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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    MAGNETOHYDRODYNAMIC SIMULATIONS OF BLACK HOLE ACCRETION
    (2017) Avara, Mark James; Reynolds, Christopher S; McKinney, Jonathan; Astronomy; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
    Black holes embody one of the few, simple, solutions to the Einstein field equations that describe our modern understanding of gravitation. In isolation they are small, dark, and elusive. However, when a gas cloud or star wanders too close, they light up our universe in a way no other cosmic object can. The processes of magnetohydrodynamics which describe the accretion inflow and outflows of plasma around black holes are highly coupled and nonlinear and so require numerical experiments for elucidation. These processes are at the heart of astrophysics since black holes, once they somehow reach super-massive status, influence the evolution of the largest structures in the universe. It has been my goal, with the body of work comprising this thesis, to explore the ways in which the influence of black holes on their surroundings differs from the predictions of standard accretion models. I have especially focused on how magnetization of the greater black hole environment can impact accretion systems.
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    Probing the Central Regions of Active Galactic Nuclei
    (2014) Lohfink, Anne; Reynolds, Christopher S; Astronomy; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
    Active Galactic Nuclei (AGN) are one of the key players in the Universe. Their energy output can strongly affect the growth of their host galaxy and can promote or suppress star formation on galactic scales. Most of the processes that determine the power of an AGN as well as the form in which that power is released take place in the immediate surroundings of its supermassive black hole, a region that is still not entirely understood. A comprehension of these inner regions is, however, crucial to any ultimate understanding of the AGN's vast influence. This dissertation explores these close-in environments of the black hole using two approaches: X-ray spectroscopy and variability studies. We begin by summarizing our current understanding of why AGN play such a significant role in galaxy formation. This is followed by a discussion of why X-ray spectroscopy is one of the best means to investigate them. We point out that, in particular, the X-ray reflection spectrum is interesting as it can directly probe parameters such as the black hole spin or the inclination of the accretion disk. Since the reflection spectrum is a broad band component, that usually only contributes a fraction of the total observed X-ray flux, the entire X-ray spectrum requires careful modeling. To perform such modeling and gain access to the parameters of the reflection spectrum, we first select a target in which the spectral decomposition is simplified by the absence of absorption - the Seyfert 1 galaxy Fairall 9. We apply a multi-epoch fitting method that uses more than one spectrum at a time to get the best possible results on the parameters of the reflection spectrum that are invariant on human timescales. This technique enables us to tightly constrain the reflection parameters and leads us to conclude that Fairall 9 most likely possesses a composite soft X-ray excess, consisting of blurred reflection and a separate component such as Comptonization. The reflection spectrum also provides a way to enhance our knowledge of jet formation. We present a multi-wavelength study of the broad line radio galaxy 3C120 centered around a study of the reflection spectrum from two Suzaku and one XMM observation. Our results confirm that jet formation is linked to changes in (and possibly the destruction of) the inner accretion disk, and the high measured spin suggests that the rotational energy could very well be the energy source required to launch the jet. Finally, we present results from variability studies, which present another window into the processes taking place close to the black hole. A 10 year RXTE monitoring of Fairall 9 allows us to discover very rapid flux dips in the X-ray band which only last 5-15 days. While we are unable to determine the exact nature of the dips, we discuss a range of possible models, including the idea that the accretion disk in this radio-quiet AGN may be undergoing sporadic disruptions (via some yet-to-be-determined global instability) in much the same manner as is inferred to occur in 3C120 and other broad-line galaxies. Lastly we turn to the UV variability of Fairall~9 and its connection to the X-ray variability. From 2.5 months of Swift monitoring, we find that Fairall~9 shows significant variability on 4 day timescales, and the analysis of XMM-OM data shows that variability is present even on the time scales of hours. Folding in the X-ray variability, we determined that this fast UV variability can be explained as reprocessing of X-rays. We conclude by explaining how these studies fit into the field of AGN science as a whole and how they can be followed up with future observations.
