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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Item From Tantrums to Transformations: AGN Transients Discovered with the Zwicky Transient Facility(2021) Frederick, Sara; Gezari, Suvi; Astronomy; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)This dissertation work has consisted of searches for extreme AGN-related outbursts during Phase I of the Zwicky Transient Facility (ZTF) survey, which has been a ground-breaking wide-field instrument for the real-time detection and regular cadence monitoring of transients in the Northern Sky. Transients found to be nuclear through photometric filtering were vetted by humans and coordinated for prompt follow-up with various rapid robotic, spectroscopic, and high energy resources, to understand the nature of the galaxy centers undergoing flares and the appearance of spectral features. Findings from this unprecedentedly high-volume data stream were often serendipitous, and led to surprising new avenues for study, including 1) the establishment of a new observational class of quiescent galaxies caught turning into quasars, 2) the discovery of a preponderance of smooth and high-amplitude optical transients hosted in NLSy1s, and 3) a framework for distinguishing extreme AGN variability from other transients in AGN. We present the results of these observations, including candidates for TDEs in AGN, changing-look AGN caught "turning-on", as well as members of the new emerging observational class of flares in Narrow-Line Seyfert 1 (NLSy1) galaxies associated with enhanced accretion (Trakhtenbrot et al. 2019). We compared the properties of these samples of flares to previously reported changing-look quasars and Seyfert galaxies, confirmed that they are a unique observational class of transients related to physical processes associated with the central supermassive black hole's accretion state, and considered the observations in the context of the physical interpretations for a range of related transients from the literature. With these unique sample sets, we also aim to understand why we have found certain galaxy types to preferentially host the sites of such rapid enhanced flaring activity, and attempt to map out the innermost environment of the accretion events. These pathfinding studies enabled with ZTF have the potential to guide how these exceptional moments of AGN evolution will be systematically discovered in future large area surveys such as the Vera C. Rubin Observatory.Item Detecting High-Energy Emission from Gamma-Ray Bursts with EGRET and GLAST(2005-07-25) Wren, David Nathan; Ritz, Steven M; Sullivan, Gregory W; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)The research described in this dissertation explores the detection of high-energy emission from gamma-ray bursts (GRBs) with EGRET and GLAST. Data from the EGRET experiment were searched for evidence of ~1-250 MeV emission that preceded or followed gamma-ray bursts on a time scale of hours. This led to the discovery of a gamma-ray burst with high-energy, post-quiescent emission from the prompt phase that was coincident with lower-energy (keV) emission. To do detailed event filtering studies for the GLAST Large Area Telescope (LAT), the flight software event filter was embedded in the standard science analysis environment. The event trigger rate, reasons why it must be reduced, and hardware-level methods of reducing it are studied. Much work was done to improve the performance of the prototype event filter, and additional work was done to develop algorithms to allow the LAT to distinguish Earth albedo photons from celestial gamma-rays, and to eliminate albedo events from the data stream. It is shown that it is possible to reduce the background rate to meet LAT mission requirements, while simultaneously keeping the gamma-ray acceptance rate high enough to exceed the relevant LAT requirements for those events. Using the onboard event filter, real-time, onboard, gamma-ray burst detection was then studied. A detection algorithm had been developed by members of the LAT collaboration, but the algorithm required a lower onboard background rate than the basic LAT requirement for downlink, in addition to knowledge of incident gamma-ray directions. Therefore, several methods of reducing the background rate to acceptable levels were provided, and onboard track reconstruction methods were created and tested. GRB detection was tested for two background filters and two track reconstruction methods for simulated bursts that had realistic light curves and spectral characteristics. With prototype background cuts, track reconstruction, and burst detection algorithms, the LAT burst detection requirements were exceeded. Suggestions were offered about how to enhance burst detection performance in the coming months before GLAST is launched.