Theses and Dissertations from UMD
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Item Optical Time Domain and Radio Imaging Analyses of the Dynamic Hearts of AGN(2017) Smith, Krista Lynne; Mushotzky, Richard; Astronomy; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Active galactic nuclei (AGN) are among the most extreme objects in the universe: galaxies with a central supermassive black hole feeding on gas from a hot accretion disk. Despite their potential as powerful tools to study topics ranging from relativity to cosmology, they remain quite mysterious. In the first portion of this thesis, we explore how an AGN may influence the formation of stars in its host galaxy. Using high-resolution 22 GHz radio imaging of an X-ray selected sample of radio-quiet AGN, we find that the far-infrared radio correlation for normal star forming galaxies remains valid within a few hundred parsecs of the central engine. Because the core flux is often spatially isolated from star formation, we can also determine that the radio emission in radio-quiet AGN is consistent with both coronal and disk-jet coupling models. Finally, we find that AGN with jet-like radio morphologies have suppressed star formation, possibly indicating ongoing feedback. The second portion of this thesis uses optical AGN light curves to study the physics of accretion. The Kepler spacecraft produces groundbreaking light curves, but its fixed field of view only contained a handful of known AGN. We conduct an X-ray survey of this field, yielding 93 unique X-ray sources identified by optical follow-up spectroscopy as a mixture of AGN and stars. For the AGN, we spectroscopically measure black hole masses and accretion rates. We then analyze a sample of 22 Kepler AGN light curves. We develop a customized pipeline for AGN science with Kepler, a necessary step since the initial data was optimized for the unique goal of exoplanet detection. The light curves display an astonishing variety of behaviors in a new regime of optical variability inaccessible with previous facilities. We find power spectral slopes inconsistent with the damped random walk model, characteristic variability timescales, correlations of variability properties with physical parameters, and bimodal flux distributions possibly consistent with passing obscuring material. We also conclude that this regime of optical variability is not produced by simple X-ray reprocessing. Finally, we explain how this work supports future robust accretion studies with upcoming large timing surveys.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 Hydrodynamic Models of AGN Feedback in Cooling Core Clusters(2008-05-20) Vernaleo, John C.; Reynolds, Christopher S.; Astronomy; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)X-ray observations show that the Intra Cluster Medium (ICM) in many galaxy clusters is cooling at a rapid rate, often to the point that it should have radiated away all of its energy in less than the age of the cluster. There is however a very clear lack of enough cool end products of this gas in the centers of the clusters. Energetic arguments indicate that Active Galactic Nuclei (AGN) should be capable of heating the inner regions of clusters enough to offset the radiative cooling; truncating massive galaxy formation and solving the cooling flow problem. We present three sets of high resolution, ideal hydrodynamic simulations with the ZEUS code to test this AGN heating paradigm. For the first set of simulations, we study the dependence of the interaction between the AGN jets and the ICM on the parameters of the jets themselves. We present a parameter survey of two-dimensional (axisymmetric) models of back-to-back jets injected into a cluster atmosphere. We follow the passive evolution of the resulting structures. These simulations fall into roughly two classes, cocoon-bounded and non-cocoon bounded. We find that the cocoon-bounded sources inject significantly more entropy into the core regions of the ICM atmosphere, even though the efficiency with which the energy is thermalized is independent of the morphological class. In all cases, a large fraction of the energy injected by the jet ends up as gravitational potential energy due to the expansion of the atmosphere. For the second set, we present three-dimensional simulations of jetted AGN that act in response to cooling-mediated accretion of an ICM atmosphere. We find that our models are incapable of producing a long term balance of heating and cooling; catastrophic cooling can be delayed by the jet action but inevitably takes hold. At the heart of the failure of these models is the formation of a low density channel through which the jet can freely flow, carrying its energy out of the cooling core. Finally, we present a set of simulations with both feedback and precessing jets. The addition of jet precession is not sufficient to couple the jets to the ICM energetically although it can deposit a large amount of energy in sound waves. These sound waves are lost to the system in ideal hydrodynamics, but ultimately may provide a powerful heating mechanism for clusters cores by AGN when additional physical effects are taken into account.Item A Decade of Rossi X-ray Timing Explorer Seyfert Observations:An RXTE Seyfert Spectral Database(2008-04-24) Mattson, Barbara; Weaver, Kimberly; Reynolds, Christopher; Astronomy; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)With over forty years of X-ray observations, we should have a grasp on the X-ray nature of active galactic nuclei (AGN). The unification model of Antonucci and Miller (1985) offered a context for understanding observations by defining a "typical" AGN geometry, with observed spectral differences explained by line-of-sight effects. However, the emerging picture is that the central AGN is more complex than unification alone can describe. We explore the unified model with a systematic X-ray spectral study of bright Seyfert galaxies observed by the Rossi X-Ray Timing Explorer (RXTE) over its first 10 years. We develop a spectral-fit database of 821 time-resolved spectra from 39 Seyfert galaxies fitted to a model describing the effects of an X-ray power-law spectrum reprocessed and absorbed by material in the central AGN region. We observe a relationship between radio and X-ray properties for Seyfert 1s, with the spectral parameters differing between radio-loud and radio-quiet Seyfert 1s. We also find a complex relationship between the Fe K equivalent width (EW) and the power-law photon index (Gamma) for the Seyfert 1s, with a correlation for the radio-loud sources and an anti-correlation for the radio-quiet sources. These results can be explained if X-rays from the relativistic jet in radio-loud sources contribute significantly to the observed spectrum. We observe scatter in the EW-Gamma relationship for the Seyfert 2s, suggesting complex environments that unification alone cannot explain. We see a strong correlation between Gamma and the reflection fraction (R) in the Seyfert 1 and 2 samples, but modeling degeneracies are present, so this relationship cannot be trusted as instructive of the AGN physics. For the Seyfert 1 sample, we find an anticorrelation between EW and the 2 to 10 keV luminosity (LX), also known as the X-ray Baldwin effect. This may suggest that higher luminosity sources contain less material or may be due to a time-lag effect. We do not observe the previously reported relationship between Gamma and the ratio of LX to the Eddington luminosity.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.