<p> Cosmic rays, with an energy density of &sim; 1eVcm^&ndash;3, play an important role in the evolution of our Galaxy. Very high energy (TeV) gamma rays provide unique information about the acceleration sites of Galactic cosmic rays. The High Altitude Water Cherenkov (HAWC) Gamma-Ray Observatory is an all-sky surveying instrument sensitive to gamma rays from 100,GeV to 100,TeV with a 2steradian instantaneous field of view and a duty cycle of >95%. The array is located in Sierra Negra, Mexico at an elevation of 4,100m and was inaugurated in March 2015. Thanks to its modular design, science operation began in Summer 2013 with one third of the array. Using this data, a survey of the inner Galaxy region of Galactic longitude <i>l</i> &isin; [+15&deg;, +50&deg;] and latitude <i>b</i> &isin; [&ndash;4&deg;, +4&deg;] is performed. To address the ambiguities arising from unresolved sources in the data, a maximum likelihood technique is used to identify point source candidates. Ten sources and candidate sources are identified in this analysis. Eight of these are associated with known TeV sources but not all have differential fluxes compatible with previous measurements. Three sources are detected with significances >5&sigma; after accounting for statistical trials, and are associated with known TeV sources. With data taken with the full array and improved reconstruction algorithms, the significance on the Crab nebula increases from 3.1&sigma;&radic;day to 5.5&sigma;&radic;day, which allows more sensitive sky surveys and more precise spectral and morphological analyses on individual sources.</p>

<p>Variable star analysis and classification is an important task in the understanding of stellar features and processes. While historically classifications have been done manually by highly skilled experts, the recent and rapid expansion in the quantity and quality of data has demanded new techniques, most notably automatic classification through supervised machine learning. I present a study on variable stars in the Kepler field using these techniques, and the novel work of unsupervised learning. I use new methods of characterization and multiple independent classifiers to produce an ensemble classifier that equals or matches existing classification abilities. I also explore the possibilities of unsupervised learning in making novel feature discovery in stars.

<p> Gamma-ray bursts (GRBs) are some of the most energetic explosions in the universe. They come from the core collapses of massive stars and the mergers of compact objects, and are observed as bright flashes of gamma rays (prompt emission) followed by long-lived, fading emission (afterglow) across the electromagnetic spectrum. The instruments on the <i>Fermi</i> Gamma-ray Space Telescope provide excellent observations of GRBs across a large energy range. The Gamma-ray Burst Monitor (GBM, 8 keV to 40 MeV) is currently the most prolific detector of GRBs, and the Large Area Telescope (LAT, &sim;20 MeV to >300 GeV) has opened up the field of GRB observations to high-energy gamma rays. </p><p> In this thesis, I present studies on improving the LAT&rsquo;s capability to detect GRBs onboard in realtime, and analyses of both a single, extraordinary burst (the record-breaking GRB 130427A) and the population of GBM GRBs with precursors in their lightcurves. In a small fraction of GRBs, a dim peak appears before the much brighter peaks that are normally observed during the prompt emission. I explore whether the properties of GRBs with precursors suggests that precursors have a distinct physical origin from the rest of the prompt emission, and discuss the implications for models of GRB precursor emission. </p>

<p> I present four papers exploring different topics in the area of characterizing the atmospheric and bulk properties of extrasolar planets. In these papers, I present two new codes, in various forms, for modeling these objects. A code to generate theoretical models of transit spectra of exoplanets is featured in the first paper and is refined and expanded into the APOLLO code for spectral modeling and parameter retrieval in the fourth paper. Another code to model the internal structure and evolution of planets is featured in the second and third papers. The first paper presents transit spectra models of GJ 1214b and other super-Earth and mini-Neptune type planets&mdash;planets with a &ldquo;solid&rdquo;, terrestrial composition and relatively small planets with a thick hydrogen-helium atmosphere, respectively&mdash;and fit them to observational data to estimate the atmospheric compositions and cloud properties of these planets. The second paper presents structural models of super-Earth and mini-Neptune type planets and estimates their bulk compositions from mass and radius estimates. The third paper refines these models with evolutionary calculations of thermal contraction and ultraviolet-driven mass loss. Here, we estimate the boundaries of the parameter space in which planets lose their initial hydrogen-helium atmospheres completely, and we also present formation and evolution scenarios for the planets in the Kepler-11 system. The fourth paper uses more refined transit spectra models, this time for hot jupiter type planets, to explore the methods to design optimal observing programs for the <i>James Webb Space Telescope</i> to quantitatively measure the atmospheric compositions and other properties of these planets.</p>

