UMD Theses and Dissertations
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Item An All-Sky Search for Bursts of Very High Energy Gamma Rays with HAWC(2016) Wood, Joshua Randall; Goodman, Jordan; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)A new ground-based wide-field extensive air shower array known as the High-Altitude Water Cherenkov (HAWC) Observatory promises a new window to monitoring the ~100 GeV gamma-ray sky with the potential for detecting a high energy spectral cutoff in gamma-ray bursts (GRBs). It represents a roughly 15 times sensitivity gain over the previous generation of wide-field gamma-ray air shower instruments and is able to detect the Crab Nebula at high significance (>5 sigma) with each daily transit. Its wide field-of-view (~2 sr) and >95% uptime make it an ideal instrument for detecting GRB emission at ~100 GeV with an expectation for observing ~1 GRB per year based on existing measurements of GRB emission. An all-sky, self-triggered search for VHE emission produced by GRBs with HAWC has been developed. We present the results of this search on three characteristic GRB emission timescales, 0.2 seconds, 1 second, and 10 seconds, in the first year of the fully-populated HAWC detector which is the most sensitive dataset to date. No significant detections were found, allowing us to place upper limits on the rate of GRBs containing appreciable emission in the ~100 GeV band. These constraints exclude previously unexamined parameter space.Item 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.Item A search for muon neutrinos coincident with Gamma-ray Bursts with the IceCube 59-String detector(2011) Redl, Peter Christian; Sullivan, Gregory; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Gamma-Ray Bursts (GRBs) are believed to be prime candidates to produce the cosmic ray flux above 10^18 eV. Cosmic rays are deflected by galactic and inter-galactic magnetic fields and do not point back to their source, therefore cosmic ray observations cannot confirm or rule out GRBs as a source. Leading theories predict that if GRBs are indeed responsible for the highest energy cosmic rays, then they would produce a detectable TeV-scale neutrino flux in a km^3 sized neutrino detector. Neutrinos are not deflected by magnetic fields and point back to their source, making it possible to correlate a neutrino flux with its source. The detection of a neutrino flux from GRBs would be strong evidence that GRBs are a source of the highest energy cosmic rays. IceCube is the first km^3 sized neutrino detector in the world and is therefore sensitive to the predicted TeV neutrino flux from GRBs. The finished detector consists of 5160 light-sensitive Digital Optical Modules (DOM) arranged on 86 Strings. There are 60 DOMs on a single string deployed at depths between 1450 and 2450 meters below the surface. The first IceCube String was deployed during the South Pole summer of 2004-2005 with construction of the IceCube detector finishing during the austral summer of 2011. The results presented here are from the 59-string detector, which operated from May 2009 to May 2010. IceCube is able to detect charged particles moving through its instrumented volume near the speed of light by detecting the Cherenkov light given off by those charged particles. Muon and anti-muon neutrinos produce secondary muons if they interact with a nucleon. If this interaction happens in or near the instrumented volume IceCube can detect those secondary muons. By searching for a neutrino signal coincident in time and space with satellite detected gamma rays from GRBs, the analysis presented here pushes the sensitivity for neutrinos from GRBs to 0.46 times the theoretically predicted neutrino flux. The result is combined with the previous search and a combined 90% upper limit of 0.22 times the theoretical predicted flux is set. The implication of this stringent limit on the model is discussed and future IceCube sensitivities are outlined. IceCube is the largest neutrino detector in the world and with this result has entered the era of neutrino astrophysics by constraining long standing astrophysical neutrino production models.Item A Search for Muon Neutrinos from Gamma-Ray Bursts wih the IceCube 22-String Detector(2009) Roth, A Philip; Hoffman, Kara; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Two searches are conducted for muon neutrinos from Gamma-Ray Bursts (GRBs) using the IceCube detector. Gamma-Ray Bursts are brief and transient emissions of keV/MeV radiation occuring with a rate of a few per day uniformly in the sky. Swift and other satellites of the Third Interplanetary Network (IPN3) detect these GRBs and send notices out via the GRB Coordinate Network (GCN). The fireball model describing the physics of GRBs predicts the emission of muon neutrinos from these bursts. IceCube is a cubic kilometer neutrino detector buried in the deep antarctic ice at the South Pole that can be used to find these prediceted but still unobserved neutrinos. It is sensitive to them by detecting Cherenkov light from secondary muons produced when the neutrinos interact