Thesis (Ph. D.)--University of Michigan, 1916. / "Reprinted from the Publications of the Observatory of the University of Michigan, vol. II."

Thesis (Ph. D.)--University of Chicago, 1914. / "Private edition, distributed by the University of Chicago libraries, Chicago, Illinois." "Reprinted from the Astrophysical journal, vol. L, no. 3, October, 1919." Includes bibliographical references.

Thesis (Ph. D.)--University of Michigan, 1932. / Caption title. Thesis note on p. 9.

Thesis (PH. D.)--University of Michigan, 1916. / "Reprinted from the Publications of the Observatory of the University of Michigan, vol. II."

Thesis (Ph. D.)--University of Chicago, 1914. / "Private edition, distributed by the University of Chicago libraries, Chicago, Illinois." "Reprinted from the Astrophysical journal, vol. L, no. 3, October, 1919." Includes bibliographical references.

<p>This article-based dissertation provides a review on the broad subject of magnetars-their characteristics, giant flares (GFs) and associated observations of X-ray, gamma-ray, and radio emissions and their proposed physical mechanisms. The primary purpose of this dissertation is to provide an extensive description of the two research projects I undertook during my tenure as a Graduate Research Assistant, under the guidance of my advisor. Broadly, my research was focused on building analytical models and running three-dimensional (3-D), high-resolution magnetohydrodynamic (MHD) simulations using the astrophysical PLUTO code to investigate the physical mechanisms behind high-energy (X-ray and gamma-ray) and radio emissions associated with magnetar GFs using observational constraints. This, in turn, aided in either validating or disfavoring existing theories behind such energetic explosions.</p><p>Chapter 1 provides a review on magnetars, their GFs and associated high-energy and radio emissions, largely based on excellent reviews by [1]–[5]. I summarize interesting observational features of magnetars, specifically those of soft gamma-ray repeaters (SGRs) and anomalous X-ray pulsars (AXPs), along with known aspects of their X-ray and gamma-ray activity. I focus on the December 27, 2004 GF emitted by SGR 1806-20, the most energetic GF out of the three that occurred to date, describe its energetics and summarize existing theories behind the physical mechanisms that give rise to two emission characteristics associated with the GF - (i) quasi-periodic oscillations (QPOs) seen in the tail, and (ii) a radio afterglow detected a week after the GF. Lastly, I describe the methods I used to hypothesize the physical mechanisms behind QPOs and the radio emission and compare and contrast them with those suggested previously.</p><p>In chapter 2, I present a version of the research article in preparation and pending publication in the Monthly Notices of the Royal Astronomical Society. The work titled “Radio afterglow of magnetars’ giant flares”, undertaken under the supervision of Dr. Maxim Lyutikov and in collaboration with Dr. Maxim Barkov, explores the possible physical mechanisms behind the radio afterglow associated with the SGR 1806-20 GF using high-resolution 3-D MHD simulations.</p><p>In chapter 3, I present a version of the research article previously published by the Journal of Plasma Physics. The work titled “Tilting instability of magnetically confined spheromaks”, undertaken under the supervision of Dr. Maxim Lyutikov, in collaboration with Dr. Lorenzo Sironi and Dr. Maxim Barkov, investigates the tilting instability of a magnetically confined spheromak using 3-D MHD and relativistic particle-in-cell (PIC) simulations with an application to astrophysical plasmas, specifically to explain the QPOs arising in the tail of the SGR 1806-20 GF.</p><p>I summarize the main results and conclusions of the two research projects and describe future prospects in chapter 4, followed by appendices A and B which describe additional theoretical concepts and simulation results for a better understanding of the nature of radio afterglows associated with GFs, and structure of spheromaks. References are compiled after the appendices in order that they are first cited, followed by a brief autobiographical sketch, and a list of publications.<br></p>

Some of the brightest and most energetic events in the Universe are associated with the death of stars. These stellar deaths power transient electromagnetic emission which are routinely observed on Earth. This dissertation presents our research on various such transients. Its topics includes, supernova remnants, kilonovae, gamma-ray bursts (GRBs): The "long'' type produced from core-collapse supernovae and the "short'' type associated with neutron star merger events. It also focuses on the disruption of stars by the tidal forces of supermassive black holes i.e., tidal disruption events (TDEs). We model the emission from these transients and compare them to observations in order to draw a number of conclusions and make predictions for future detections. For example, we find that the non-thermal emission from supernovae and kilonovae associated with GRBs can produce long term emission which may be detected as a re-brightening in the overall emission. The sharp cut off observed in some TDE flares can be caused by a pre-existing accretion disk present around a supermassive black hole, which is expected in active galactic nuclei. Our work successfully predicted the nature of the very first electromagnetic detection from a neutron star merger, and was able to reproduce the emission that had been observed for more than one hundred days after the merger. This dissertation also provides frameworks on how the observable features of these transients can be leveraged to probe the properties of the progenitor system and their environment. <br>