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    Novel Techniques for Simulation and Analysis of Black Hole Mergers
    (2011) Boggs, William Darian; Tiglio, Manuel; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
    This dissertation consists of three research topics from numerical relativity: waveforms from inspiral mergers of black hole binaries, recoils from head-on mergers of black holes, and a new computational technique for error-reduction. The first two topics present research from journal articles that I coauthored with my colleagues in the NASA Goddard Numerical Relativity research group. Chapter 2 discusses a heuristic model of black hole binary mergers and the waveforms produced by them, based on simulations of nonspinning black holes. The gravitational radiation is interpreted as the result of an implicit rotating source that generates the radiation modes as the source multipoles rotate coherently. This interpretation of the waveform phase evolution provides a unified physical picture of the inspiral, plunge, and ringdown of the binaries, and it is the basis of an analytic model of the late-time frequency evolution. Chapter 3 presents a study of kicks in head-on black hole mergers, emphasizing the distinct contributions of spin and mass ratio, as well as their combined effects, to these radiation-induced recoils. The simpler dynamics of head-on mergers allow a more clear separation of the two types of kick and a validation of post-Newtonian predictions for the spin scaling of kicks. Finally, Chapter 4 presents a technique I developed to improve the accuracy of the field evolution in numerical relativity simulations. This "moving patches" technique uses local coordinate frames to minimize black hole motion and reduce error due to advection terms. In tests of the technique, I demonstrate reduction in constraint violations and in errors in the orbital frequency derived from the black holes' motions. I also demonstrate an accuracy gain in a new diagnostic quantity based on orbital angular momentum. I developed this diagnostic for evaluating the moving patches technique, but it has broader applicability. Though the moving patches technique has significant performance costs, these limitations are specific to the current implementation, and it promises greater efficiency and accuracy in the future.
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    How to Prove a Differential Form of the Generalized Second Law
    (2011) Wall, Aron Clark; Jacobson, Theodore A; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
    A new method is given for proving the semiclassical generalized second law (GSL) of horizon thermodynamics. Unlike previous methods, this method can be used to prove that entropy increases for arbitrary slices of causal horizons, even when the matter fields falling across the horizon are rapidly changing with time. Chapter I discusses how to define the GSL, and critically reviews previous proofs in the literature. Chapter II describes the proof method in the special case of flat planar slices of Rindler horizons, assuming the existence of a valid renormalization scheme. Chapter III generalizes the proof method to arbitrary slices of semiclassical causal horizons, by the technique of restricting the fields to the horizon itself. In the case of free fields it is clear that this restriction is possible, but for interacting fields the situation is murkier. Each of the three parts has been, or will be, separately published elsewhere.
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    SELF-FORCE AND NOISE-KERNEL IN CURVED SPACE-TIME USING QUASI-LOCAL EXPANSION METHODS
    (2007-04-29) Eftekharzadeh, Ardeshir; Hu, Bei-Lok B; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
    We find a quasi-local expansion for the tail term of the Green's function for a particle with scalar charge moving outside the event horizon of a black hole of mass M. To do that we use a WKB-like ansatz for the mode functions and we solve the resulted differential equation by iteration. We then sum the mode contributions using Plana sum rule. The fact that we find the tail term as an analytic expression is important. We then use our expressions to calculate the self-force exerted upon a particle of scalar charge that has been held at rest from infinite past to some time after which it moves on a general geodesic of the space-time. We perform this computation first for the radial path of a particle released from rest and then generalize the method for a particle launched on a general geodesic. We then turn to computing the noise kernel. The problem we are primarily concerned with is that of a massless, conformally coupled scalar field in the optical Schwarzschild (the ultrastatic spacetime conformal to the Schwarzschild black hole). In contrast to previous work done on this topic, we keep the two points separate, and as a result work with non-renormalized Wightman functions. We give an expression in terms of an expansion in coordinate separation and conclude with an outlook.