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

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    A Blind Search for Bursts of Very High Energy Gamma Rays with Milagro
    (2008-08-03) Vasileiou, Vlasios; Goodman, Jordan A; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
    Milagro is a water-Cherenkov detector that observes the extended air showers produced by cosmic gamma rays of energies E>100GeV. The effective area of Milagro peaks at energies E>10TeV, however it is still large even down to a few hundred GeV (~10m^2 at 100GeV). The wide field of view (~2sr) and high duty cycle (>90%) of Milagro make it ideal for continuously monitoring the overhead sky for transient Very High Energy (VHE) emissions. This study searched the Milagro data for such emissions. Even though the search was optimized primarily for detecting the emission from Gamma-Ray Bursts (GRBs), it was still sensitive to the emission from the last stages of the evaporation of Primordial Black Holes (PBHs) or to any other kind of phenomena that produce bursts of VHE gamma rays. Measurements of the GRB spectra by satellites up to few tens of GeV showed no signs of a cutoff. Even though multiple instruments sensitive to GeV/TeV gamma rays have performed observations of GRBs, there has not yet been a definitive detection of such an emission yet. One of the reasons for that is that gamma rays with energies E>100GeV are attenuated by interactions with the extragalactic background light or are absorbed internally at the site of the burst. There are many models that predict VHE gamma-ray emission from GRBs. A detection or a constraint of such an emission can provide useful information on the mechanism and environment of GRBs. This study performed a blind search of the Milagro data of the last five years for bursts of VHE gamma rays with durations ranging from 100 micro seconds to 316 seconds. No GRB localization was provided by an external instrument. Instead, the whole dataset was thoroughly searched in time, space, and duration. No significant events were detected. Upper limits were placed on the VHE emission from GRBs.
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    A Search for TeV Emission from Active Galaxies using the Milagro Observatory
    (2004-08-06) Hays, Elizabeth A; Sullivan, Gregory W; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
    Milagro is a unique instrument that observes very high energy gamma rays (100 GeV to 100 TeV) using the water-Cerenkov technique. The instrument has a large field of view, which covers the entire overhead sky (~2 sr). Located in northern New Mexico, Milagro observes most of the Northern Hemisphere over the course of a day. The high duty cycle (>90%) permits searches for TeV sources over a variety of time exposures. This thesis presents the results of two separate searches of the Milagro data for TeV emission. A real-time search of the entire field of view has been running since 2002. The real-time search provides early notification of significant transient behavior for time integrations from two hours to one month. No new gamma-ray sources have been found although two known sources are detected at significant levels. A longer and more refined search is conducted of archived data to monitor a set of active galaxies selected as TeV candidates. Active galaxies have been observed to be highly variable at TeV energies. To test for episodic emission, a data set is constructed using observations taken from December 2000 through September 2003 and divided into six sets of shorter time integrations. No significant emission is detected, and upper limits are set on the maximum time-averaged flux from each object for each time integration. The flux limits are also calculated to include the effects of absorption of TeV gamma rays by extragalactic background light. Predictions for the spectrum are used to set flux limits that can be compared to predictions for the flux from each source. The Milagro flux limits constrain some predictions for the TeV emission from the sources. The amount of absorption expected for two of the selected sources does not explain the discrepancy between the predictions and the flux upper limits. The implications of the constraints are discussed.