Theses and Dissertations from UMD
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Item Radiative Plasmas in Pulsar Magnetospheres(2024) Chernoglazov, Alexander; Philippov, Alexander; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Pulsars are highly magnetized rotating neutron stars known for their periodic bursts of radio emission. Decades of astronomical observations revealed that pulsars produce non-thermal radiation in all energy bands, from radio to gamma rays, covering more than 20 decades in photon energy. Modern theories consider strongly magnetized relativistic electron-positron plasmas to be the source of the observed emission. In my Thesis, I investigate physical processes that can be responsible for plasma production and the observed high-energy emission in the wide range of photon energies, from eV to TeV. In the first Chapter of my Thesis, I investigate relativistic magnetic reconnection with strong synchrotron cooling using three-dimensional particle-in-cell kinetic plasma simulations. I characterize the spectrum of accelerated particles and emitted synchrotron photons for varying strengths of synchrotron cooling. I show that the cutoff energy of the synchrotron spectrum can significantly exceed the theoretical limit of 16 MeV if the plasma magnetization parameter exceeds the radiation reaction limit. Additionally, I demonstrate that a small fraction of ions present in the current sheet can be accelerated to the highest energies, making relativistic radiative reconnection a promising mechanism for the acceleration of high-energy cosmic rays. In the second Chapter, I present the first multi-dimensional simulations of the QED pair production discharge that occurs in the polar region of the neutron star. This process is believed to be the primary source of the pair plasma in pulsar magnetospheres and also the source of the radio emission. In this work, I focus on the self-consistently emerging synchronization of the discharges in different parts of the polar region. I find that pair discharges on neighboring magnetic field lines synchronize on a scale comparable to the height of the pair production region. I also demonstrate that the popular “spark” model of pair discharges is incompatible with the universally adopted force-free magnetospheric model: intermittent discharges fill the entire polar region that allows pair production, leaving no space for discharge-free regions. My findings disprove the key assumption of the spark model about the existence of distinct discharge columns. In the third Chapter, I demonstrate how the key findings of two previous chapters can provide a self-consistent explanation of the recently discovered very-high-energy, reaching 20TeV, pulsed emission in Vela pulsar. Motivated by the results of recent global simulations of pulsar magnetospheres, I propose that this radiation is produced in the magnetospheric current sheet undergoing radiative relativistic reconnection. I show that high-energy synchrotron photons emitted by reconnection-accelerated particles efficiently produce electron-positron pairs. The density of secondary pairs exceeds the supply from the polar cap and results in a self-regulated plasma magnetization parameter of $\sim 10^7$. Electrons and positrons accelerate to Lorentz factors comparable to $\sim 10^7$ and emit the observed GeV radiation via the synchrotron process and ~10 TeV photons by Compton scattering of the soft synchrotron photons emitted by secondary pairs. My model self-consistently accounts for the ratio of the gamma-ray and TeV luminosities.Item Theoretical, Experimental, and Observational Studies of Iron X-ray Spectra: From the Laboratory to the Universe(2024) Grell, Gabriel Jonathan; Mushotzky, Richard; Astronomy; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)The spectral lines of iron ions, particularly the dominant neon-like Fe XVII charge state, provide crucial diagnostics for the physical conditions of hot astrophysical plasmas in the X-ray regime. However, the diagnostic utility of these lines are hampered by significant discrepancies at the ~20% level between spectral observations, laboratory experiments, and theoretical calculations of the astrophysically important Fe XVII transitions, an issue that has been observed in numerous studies over several decades. Understanding the source of these discrepancies is critical for the improvement of both theoretical atomic models and laboratory experiment data on transition energies and cross sections of electron-ion processes, which themselves will be key for comparison to observations from X-ray spectroscopy missions such as XRISM, Line Emission Mapper (LEM), Arcus, and Athena. My dissertation encapsulates the main branches of X-ray astrophysics by focusing on the use of theoretical models and experimental measurements to further the diagnostic use, understanding, and interpretation of spectroscopic observations of iron transition lines. I modeled the effects of UV photoexcitation in O-type stars on a spectral line ratio of the Fe XVII 3s – 2p transitions in an attempt to explain an anomalous value found for the X-ray spectra of the O star ζ Puppis. I conjectured that the strong UV field of ζ Pup produces the observed ratio by depopulation of metastable 3s excited states, and that the ratio can potentially be used as an independent diagnostic of the radial distribution of X-ray-emitting plasma. Using the Flexible Atomic Code (FAC) collisional-radiative model to model the effect of