes may still serve as first highly mathematically idealised approximations for the spacetimes of a linearly accelerating black hole or a black hole with gravitomagnetic charge.
In this thesis we will address the question how we can use gravitational lensing for identifying if an astrophysical black hole can be described by one of the charged C-de Sitter metrics or one of the charged NUT-de Sitter metrics. We will first analytically solve the equations of motion using elementary as well as Jacobi’s elliptic functions and Legendre’s elliptic integrals. Then we will rederive the angular radius of the shadow, formulate an exact analytic lens equation and derive the redshift and the travel time of the light rays
Relativistische Akkretion auf Kompakte Objekte
No full textThe present PhD thesis summarises results on analytical accretion models obtained during my PhD studies in 2014a 2018. I present a new ansatz for specific angular momentum-angular velocity dependence for perfect fluid in circular motion around a Kerr black hole. With this ansatz new solutions for toroidal configurations were obtained. I discuss here in detail the dynamical properties of these configurations and results of their application as initial conditions for accretion simulations with 2-D HARM code. For the case of circular motion in the NUT spacetime I present my results on characteristic circular geodesics radii in it. These results are further used for modelling perfect fluid tori with constant specific angular momentum (Polish Doughnuts). Characteristic changes in the patterns of circular motion of perfect fluid due to presence of the NUT parameter are discussed. The last part of the work presents a pre-study of accretion of spinning particles/fluids onto Kerr black holes. I discuss our results on spin-induced advance in last stable orbits and its relevance for accretion models
Investigation of the gravitational lens effect with differential topology
No full textIn this thesis, the methods of differential topology are used to study the gravitational lensing effect. First, the reader is taken on a brief tour through the basics of black holes, wormholes, gravitational lensing and the mathematical tools of Fermat's principle, Morse theory and the Gauss-Bonnet theorem. Then, the understanding is deepened by applying Morse theory and the Gauss-Bonnet theorem to gravitational lensing.
In the first part, we have fixed an observation event p and the worldline of a light source gamma and identified the set of all past-oriented lightlike geodesics from p to gamma. Since each such geodesic corresponds to an image of the light source on the observer’s sky, this allows us to examine the lensing properties of wormholes. As key results, we have proven with the help of Morse theory that under very mild conditions on gamma, the observer is able to see infinitely many images of gamma. Moreover, we have studied some qualitative features of the lightlike geodesics with the help of two potentials that determine the sum of the centrifugal and Coriolis forces of observers in circular motion for the case that the observers’ velocity approaches the velocity of light. We have exemplified the general results with two specific wormhole spacetimes.
In the second part, we have shown with the help of Fermat’s principle that every lightlike geodesic in the Brill metric projects onto a geodesic of a two-dimensional Riemannian metric, the so-called optical metric. The optical metric is defined on a (coordinate) cone, whose opening angle is determined by the impact parameter of the lightlike geodesic. We have shown that the optical metrics on cones with different opening angles are locally isometric. With the help of the Gauss-Bonnet theorem, we have demonstrated that the deflection angle of a lightlike geodesic is determined by an area integral over the Gaussian curvature of the optical metric.
Last but not least, we have investigated gravitational lensing of Brill wormholes by examining the existence of photon circles and propagation possibilities of light rays
Influence of the spin on the radiation of a spinning emitter orbiting compact objects
No full textIn this thesis, I develop a theoretical description of the radiation of an extended, spinning light source on a circular orbit in the symmetry plane of a stationary, axially symmetric and asymptotically flat spacetime. The light source is assumed to be a test particle, in order to neglect its gravitational influence on the background spacetime. I derive the necessary transformations for a reference frame that is at rest on the surface of the rotating emitter, and link the emission angles, relative to the surface of the emitter, to the constants of motion of the light ray. Two emitter geometries are considered: a sphere and a Maclaurin spheroid, which is flattened as a result of its spin. In particular, I apply this theory to an emitter in orbit around a Schwarzschild object, as well as an emitter orbiting a Kerr black hole. In this context, I investigate the influence of the emitter spin on the observables, specifically the polarization plane, redshift and flux, as well as the influence of the black hole rotation.
Notably, the position of the emitter orbit where the maximum flux is observed depends on the spin, and non-monotony is observed in the amount of observed flux, varying the spin of the emitter. This theory may find application in describing the emissions of spinning hot spots in accretion disks or neutron stars
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
Full text linkWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
Allgemein relativistische Dynamik klassischer Spinteilchen
No full textCoalescing binary systems are supposed to be good sources for gravitational radiation. The data analysis of gravitational wave signals is very much involved with matched filtering procedures. Thus, a detailed theoretical understanding is an essential pillar of gravitational wave astronomy. This thesis is devoted to improve the theoretical description of binary systems consisting of spinning objects. It starts with a study of the dynamical properties of spinning test particles as described by the Mathisson-Papapetrou equations. Provided that the frequencies offer a straight link to observations the pairs of geometrically different timelike geodesics with the same radial and azimuthal frequencies is examined for spinning test particles moving in Schwarzschild-de Sitter spacetime. Then this thesis deals with a Hamiltonian formulation of spinning particles in general relativity. Due to the spin condition the derivation of a Hamiltonian involves the implementation of constraints. A Hamiltonian function linearised in the particle s spin that includes the constraints by means of Dirac brackets is analysed. Since the Hamiltonian offers a wide range of applications to dynamical systems, the significance of the approximation in the spin is investigated. In order to improve the Hamiltonian formulation and expand it to higher orders in the particle s spin an action approach is employed to impose the constraints at the level of the action. At the end applications to future work and implications on observations are discussed
Gravitational lensing from a spacetime perspective.
Full text linkThe theory of gravitational lensing is reviewed from a spacetime perspective, without quasi-Newtonian approximations. More precisely, the review covers all aspects of gravitational lensing where light propagation is described in terms of lightlike geodesics of a metric of Lorentzian signature. It includes the basic equations and the relevant techniques for calculating the position, the shape, and the brightness of images in an arbitrary general-relativistic spacetime. It also includes general theorems on the classification of caustics, on criteria for multiple imaging, and on the possible number of images. The general results are illustrated with examples of spacetimes where the lensing features can be explicitly calculated, including the Schwarzschild spacetime, the Kerr spacetime, the spacetime of a straight string, plane gravitational waves, and others
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