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.

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    Measurement of the cosmic-ray proton spectrum from 54 GeV to 9.5 TeV with the Fermi Large Area Telescope
    (2016) Green, David Michael; Hoffman, Kara; Hays, Elizabeth A; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
    Cosmic rays are a near-isotropic continuous flux of energetic particles of extraterrestrial origin. First discovered in 1912, cosmic rays span over 10 decades of energy and originate from Galactic and extragalactic sources. The Fermi Gamma-ray Space Telescope observations have recently confirmed supernova remnants (SNR) as a source class for Galactic cosmic-ray protons. Additionally, recent measurements made by AMS-02 of the cosmic-ray proton spectrum to 1.8 TeV in kinetic energy have shown an unexpected spectral break at 415 ± 117 GeV with a primary spectral index of −2.794±0.006 and a secondary spectral index of −2.702±0.047. The Fermi Large Area Telescope (LAT), one of two instruments on Fermi, has an ideal energy range for confirming a spectral break and extending a space-based cosmic-ray proton spectrum measurement to overlap with higher energy balloon-borne measurements. In this thesis, I present the measurement of the cosmic-ray proton spectrum from 54 GeV to 9.5 TeV with the Fermi-LAT. Using the LAT’s anti-coincidence detector and tracker as two independent measures of charge, I estimated a residual contamination in our proton data set of less than 5% primarily from cosmic-ray electrons and positrons. The LAT calorimeter provides an energy estimation of the electromagnetic fraction of an induced cosmic-ray proton shower. I use the charge and energy measurements to build instrument response functions, such as acceptance and response for the LAT, and measure cosmic-ray proton flux. I estimate the systematic uncertainties associated with the acceptance and the energy measurement. Using a broken power-law spectrum, I find a primary spectral index of −2.80 ± 0.03, a secondary spectral index of −2.60 ± 0.04, and an energy break of 467 ± 144 GeV. I discuss possible astrophysical and cosmic-ray physics interpretations for the observed spectral break.
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    Precursors in Gamma-ray Bursts Observed by Fermi
    (2015) Zhu, Sylvia; Shawhan, Peter; McEnery, Julie; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
    Gamma-ray bursts (GRBs) are some of the most energetic explosions in the universe. They come from the core collapses of massive stars and the mergers of compact objects, and are observed as bright flashes of gamma rays (prompt emission) followed by long-lived, fading emission (afterglow) across the electromagnetic spectrum. The instruments on the Fermi Gamma-ray Space Telescope provide excellent observations of GRBs across a large energy range. The Gamma-ray Burst Monitor (GBM, 8 keV to 40 MeV) is currently the most prolific detector of GRBs, and the Large Area Telescope (LAT, ∼20 MeV to >300 GeV) has opened up the field of GRB observations to high-energy gamma rays. In this thesis, I present studies on improving the LAT’s capability to detect GRBs onboard in realtime, and analyses of both a single, extraordinary burst (the record-breaking GRB 130427A) and the population of GBM GRBs with precursors in their lightcurves. In a small fraction of GRBs, a dim peak appears before the much brighter peaks that are normally observed during the prompt emission. I explore whether the properties of GRBs with precursors suggests that precursors have a distinct physical origin from the rest of the prompt emission, and discuss the implications for models of GRB precursor emission.