Theses and Dissertations from UMD

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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.

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    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.
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    Tracking Spectral Changes in Blazars with the Energetic Gamma Ray Experiment Telescope (EGRET)
    (2007-11-27) Nandikotkur, Giridhar; Goodman, Jordan; Jahoda, Keith M; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
    I analyze the entire blazar data from the Energetic Gamma Ray Experiment Telescope (EGRET) on board the Compton Gamma Ray Observatory (CGRO), using the skymaps that were regenerated to include the changes in performance during the mission. The sample of 98 sources consists of 66 flat spectrum radio quasars (FSRQs), 17 low-frequency peaked BL Lac objects (LBLs), 4 high-frequency peaked BL Lac objects (HBLs), 10 flat spectrum radio sources and 1 radio galaxy. I do not detect any clear pattern in the variation of spectral index with flux. Some of the blazars do not show any statistical evidence for spectral variability. The spectrum hardens with increasing flux in a few cases. There is also evidence for a flux-hardness anticorrelation at low fluxes in five blazars. I examine the EGRET spectral energy distribution (SED) for all the sources to identify these trends. I also observe a previously unreported spectral hysteresis in the spectral index Vs. flux space at weekly timescales, in all the three FSRQs for which data from flares lasting 3-4 weeks were available. All three sources show a counterclockwise rotation despite the widely different flux profiles. The time-averaged spectra of the HBLs are inconsistent with the predictions of the current theoretical models that have had success in describing simultaneous X-ray/TeV observations, and suggest additional components in the GeV band, as well as complex time variability. Current theoretical pictures explain the GeV emission as comptonization of the synchrotron photons in the jet, and predict hard spectra that should join smoothly with the TeV emission. The current analysis shows that the situation is more complex. The spectrum ranges from hard to soft during individual epochs, and the Mrk 421 SED shows a convex break in the aggregated data. The mission averaged EGRET spectrum for PKS~2155-304 also shows a similar (but not as pronounced) convex curvature. Simultaneous GLAST and X-ray observations of high X-ray states will address the issue of the convex curvature in the future. Such data will also explore the possibility of the steep EGRET emission originating from photons produced by electrons accelerated close to the limit of diffusive shock acceleration.