UV photoexcitation on the Fe XVII lines, I compared the model calculations to archival spectra of coronal and hot stars from the Chandra HETGS and XMM-Newton RGS. The calculations showed that UV photoexcitation does not produce a sufficiently large dynamic range in the Fe XVII line ratio to explain the difference in the observed ratio between coronal stars and ζ Pup. I used FAC to compute steady-state populations of Fe XVII states and calculate cross sections for the dielectronic recombination (DR) and direct electron-impact excitation (DE) line formation channels of Fe XVII, and benchmarked the model predictions with experimental cross sections of Fe XVII resonances that were mono-energetically excited in an electron beam ion trap (EBIT) experiment. I extended the benchmark to all resolved DR and DE channels in the experimental dataset with a focus on the n ≥ 4 DR resonances, finding that the DR and DE absolute cross section predictions for the higher n complexes disagree considerably with experimental results when using the same methods as in previous works. However, agreement within ∼10% of the experimental results was achieved by an approach whereby I doubly convolve the predicted cross sections with both the spread of the electron-beam energy and the photon-energy resolution of the EBIT experiment. I also calculated rate coefficients from the experimental and theoretical cross sections, finding general agreement within 2σ with the rates found in the OPEN-ADAS atomic database. Circling back to the ζ Pup Fe XVII ratio, I probed the potential significance of the process of resonant Auger destruction (RAD), which occurs when a photon emitted by an ion is absorbed in a neighboring cooler part of the stellar wind by near-coincident inner-shell transitions of lower charge state ions. The inner-shell excited ion then undergoes Auger decay, in which the energy is transferred to an outer electron that is subsequently ejected from the atom by autoionization. EBIT measurements at a synchrotron beamline determined that 3d – 2p transitions of the lower iron charge state Fe VI is nearly coincident in transition energy with the Fe XVII 3G line, which would enable possible destruction of Fe XVII 3G photons and thus a potential explanation of the lower line intensity ratio found in ζ Pup. Model calculations show a noticeable amount of optical thickness for the Fe VI line, but the calculated X-ray line profile model does not show nearly enough reduction of the Fe XVII 3G line to suggest that RAD by Fe VI lines is causing the ratio anomaly in ζ Pup. Finally, I introduce preliminary steps for the analysis of XRISM spectral observations of Fe Kα lines from the starburst galaxy Messier 82. The key unsolved questions regarding M82 are what drives the hot wind and how much gas escapes the galaxy. Understanding the hot wind requires accurate measurements of its energy content, which requires obtaining constraints for the density, temperature, and velocity at the wind’s base. In order to sufficiently constrain the hot component velocity, the 6.7 keV Fe XXV line width and center must be determined to better than 10%. This accuracy requires an energy resolution ΔE ≤ 5 eV, which can be achieved by the high-resolution X-ray measurements with the XRISM Resolve calorimeter array. The M82 observation and subsequent analysis will confirm whether hot gas pressure is the primary driver of the galactic wind by measuring the energy contained in the T ∼ 10^8 K hot gas, and will constrain the mass-loading rate by measuring the velocity of the superheated nuclear gas using the Fe XXV line width. By completing these works, I will have successfully contributed to the refinement and advancement of theoretical, laboratory, and observational X-ray astrophysical data for iron transition lines.Item The Lives and Times of Stars and Black Holes in the Disks of Active Galactic Nuclei(2024) Dittmann, Alexander Joseph; Miller, Michael C; Astronomy; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Enormous disks of gas are thought to feed the supermassive black holes at the centers of active galaxies; these disks may capture stars from nuclear clusters, or form stars in situ after collapsing under their own gravity. Such stellar populations may enrich these accretion disks with fusion byproducts, cause giant flares in these active galaxies, and leave behind compact remnants detected on earth through gravitational waves emitted as they merge with one another. This dissertation charts a theoretical expedition into these phenomena, from studying the implications of star-forming accretion disks for the growth of black holes in the early universe, to simulating the flow of gas around black hole binaries to ascertain their orbital evolution. After a brief observational and theoretical overview of stars and active galactic nuclei, this dissertation delves into the development of simplified models of accretion disk structure, including the effects of stars and black holes embedded within accretion disks. The ultimate goal of this chapter was to determine if gravitational instability in the outer regions of these accretion disks might lead to the formation of large numbers of black holes, which might go on to merge with the central supermassive black hole; this process might decrease the effective radiative efficiency of accretion onto supermassive black holes, facilitating the rapid growth of black holes in the early universe, which defies conventional explanation. Along the