<p> The Antarctic Impulsive Transient Antenna (ANITA) is a balloon-based experiment designed to search for ultra-high energy(UHE) neutrinos and cosmic rays in Antarctica. A successful detection would be an important step in understanding the most energetic cosmic accelerators in the universe. The fourth flight of ANITA (ANITA-IV) funded by NASA took place in December 2016. It uses a radio antenna array designed to detect Askaryan radiation from UHE neutrino-induced showers in ice and geomagnetic radiation from Extensive Air Showers (EAS) induced by cosmic rays. </p><p> In this dissertation, I present my analysis of the ANITA-IV flight data with the objective to search for ultra-high energy neutrino and cosmic ray events. The data analysis includes six major aspects: instrument calibrations, event direction reconstruction, event quality cuts, thermal events cut, anthropogenic events cut (clustering), and background estimation. Twenty-four cosmic ray candidates and one neutrino candidate events were found each with an estimated background of 0.34 event. Although the signal significance for a single neutrino event is consistent with background, it still leads to the world's best limit on the ultra-high energy neutrino flux for energy above 4 &times; 10¹⁹ eV.</p><p>

Accretion theory is essential for understanding a multitude of varied astronomical observations such as X-ray binaries, active galactic nuclei and gamma-ray bursts. In this document three works on accretion will be presented.The first one is on the structure of an inviscid accretion disc with small angular momentum around a rotating black hole. This regime of accretion may occur in X-ray binaries and GRBs. The second work is on neutrino antineutrino annihilation in the vicinity of a hyper-accreting black hole. This work is relevant for the study of GRBs, in particular it singles out when neutrinos may be responsible for powering GRBs. The third work studies the tidal stripping of a white dwarf spiraling into a massive black hole. The stripped matter accretes onto the black hole producing a transient emission, presumably periodic, observable with X-ray and optical telescopes. At the same time the white dwarf emits gravitational waves as it spirals into the black hole.

Neutron stars and black holes are generally surrounded by magnetospheres of highly conducting plasma in which the magnetic flux density is so high that hydrodynamic forces are irrelevant. In this vanishing-inertia---or ultra-relativistic---limit, magnetohydrodynamics becomes force-free electrodynamics, a system of equations comprising only the magnetic and electric fields, and in which the plasma response is effected by a nonlinear current density term. In this dissertation I describe a new pseudospectral simulation code, designed for studying the dynamic magnetospheres of compact objects. A detailed description of the code and several numerical test problems are given. I first apply the code to the aligned rotator problem, in which a star with a dipole magnetic field is set rotating about its magnetic axis. The solution evolves to a steady state, which is nearly ideal and dissipationless everywhere except in a current sheet, or magnetic field discontinuity, at the equator, into which electromagnetic energy flows and is dissipated. Magnetars are believed to have twisted magnetospheres, due to internal magnetic evolution which deforms the crust, dragging the footpoints of external magnetic field lines. This twisting may be able to explain both magnetars' persistent hard X-ray emission and their energetic bursts and flares. Using the new code, I simulate the evolution of relativistic magnetospheres subjected to slow twisting through large angles. The field lines expand outward, forming a strong current layer; eventually the configuration loses equilibrium and a dynamic rearrangement occurs, involving large-scale rapid magnetic reconnection and dissipation of the free energy of the twisted magnetic field. When the star is rotating, the magnetospheric twisting leads to a large increase in the stellar spin-down rate, which may take place on the long twisting timescale or in brief explosive events, depending on where the twisting is applied and the history of the system. One such explosive field-expansion and reconnection event may have been responsible for the 27 August 1998 giant flare from SGR 1900+14, and the coincident sudden increase in spin period, or "braking glitch." The inner magnetospheres of relativistic compact objects are in strongly curved spacetimes. I describe the extension of the code to general-relativistic simulations, including the hypersurface foliation method and the 3+1 equations of force-free electrodynamics in curved, evolving spacetimes. A simple test problem for dynamical behavior in the Schwarzschild metric is presented, and the evolutions of the magnetospheres surrounding neutron stars and black holes, in vacuum and in force-free plasma, are compared.

