Accretion onto compact objects plays a central role in high-energy astrophysics. The presence of a compact object considerably alters the structure and dynamics of the accreting plasma, as well as its radiative emissions. For accreting black holes in Active Galactic Nuclei (AGN) this is especially true. A significant fraction of the emission may originate or pass near the event horizon. Strong gravity modifies the radiation from an emission source. Photons no longer propagate in straight lines and experience frequency shifts. Gravitational lensing creates multiple images of an emission source, further modifying its temporal and spectral properties. Addressing these effects, the first part of this thesis formulates the equations of radiative transfer for particles with and without mass in a manifestly covariant form. Using ray-tracing, the observed images and line emission from accretion disks and tori are calculated. The effects of absorption, emission and optical depth gradients are investigated. The second part of this thesis examines scattering in general relativity. The general relativistic Compton scattering kernel and its angular moments are expressed in closed-form for the first time, in terms of hypergeometric functions. This has the advantage of being fast, accurate and not restricted by specific energy ranges. The results are in perfect agreement with semi-analytic calculations and Monte-Carlo simulations of Compton scattering of monochromatic emission lines. Finally, I investigate the effects of variability in the accretion flow. Two models are considered: a plasmoid on a Keplerian orbit around a black hole and a magnetically-driven plasmoid ejection from the disk corona. Deriving a new time-dependent radiative transfer formulation, I calculate this variable emission, presenting the results in the form of spectrograms and lightcurves.
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:626564 |
| Date | January 2014 |
| Creators | Younsi, Z. |
| Publisher | University College London (University of London) |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
| Source | http://discovery.ucl.ac.uk/1426969/ |
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