way, this work developed a new flavor of model to describe these disks, accounting for the pressure support provided by feedback from disk-embedded stellar-mass black holes, developed a number of semi-analytical estimates for how stars might evolve within these accretion disks, and estimated the typical timescales for objects to move through the disk. Together, these estimates showed that accelerated supermassive growth in the early universe was indeed feasible, although this estimate hinged on a number of yet-untested assumptions. Subsequently, this dissertation advances to the question of how stars evolve when embedded within hot, dense disks of gas accreting onto supermassive black holes. Moving beyond the semi-analytical models of the preceding section, the third chapter reviews simulations of stellar evolution subject to the extreme conditions within these accretion disks. Stellar evolution calculations, due to the enormous spatial and time-scales involved, are virtually always restricted to one spatial dimension. This chapter investigates a number of the ways to account for the deviations in spherical symmetry inherent to accretion disks in these calculations, before reviewing how stellar rotation and the chemical composition of these accretion disks can affect the evolution of stars embedded therein. This work developed analytical criteria governing different regimes in stellar evolution, such as the balance between the stellar accretion and nuclear burning timescales, the relationship between gas composition and gas opacity, and the limiting effect of the central supermassive black hole's gravity on stellar accretion as the two compete for gravitational influence on the gas within the disk. Ultimately, the precise, quantitative details of these simulations depend on the specific 3D-inspired prescriptions implemented, but the overall trends identified are robust. The final study presented in this dissertation investigates the feasibility of these accretion disks as the host sites for the stellar-mass black hole mergers detected by the Laser Interferometer Gravitational-Wave Observatory. One of the primary uncertainties of this scenario is whether binaries formed within the disk will tend to spiral inward after formation, or instead be driven via hydrodynamic interactions to spiral outward to the point where chaotic three-body interactions would separate the binary. To address the feasibility of this gravitational wave progenation channel, we conducted three-dimensional hydrodynamical simulations of black hole binaries embedded within these accretion disks, at orbital separations slightly smaller than the limit for dynamical instability. This chapter focused on initially circular binaries over a range of orbital inclinations with respect to the midplane of the disk, finding that binaries with orbits at all misaligned with the disk midplane are gradually realigned, and that retrograde binaries can inspiral appreciably faster than prograde ones. Although the simulations were physically incomplete, in particular neglecting magnetohydrodynamic and radiative effects, they suggest that AGN disks could indeed host the binary black hole mergers detected via gravitational waves.Item EXCITED DYONIC STATES OF MONOPOLES AND ASTRONOMICAL BOUNDS ON AN AXION-PHOTON-DARK PHOTON INTERACTION(2024) Ristow, Clayton James; Hook, Anson; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)The study of beyond the standard model physics can largely be broken into twocategories: theoretical and phenomenological. In the former, we study theories in depth to better understand their implications while in the latter, we hold models of our physical world to scrutiny against experimental evidence. Both are crucial to understanding physics beyond the standard model. To reflect this dichotomy, this thesis is broken into two acts, one covering theoretic research and the other discussing progress made on the phenomenological front. Chapter 2, comprising the entirety of Act 1 of this thesis, concerns the theory of magnetic monopoles. In the mid-1970’s t’Hooft and Polyakov discovered magnetic monopoles exist as generic solutions in spontaneously broken gauge theories. Since then much progress has been made in understanding these monopoles, most notably by Callan who argued that the fermion vacuum is non-trivial around the core of the magnetic monopole. These non-trivial vacuua can be interpreted as bound states of fermions with fractional fermion number. In this work, we explicitly compute these fermion bound states in an SU (2) gauge theory coupled to Nf fermions. We demonstrate there are two unique ways to grant mass to the fermions in the SU (2) theory which, after symmetry breaking, give the same UEM (1) theory of fermions. Despite this low energy equivalence, we show that the two theories exhibit very different physics at low energy scales around a magnetic monopole. We show that there may exist stable excited dyonic states with differing charges and energies between the two theories. We find the ground states can also differ in energy and charge between the two theories. We demonstrate the monopole can inherit a mass correction and charge distribution that depends on the topological θ angle even if one of the fermions is massless. This effect is present in one of the theories and is completely absent in the other. Finally, we discuss the implications of these effects on the SU (5) GUT monopole. Act two, comprising of chapters 3 and 4, focuses on