Andrew West


Date of Award

Spring 5-15-2023

Author's School

Graduate School of Arts and Sciences

Author's Department


Degree Name

Doctor of Philosophy (PhD)

Degree Type



Black holes are among the most exotic phenomena in our universe, physical objects so dense and compact that within their horizon not even light can escape. In binary systems, black holes can accrete material from their companion star, forming an accretion disk that is subjected to extreme physical conditions. Often, these disks are accompanied by jets - highly relativistic outflows of material moving at significant fractions of the speed of light. This extreme environment is the source of some of the most luminous and energetic processes in our universe. Black hole accretion disks are often modeled as a steady state, razor thin disk aligned in the equatorial plane of the black hole spin. In reality, however, this approximation is not always valid as these systems are highly dynamic, with their energy spectra varying as the state of the accretion flow changes. These changes of state occur on timescales of days to weeks to months, during which the luminosity can increase or decrease by orders of magnitude. Simultaneously, variations in the luminosity are also observed on sub-second timescales, a phenomenon known as quasi-periodic oscillations (QPOs). While the characteristics of these state transitions and QPOs have been well typified by modern observations, the physical mechanisms underlying them are only beginning to be understood.

This thesis contains work I have completed in to better understand and explain the observational characteristics of accretion disks which deviate from the thin disk model. In the first part of this thesis I explore the effect of finite geometric thickness on the polarization spectra produced by accreting black holes, achieved by introducing a modified geometry to the raytracing code xTrack. I find that, in general, disks of geometric thickness produce higher polarization signatures than thinner disks. The remainder of the thesis focuses on the time domain characteristics, reflection spectra, and polarization spectra of a dynamically evolving accretion disk undergoing tearing events. To achieve this, I developed a raytracing code based on xTrack that utilizes the output of the General Relativistic Magneto-Hydrodynamical simulation H-AMR as initial conditions and raytraces the spectral emission through the evolving geometry. I show, for the first time, spectral high frequency QPOs that result from tearing events in the disk itself. Additionally, I show that precession of an inner, quasi- Bardeen-Petterson aligned disk produces low frequency QPOs. I also explore the dynamic behavior of the both the polarization and Fe-K$\alpha$ line emission during these tearing events. These novel results lay the groundwork for developing new methods for measuring the spin of accreting black holes and are an important step toward explaining the dynamics of the accreting plasma during the state transitions observed in stellar mass black holes.


English (en)

Chair and Committee

Henric Krawczynski

Committee Members

Ramanath Cowsik, Michael Nowak, James Steiner, Yajie Yuan,