Ray-Tracing Based Polarized Radiative Transfer in General Spacetimes

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Title: Ray-Tracing Based Polarized Radiative Transfer in General Spacetimes
Author: Mannerkoski, Matias
Contributor: University of Helsinki, Faculty of Science
Thesis level: master's thesis
Abstract: This thesis presents a ray-tracing based method for performing polarized radiative transfer in arbitrary spacetimes and a numerical implementation of said method. This method correctly accounts for general relativistic effects on the propagation of radiation, and the polarized im- ages and spectra it produces can be directly compared with observations. Thus it is well suited for studying systems where relativistic effects are significant, such as compact astrophysical objects. The ray-tracing method is based on several approximations, which are discussed in depth. The most important one of these is the geometric optics approximation, which is derived starting from Maxwell’s equations. In the geometric optics approximation, high frequency radiation is described as amplitudes or intensities which are propagated along geodesic rays. Additional assumptions about the properties of the radiation field allow describing it and its interaction with matter using the formalism of kinetic theory, which leads to a simple transfer equation along rays. This transfer equation is valid in arbitrary spacetimes, and forms the basis for the ray-tracing method. The ray-tracing method presented in this work and various similar methods described in the literature are not suited for analytic computations using realistic models. Instead numerical methods are needed. Such numerical methods are implemented in a general fashion in the Arcmancer library (paper in preparation), of which large parts were implemented as a part of this work. The implementation details of Arcmancer are described and its features are compared to those available in other similar codes. Tests of the accuracy of the numerical methods as well as example applications are also presented, including a novel computation of a gravitational lensing event in a binary black hole system. The implementation is found to be correct and easily applicable to a variety of problems.
URI: URN:NBN:fi:hulib-201803011364
Date: 2018-03-01
Discipline: Teoreettinen fysiikka

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