the phenomoenological side of beyond the standard model physics. In these chapters, we consider two highly motivated beyond the standard model particles, the axion, φ, and the dark photon AD which are coupled to the standard model photon via a coupling φF ̃FD. In some models, this coupling can provide the leading order coupling between our sector and the dark sector containing the axion and dark photon. In chapter 2, we demonstrate the effect this coupling has on the Cosmic Microwave Background (CMB) in the scenario where either the axion or the dark photon constitutes dark matter. Depending on which we choose to be dark matter, we show that this interaction leads to the conversion of the CMB photons into the other dark sector particle, leading to a distortion in the CMB spectrum. We present the details of these unique distortion signatures and the resulting constraints on the φF ̃FD coupling. In particular, we find that for a wide range of masses, the constraints from this effect are stronger than on the more widely studied axion-photon-photon coupling. We also demonstrate that CMB distortions of this type can a exhibit unique, non-thermal frequency profile which could be detected by future experiments. In chapter 3, we consider the astrophysical effects of the φF ̃FD coupling, in particular, its effect on supernova cooling rates. We show that the bound on this interaction due to supernova cooling exhibits two unusual features. If there is a large mass difference between the axion and dark photon, we show both production and scattering become suppressed and the bounds from bulk (volume) emission and trapped (area) emission both weaken exponentially. We show that these bounds do not intersect leading to a larger area of excluded parameter space than may have otherwise been expected. The other unusual feature occurs because the longitudinal modes of light dark photons couple more weakly than their transverse modes. As a consequence, the longitudinal modes can still cause excessive cooling even if the transverse modes are trapped. Thus, the supernova constraints for massive dark photons look like two independent supernova bounds super-imposed on top of each other. We also briefly consider the effect of this interaction on white dwarf cooling and Big Bang Nucleosynthesis.Item Multi-messenger search for galactic PeVatron with HAWC and IceCube(2024) Fan, Kwok Lung; Goodman, Jordan A; Sullivan, Gregory W; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)In recent years, many advancements in astrophysics have brought astrophysicists new tools to study the universe. Specifically, the discovery of astrophysical neutrinos by the IceCube Neutrino Observatory and Gravitational waves by the LIGO/Virgo collaboration has started the era of multi-messenger astronomy. Scientists can finally use messengers other than electromagnetic waves to study astrophysical phenomena. With the addition of new messengers, it is crucial that data from multiple instruments and messengers can be jointly analyzed through a unified framework using one physics model. Many efforts have been put into jointly analyzing electromagnetic waves of different wavelengths from different instruments, but the ability to jointly fit other messengers to a single physics model is still missing. In this work, we present a method to jointly analyze data from HAWC Gamma-ray Observatory and IceCube Neutrino Observatory by using a newly developed IceCube likelihood software called i3mla and the existing HAWC likelihood software called HAL. Together with the Multi-Mission Maximum Likelihood framework (3ML), we are able to jointly fit the gamma-ray emission model and neutrino emission model simultaneously with the HAWC gamma-ray and IceCube neutrino data. We apply the method to search for Galactic PeVatrons. Galactic PeVatrons are sources of PeV galactic cosmic rays. When the cosmic ray interacts with nearby material, it will produce both gamma rays and neutrinos with the same morphology and spectral shape. While gamma rays could also be produced from other interactions, neutrinos can only be produced by hadronic interactions of the cosmic ray. Therefore, it is natural to search for neutrino emissions from gamma-ray sources. We first perform a search for neutrino emissions from the 12 known gamma-ray sources detected by LHAASO. No significant detection was found and we put constraints on the neutrino emission on the sources. Second, a more detailed multimessenger search of Galactic PeVatrons candidates using simultaneously the HAWC data and IceCube neutrino data is conducted. We model the gamma-ray emission using the HAWC data and jointly fit a unified model to both the gamma-ray and neutrino data. No significant detection was found and we put constraints on the fraction of the gamma rays due to hadronic interactions.Item Interactions between Massive Stellar Feedback and Interstellar Gas in the Eagle Nebula(2024) Karim, Ramsey Lee; Mundy, Lee G; Pound, Marc W; Astronomy; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)My thesis describes multi-scale stellar feedback processes observed in the Eagle Nebula star forming region in our Milky Way galaxy. Stellar feedback from massive stars encompasses bright ultraviolet radiation which ionizes atoms and dissociates molecules in gas surrounding the stars as well as supersonic winds which impact the gas and create hot shocked layers. I study the interaction of stellar radiative and mechanical feedback with pre-existing density