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ItemResonant and Secular Evolution of Three Body Systems – With Applications on Planetary Systems and Gravitational Wave Sources(Georgia Institute of Technology, 2023-07-24) Bhaskar, Hareesh GauthamThis work focuses on the gravitational interactions of astrophysical systems. In particular, we focus on the triple system dynamics, including mildly hierarchical three body secular dynamics, as well as precession induced resonances of binaries under the perturbation of a third companion. We apply our theoretical investigations of these physical processes to wide-orbit planetary systems and black hole binaries embedded in AGN disks. More specifically, we consider the secular dynamics of a test particle in a mildly-hierarchical configuration. We find the limit within which the secular approximation is reliable, present resonances and chaotic regions using surface of sections, and characterize regions of phase space that allow large eccentricity and inclination variations. Finally, we apply the secular results to the outer solar system. We focus on the distribution of extreme trans-neptunian objects (eTNOs) under the perturbation of a possible outer planet (Planet-9), and find that in addition to a low inclination Planet-9, a polar or a counter-orbiting one could also produce pericenter clustering of eTNOs, while the polar one leads to a wider spread of eTNO inclinations. Beyond the secular mildly hierarchical triple dynamics, we also propose a novel pathway through which compact binaries could merge due to eccentricity excitation, including in a near coplanar configuration. Mechanisms have been proposed to enhance the merger rate of stellar mass black hole binaries, such as the von Zeipel-Lidov-Kozai mechanism (vZLK). However, high inclinations are required in order to greatly excite the eccentricity and to reduce the merger time through vZLK. Specifically, a compact binary migrating in an AGN disk could be captured in a precession-induced resonance, when the apsidial and nodal precession rates of the binary are commensurable to the orbital period around the supermassive black hole. We find 8 such resonances upto quardupole order of the Hamiltonian. We show that if a binary is captured in these resonances and is migrating towards the companion, it can experience large eccentricity and inclination variations. Eccentricity is excited when the binary sweeps through the resonance which happens only when it migrates on a timescale 10-100 times the libration timescale of the resonance. Libration timescale decreases as the mass of the disk increases. The eccentricity excitation of the binary can reduce the merger timescale by a factor up to $10^{3−5}$.
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ItemElectromagnetic and Gravitational Wave Signatures of Massive Black Hole Binaries in Merger Galaxies(Georgia Institute of Technology, 2023-05-16) Li, KunyangMotivated by observational searches for potential gravitational-wave (GW) signals from massive black binary (MBHB) coalescences, we developed a model to describe orbital evolution of MBHBs. In this thesis, we use the model developed to determine how the properties of the merger remnant galaxy and the orbital configuration of MBHs affect the likelihood for and timescale to a coalescence. By varying galactic properties and orbital configurations of MBHBs in the model, we built a parameter space that contains 40,000 model galaxies, spans a wide range in initial orbital eccentricities and includes both prograde and retrograde orbits. We used these models to acquire a comprehensive view of how different types of orbital decay mechanisms impact the MBHB evolution. We estimated the LISA detection rates for different binary orbital configurations in the absence and presence of radiation feedback and explored the properties of MBHBs that are most likely to be detected as GW sources by applying the model on MBH pairs from the IllustrisTNG simulation. Finally, we use the model to quantify the electromagnetic (EM) detectability of dual active galactic nuclei (dAGNs). By tracking how the EM detectability varies with galaxy and orbital properties, we provided a convenient way to select dAGN candidates that evolve into GW sources. These kinds of predictions will be crucial for the future and present EM and GW observatories, for they will indicate where to look for possible MBH coalescences or the most detectable dAGNs.
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ItemNumerical Relativity Studies in Black Hole Astrophysics(Georgia Institute of Technology, 2021-08-27) Evans, Christopher WilliamBlack holes have long fascinated both physicists and the general population alike. In an astrophysical context, black holes participate in interesting interactions with not only stars, but also other black holes. Moreover, the recent detections of gravitational waves from both black hole-black hole and black hole-neutron star systems have only served to amplify excitement in the field of black hole astrophysics. Over the last few decades, numerical relativity has come to be a versatile tool for studying both of these classes of encounter. In this thesis, I present a collection of numerical relativity studies of black holes in the context of binary black hole mergers and tidal disruption events.