Recent studies have shown that the number counts of peaks in weak lensing (WL) surveys contain significant cosmological information. Motivated by this finding, in the first part of the thesis, we address two questions: (i) what is the physical origin of WL peaks; and (ii) how much information do the peaks contain beyond the traditional cosmological WL observable (the power spectrum). To investigate the first question, we use a suite of ray-tracing N-body simulations, in which we identify individual dark matter halos. We study the halos' contribution to the peaks. We find that high peaks are typically dominated by a single massive halo, while low peaks are created by galaxy shape noise, but with an important contribution from a line-of-sight projection of typically 4-8 halos. For the second question, we first compare the cosmological peak count distributions to those in a Gaussian random field. We find significant differences, both in the peak-count distributions themselves, as well as in how the distributions depend on cosmology, demonstrating that the peaks contain non-Gaussian information. To explicitly quantify the information content of the peaks beyond the power spectrum, we use the Fisher matrix method to forecast errors in the three-dimensional parameters space (σ_8, w, Ω_m). We find that when we combine the peaks and the power spectrum, the marginalized errors are a factor of about two smaller than from power spectrum alone. In the second part of the thesis, we address a major theoretical systematic error: the presence of baryons -- not included in the N-body simulations -- can affect the WL statistics (both peaks and power spectrum), and the inferred cosmological parameters. We apply a simplified model, which mimics the cooling and condensation of baryons at the centers of dark matter halos. In particular, we manually steepen the density profile of each dark matter halo identified in the N-body simulations, and repeat the ray-tracing procedure create WL maps in mock "baryonic'' universes. We then compare the peak count distributions and power spectra in these baryonic models to those from the pure DM models. We find that there is a large increase in the number of high peaks, but low peaks -- which contain most of the cosmological information -- are robust to baryons. Similarly, we find that the high--l power spectrum is increased, but the change in the low--l power spectrum is relatively modest. We then utilize a Monte Carlo approach to compute the joint, and in general, biased constraints on σ_8, w, Ω_m when the baryonic model is fit by the pure DM models. We find that: (i) constraints obtained from low peaks are nearly unbiased; (ii) high peaks yield large biases, but in different directions in parameter space than the biases from the power spectrum. Our first finding suggests it may be advantageous to use low peaks for analysis until the baryonic processes are better understood. However, our second finding suggests the possibility of "self-calibration'': simultaneously fitting astrophysical "nuisance'' parameters (describing lensing halo profiles) with cosmological parameters.

abstract: This thesis contains an overview, as well as the history of optical interferometers. A new approach to interferometric measurements of stars is proposed and explored. Modern updates to the classic techniques are described along with some theoretical derivations showing why the method of single photon counting shows significant promise relative to the currently used amplitude interferometry. Description of a modular intensity interferometer system using commercially available single-photon detectors is given. Calculations on the sensitivity and \emph{uv}-plane coverage using these modules mounted on existing telescopes on Kitt Peak, Arizona is presented. Determining fundamental stellar properties is essential for testing models of stellar evolution as well as for deriving physical properties of transiting exoplanets. The proposed method shows great promise in measuring the angular size of stars. Simulations indicate that it is possible to measure stellar diameters of bright stars with AB magnitude <6 with a precision of >5% in a single night of observation. Additionally, a description is given of a custom time-to-digital converter designed to time tag individual photons from multiple single-photon detectors with high count rate, continuous data logging, and low systematics. The instrument utilizes a tapped-delay line approach on an FPGA chip which allows for sub-clock resolution of <100 ps. The TDC is implemented on a Re-configurable Open Architecture Computing Hardware Revision 2 (ROACH2) board which allows for continuous data streaming and time tagging of up to 20 million events per second. The functioning prototype is currently set-up to work with up to ten independent channels. Laboratory characterization of the system, including RF, pick up and mitigation, as well as measurement of in-lab photon correlations from an incoherent light source (artificial star), are presented. Additional improvements to the TDC will also be discussed, such as improving the data transfer rate by a factor of 10 via an SDP+ Mezzanine card and PCIe 2SFP+ 10 Gb card, as well as scaling to 64 independent channels. Furthermore, a modified nulling interferometer with image inversion is proposed, for direct imaging of exoplanets below the canonical Rayleigh resolution limit. Image inversion interferometry relies on splitting incoming radiation from a source, either spatially rotating or reflecting the electric field from one arm of the interferometer before recombining the signals and detecting the resulting images in the two output ports with an array of high-speed single-photon detectors. Sources of incoming radiation that have cylindrical symmetry and are centered on the rotation axis will cancel in one of the output ports and add in the other output port. The ability to suppress light from a host star, as well as the ability to resolve past the Rayleigh limit, enables sensitive detection of exoplanets from a stable environment without the need for a coronagraph. The expected number of photons and the corresponding variance in the measurement for different initial contrast ratios are shown, with some first-order theoretical instrumental errors. Lastly, preliminary results from a sizeable photometric survey are presented. This survey is used to derive bolometric flux alongside from angular size measurements and the effective stellar temperatures. / Dissertation/Thesis / Doctoral Dissertation Astrophysics and Astronomy 2018

published_or_final_version / Physics / Master / Master of Philosophy