inhomogeneities in the molecular cloud in order to learn about the effects of the interstellar environment on the relative efficiency of various forms of feedback. This work informs our understanding of the life cycle of interstellar gas: gas forms stars and is then exposed to their winds and radiation, and we would like to know how that affects the formation of future generations of stars. The Eagle Nebula's relative proximity to us means we observe the H II region with high spatial resolution. Extra-galactic studies observe many H II regions simultaneously and at a variety of cosmic ages, but lack the resolution to understand the structure of the individual regions. High resolution studies of Galactic sources such as the Eagle serve as templates for what extra-galactic astronomers are seeing in far-away galaxies. The work also contributes to sub-grid feedback prescriptions in large-scale simulations of galaxy formation and evolution. Stars and their feedback are too small to be simulated in these contexts, so theorists require accurate approximations for the effects of stellar feedback. Massive stars form in massive molecular gas clouds and then deliver vast quantities of energy back into the clouds in the form of radiation and stellar winds. They form H II regions, 1-to-10-light-year scale areas of ionized hydrogen, which are often overpressured bubbles compared to the surrounding interstellar medium, and their supersonic winds sweep up a compressed shell of gas. Around the edge of the H II region, there lies a layer of gas which receives no >13.6 eV extreme-ultraviolet H-ionizing radiation (EUV), but is rich in 6-13.6 eV far-ultraviolet radiation (FUV) which can photodissociate molecules such as CO and H2 and ionize C. These photodissociation regions (PDRs) are heated via the photoelectric effect as FUV radiation interacts with organic molecules called polycyclic aromatic hydrocarbons (PAHs), and the regions are cooled by the collisionally excited far-infrared fine structure transitions of ionized carbon and atomic oxygen. The FEEDBACK SOFIA C+ Legacy Project (Schneider et al. 2020) studies the coupling efficiency of that energetic feedback to the gas by observing one such transition of singly ionized carbon at 158 micron referred to as C+ or [C II]. In this astrophysical context, the line is emitted primarily within PDRs. With modern heterodyne receivers and an observatory above Earth's atmosphere, we can both detect and spectroscopically resolve the [C II] line and therefore trace the morphology and kinematics of the PDR regions surrounding massive stars. We contextualize these observations with velocity-resolved observations tracing the un-illuminated molecular gas beyond the PDRs and a variety of archival data spanning the electromagnetic spectrum from radio to X-ray. I use these observations to study the Eagle Nebula, home to the iconic Pillars of Creation, and learn how pre-existing density structure evolves when exposed to stellar feedback and what that implies for the energetic coupling of the stellar feedback to the gas. My first study covers the Pillars of Creation in a detailed, multi-wavelength analysis published in the Astronomical Journal. We find that these pillars are long-lasting structures on the scale of the H II region age and that they must arise from pre-existing density structures. My second study zooms out to the greater Eagle Nebula H II region to learn how the massive stars affect the rest of the region. This analysis concludes that the primordial filamentary structure which must have led to the formation of the stellar cluster also governs the shape of the H II region and how much of the surrounding gas is affected by the feedback. Finally, I describe a software package, scoby, which I developed to aid these two studies. The software connects theoretical feedback estimates to observed star catalogs and delivers results tuned for observational studies like these. It has been used for several published analyses of other regions.Item Population Studies of Tidal Disruption Events and Their Hosts: Understanding Host Galaxy Preferences and the Origin of the Ultraviolet and Optical Emission(2024) Hammerstein, Erica; Veilleux, Sylvain; Cenko, S. Bradley; Astronomy; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)It is well-established that the majority of galaxies harbor a supermassive black hole (SMBH) in their nucleus. While some of these SMBHs are easily studied either through signatures of persistent gas-fueled accretion or direct observations of the SMBH's influence on stars and gas in its potential well, many more are elusive, providing no obvious evidence of their existence. One way to detect these dormant SMBHs is through the tidal disruption of a star that wanders too close and is torn apart under the tidal stress. These tidal disruption events (TDEs) illuminate otherwise difficult-to-study dim or distant galaxy nuclei, acting as cosmic signposts announcing the presence of the SMBH lurking there through luminous flares observed across the electromagnetic spectrum. These flares can, in principle, be used to extract information about the SMBH itself, and can therefore serve as important probes of SMBH growth and evolution. TDE host galaxies can be used to study the connection between SMBHs and their environments, an important goal in understanding the origin of SMBHs, galaxy formation, and SMBH co-evolution. My dissertation addresses both of these important facets of TDEs, their light