Teixeira, Joana Ribeiro da Cunha Gomes; (2024) General relativistic radiation transport of massive and massless particles in strong gravity. Doctoral thesis (Ph.D), UCL (University College London).

Text Joana_PhD_Thesis_submitted_version_with_corrections.pdf - Other Access restricted to UCL open access staff until 1 March 2025. Download (39MB)

Abstract

General relativity is the best theory at present to describe gravitational interactions. One of its predictions is the existence of black holes. Because general relativity is not compatible with Quantum Mechanics, some new Physics must exist to unify them. This makes systems like black holes, where both quantum mechanical and general relativistic phenomena are expected to occur, particularly interesting objects of study. The discovery of astrophysical black holes opened the doors to detailed studies of these objects, both from the point of view of understanding the consequences and limitations of general relativity and of gaining insight into the effects of strong gravity on physical processes that occur in their vicinity. To study these objects, we must resort to observations performed on Earth (or in space around the Earth). This means that we require radiation or particles to travel between these regions of the universe towards our detectors. In the transport process, radiation or particles may be absorbed, scattered, or added to the original bundle, as well as distorted by the gravitational and relativistic phenomena at play in the extreme regions of their origin. As such, it is essential to have a way of understanding this transport mechanisms and how it may affect the observations performed on Earth. Only in this way are we able to confidently make conclusions about the phenomena happening very close to these black holes. The main focus of this thesis was the development of a formalism and algorithm able to perform general relativistic transport calculations of both particles with and without mass in a selfconsistent manner. This algorithm was built on and generalized another general relativistic radiation transport (GRRT) formalism which focused on massless particles. This formalism is then applied to two distinct astrophysical scenarios. The first is related to the detection of electromagnetic flares across multiple energy bands from the vicinity of Sagittarius A*, the black hole at the centre of the Milky Way. We performed GRRT calculations for this system, performing a thorough analysis of the effects of general and special relativity in the distortion of flares, considering a time-dependent spectral evolution of the emissivity profile and taking into account the energy bands in which observations take place. The second scenario is related to the propagation of massive particles accelerated in the vicinity of black hoes and their interactions with the medium around them. We were able to identify and explain interesting trends in their behaviour and see how they were affected by different aspect of special and general relativity. We investigated how the particle’s Lorentz factor and its acceleration region influences the location and timing of where interactions would take place and found some unexpected degeneracies which may impact the interpretations made from observations associated to these systems.

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