in or near the instrumented volume. The construction of IceCube has been underway since the austral summer of 2004-2005 and will continue until 2011. The growing IceCube detector will soon be sensitivite to the high energy neutrino emission from GRBs that is predicted by the fireball model. A blind and triggered search of the 22-string IceCube data for this neutrino emission was conducted. The principal background to the observation of neutrinos in IceCube is muons generated in cosmic-ray air-showers in the atmosphere above the detector. Atmospheric neutrinos make up a separate irreducible background to the detection of extraterrestrial neutrinos. A binned stacked search of 41 bursts occuring in the northern hemisphere greatly reduces the muon background by looking for tracks moving up through the detector. The atmospheric neutrino background is greatly reduced by the temporal constraints of the search, making it effectively background free. 40 individual unbinned searches of bursts occuring in the southern hemisphere extend IceCube's sensitivity to the higher background regions above the horizon. No significant excesses over background expectations are found in either search. A 90% confidence upper limit on the neutrino fluence from northern hemisphere bursts is set at 6.52 x 10-3 erg cm-2 with 90% of the expected signal between 87.9 TeV and 10.4 PeV.Item Early Afterglow Evolution of X-Ray Flashes Observed by Swift(2006-03-08) Hullinger, Derek; Boldt, Elihu; Parsons, Ann; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Gamma-ray bursts (GRBs) are bright flashes of gamma-ray energy that originate in distant galaxies and last only a matter of seconds before fading away, never to appear again. They are accompanied by longer-wavelength "afterglows" that fade away much more gradually and can be detected for up to several days or even weeks after the gamma-ray burst has vanished. In recent years, another phenomenon has been discovered that resembles gamma-ray bursts in almost every way, except that the radiated energy comes mostly from x-rays instead of gamma rays. This new class of bursts has been dubbed "x-ray flashes" (XRFs). There is strong evidence to suggest that GRBs and XRFs are closely-related phenomena. The Swift mission, launched in November of 2004, is designed to answer many questions about GRBs and their cousins, XRFs--where they come from, what causes them, and why gamma-ray bursts and x-ray flashes differ. The key to the Swift mission is its ability to detect and determine the location of a burst in the sky and then autonomously point x-ray and optical telescopes at the burst position within seconds of the detection. This allows the measurement of the afterglow within 1 - 2 minutes after the burst, rather than several hours later, as was necessary with past missions. This early afterglow measurement is an important key to distinguishing between different theories that seek to explain the differences between XRFs and GRBs. This dissertation describes the calibration of the Burst Alert Telescope, which measures the spectral and temporal properties of GRBs and XRFs. It also presents a study of XRFs and GRBs detected by Swift, including the first analysis and comparison of the early afterglow properties of these phenomena. This study reveals interesting differences between the temporal properties of GRB and XRF afterglows and sets strong constraints on some theories that seek to explain XRF origins.Item A Search for Short Duration Very High Energy Emission from Gamma-Ray Bursts(2005-08-29) Noyes, David Carl; Sullivan, Gregory W; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Milagro is a water-Cherenkov detector capable of observing air showers produced by gamma rays with primary energies of approximately 100 GeV and higher. The wide field of view (~ 2 sr) and high duty cycle (>90%) of Milagro make it ideal for searching for transient emission from gamma-ray bursts (GRBs). The median energy of photons detected by Milagro is a few TeV, but the effective area is still relatively large at a few hundred GeV (~50 m^2 at 100 GeV). This results in a gamma-ray fluence sensitivity comparable to previous satellite detectors at keV energies. Measurements have been made of GRB spectra up to a few tens of GeV with no sign of a cutoff, however much is still unknown about the nature and existence of this Very High Energy (VHE) component. Additionally, gamma/gamma absorption from infrared background photons or from the optically thick region of the burst source complicate observations of this VHE component. However, many models predict VHE emission from GRBs through mechanisms such as synchrotron self-Compton processes. In the absence of a GRB localization provided by another instrument, the Milagro data is searched independently for VHE emission from GRBs. In 2.3 years of searching for bursts with durations ranging from 250 us to 40 s, no significant evidence was observed for VHE emission from GRBs. Models for different GRB parameters (such as redshift and isotropic energy distributions) are used to constrain the VHE spectrum of GRBs.