Physics
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Item Investigating the Origin of Gamma-ray Emission in Non-blazar AGN with the Fermi Large Area Telescope(2014) McConville, William Francis; Goodman, Jordan; McEnery, Julie; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)The Fermi Large Area Telescope (Fermi-LAT) has detected a small sample of gamma-ray loud non-blazar Active Galactic Nuclei (AGN), including the so-called misaligned AGN, whose radio jets are believed to be pointed off-axis with respect to the observer's line of sight, in contrast to the far more populous gamma-ray loud blazars, whose jets are pointed directly toward the line of sight of the observer. The origin of the gamma-ray emission in these misaligned sources has been widely attributed to the so-called "blazar zone" under the pretense of AGN unification, in which the misaligned Fanaroff-Riley type I and II objects are purported to make up the parent population of the BL Lac and FSRQ blazars, respectively. For a number of misaligned sources, the observations prove to be consistent under this scenario, in that the sources demonstrate short timescale gamma-ray variability, thus confining the emission region to a size scale consistent with the inner parsec-scale regions of the jet. Representing an even smaller percentage of non-blazar sources are those that exhibit no evidence of variable gamma-ray emission over timescales of > 3 years. Steady high energy (HE) emission over these timescales, if proven to be statistically significant, relaxes the constraint that would place the gamma-ray emission within a < 1 parsec region consistent with the size scale of the blazar zone. Three sources in particular that have demonstrated no evidence of variability in the LAT range are 4C+55.17, Fornax A, and M87. Each of these objects further demonstrates a unique set of multiwavelength properties that could potentially give rise to gamma-ray emission that is produced outside of the blazar zone. In this thesis, I conduct a detailed investigation into the origin of gamma-rays from each of these objects, and I discuss the multiwavelength properties that could give rise to a steady gamma-ray component consistent with non-blazar emission. Further improvements in LAT analysis techniques are also briefly discussed.Item 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.