Theses and Dissertations from UMD
Permanent URI for this communityhttp://hdl.handle.net/1903/2
New submissions to the thesis/dissertation collections are added automatically as they are received from the Graduate School. Currently, the Graduate School deposits all theses and dissertations from a given semester after the official graduation date. This means that there may be up to a 4 month delay in the appearance of a give thesis/dissertation in DRUM
More information is available at Theses and Dissertations at University of Maryland Libraries.
Browse
3 results
Search Results
Item Simulations of Accretion Mechanisms and Observational Signatures of Black Hole Accretion Disks(2019) Smith, Megan; McKinney, Jonathan C; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Black holes have been a subject of fascination since they were first theorized about over a century ago. There are many questions about them left unanswered. One of these questions is how matter is accreted onto these objects when the plasma around them is rotating in an accretion disk. An answer to this question is likely to be found in the magnetohydrodynamic processes that occur in the plasma, which require highly sophisticated numerical simulations to explore. In this thesis, I describe an analysis of one magnetohydrodynamic instability found in these simulations as well as the observational signatures it produces, which might be recognized in observations of these systems. For the remainder of this thesis, I will discuss the formation and evolution of a formal near-peer mentoring program for women in the University of Maryland physics department. Mentoring programs have been shown to have a number of benefits for both mentors and mentees. Primary among them is an increased sense of belonging and science identity, which is linked to increased retention. Given the so-called "leaky pipeline" problem of women leaving physics, a field where they are already underrepresented, efforts to improve retention are vital and peer mentoring is one way to do this.Item Accretion Physics Through the Lens of the Observer: Connecting Fundamental Theory with Variability from Black Holes(2018) Hogg, James Andrew; Reynolds, Christopher S; Astronomy; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Variability is a generic feature of accretion onto black holes. In both X-ray binaries and active galactic nuclei, variability is observed on nearly all accessible timescales and across the entire electromagnetic spectrum. On different timescales and at different wavelengths it has unique signatures that can be used to characterize the accretion processes generating the emission and probe the accretion disks, which would otherwise be impossible. Despite having been observed for over fifty years, interpreting this variability is difficult. Simple phenomenological models have been used to explain the behaviors and geometries of the observed accretion disk, but they have yet to be rigorously tested in a full magnetohydrodynamic framework. In this dissertation we use high-resolution numerical models to investigate: (1) ``propagating fluctuations" in mass accretion rate that give rise to the nonlinear signatures of accretion on viscous timescales, (2) the dynamics of truncated accretion disks which are invoked to explain the spectral variation of outbursting X-ray binaries and the bifurcation of AGN accretion states, and (3) the large-scale magnetic dynamo behavior in thick and thin accretion disks. We find that the structured variability readily seen in the light curves from accreting black holes (i.e. log-normal flux distributions, linear relations between the RMS and the flux, and radial coherence) quickly and naturally grows from the MRI-driven turbulence and that these properties translate into photometric variability. For the first time, we identify the large-scale magnetic dynamo as the source of the low-frequency modulations of the disk stress that cause this structure. We introduce a bistable cooling law into hydrodynamic and magnetohydrodynamic simulations to study the manifestation of a truncated accretion disk in each regime. We find that rather than a truncation edge, the transition is better described by a ``truncation zone" when the angular momentum transport and heating is governed by MRI-driven turbulence instead of a true viscosity. Additionally, we find that the hot gas in the simulation buoyantly rises in a gentle outflow and eventually fills the entire volume, instead of simply being confined to the innermost region. The outflow interacts with the disk body and enhances the magnetic stresses, which could produce stronger quasiperiodic variability. Finally, we conduct an investigation of the large-scale magnetic dynamo using a suite of four global magnetohydrodynamic disk simulations with scaleheight ratios of $h/r=\{0.05, 0.1, 0.2, 0.4\}$. Most notably, the organization that is prevalent in accretion disk simulations and described as a ``butterfly pattern" does not occur when $h/r \ge 0.2$, despite the dynamo action still operating efficiently.Item Non-Perturbative Methods in Quantum Field Theory and Quantum Gravity(2016) de la Fuente, Anton; Sundrum, Raman; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)This thesis considers non-perturbative methods in quantum field theory with applications to gravity and cosmology. In particular, there are chapters on black hole holography, inflationary model building, and the conformal bootstrap.