curves and their hosts, to understand not only the events themselves but how they can be used to study SMBHs. First, I studied a sample of 30 optically selected TDEs from the Zwicky Transient Facility (ZTF), the largest sample of TDEs discovered from a single survey yet. After performing a careful light curve analysis, I uncovered several correlations between light curve parameters which indicate that the properties of the black hole are imprinted on the light curve. I also fit the light curves using tools that yield black hole mass estimates and I found no correlation between these estimates and the host galaxy stellar mass. I found no difference between the optical light curve properties, apart from the peak luminosity, of the X-ray bright and X-ray faint TDEs in this sample. This provides clues as to the origin of the optical emission and may support a scenario where the viewing angle is responsible for the observed emission. Lastly, I presented a new spectral class of TDE, TDE-featureless, which in contrast to other events, show no broad lines in their optical spectra. This new class may be connected to the rare class of jetted TDEs. Next, I studied a subset of host galaxies in the ZTF sample of TDEs. I examined their optical colors, morphology, and star-formation histories. I found that TDE hosts can be classified as ``green'', in a phase between red, inactive galaxies and blue, star-forming galaxies. Morphologically, the TDE hosts are centrally concentrated, more so than galaxies of similar mass and color. By looking at the optical spectra of the TDE hosts, which can be used to estimate the current star formation and the star formation history, I found that TDE host populations are dominated by the rare class of E+A, or post-starburst, galaxies. In tandem with the other peculiar photometric and morphological properties, this points to mergers as the likely origin for TDE hosts. I extended this study of TDE hosts by using integral field spectroscopy to infer black hole masses via the $M_{\rm BH} - \sigma_\star$ relation and investigate large-scale stellar kinematics. I found that the black hole mass distribution for TDE hosts is consistent with the theoretical prediction that they should be dominated by lower mass SBMHs. Interestingly, one TDE-featureless object was found to have a black hole mass of $\log(M_{\rm BH}/M_\odot) = 8.01$, which is likely above the Hills mass for the disruption of a solar-type star and could necessitate a rapid spin for this particular black hole. If high spin is required to launch relativistic jets, this may further support the connection between featureless TDEs and jetted TDEs. The large-scale kinematics of a galaxy are strongly tied to its merger and star formation history. I found that TDE hosts share similar kinematic properties to E+A galaxies, which are thought to be post-merger. Lastly, I presented further observations of the jetted TDE AT2022cmc. This event, discovered in the optical, presented an opportunity to place this rare class of TDE in the context of the larger TDE population. I performed a careful light curve analysis that accounts for both the thermal and non-thermal components in the light curve. I showed that the thermal component of AT2022cmc is similar to the TDE-featureless class of events and follows correlations presented for TDE light curve properties found in this thesis.Item THE SEARCH FOR COINCIDENT GAMMA-RAY EMISSION FROM FAST RADIO BURSTS WITH THE HAWC OBSERVATORY(2024) Willox, Elijah J; Goodman, Jordan A; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)In 2007 a new class of radio transients was discovered, coming from outside of our galaxy with high fluence emitted in the radio band on millisecond timescales. These bursts of radio waves emitted within an order of magnitude of the power of the least bright gamma- ray bursts. These fast radio bursts (FRBs) have since become the target of many searches across radio observatories and multiwavelength follow-up campaigns, but their origin re- mains unknown. In order to understand more about these fascinating events, continued multiwavelength follow-ups are necessary to provide a more complete picture. The High Altitude Water Cherenkov (HAWC) observatory is a very-high-energy gamma-ray detector covering the range of 100 GeV to 300 TeV that is well suited to the detection of transient phenomena due its high live-time and wide field of view, and in particular for a follow- up search on FRBs to determine possible very high energy gamma-ray coincidences. The search for gamma-ray signals from FRBs consists of two searches: first is a persistent source search to identify if FRB emission ever comes from TeV gamma-ray emitting galax- ies, and a transient search centered on the reported burst time and location. The results of the FRB search within the HAWC data sets the most constraining limits on the widestpopulation of FRBs ever searched in the VHE band.Item Mechanical evolution of small solar system bodies(2023) Marohnic, Julian Charles; Richardson, Derek C; Astronomy; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)This dissertation presents a series of studies that develop and apply numerical modeling techniques to small bodies in the solar system. We are particularly interested in low-energy deformations, collisions, and disruptions, and our subjectsrange from near-Earth asteroids to Kuiper belt contact binaries in the farthest reaches of the solar system. We use the N-body code pkdgrav to investigate these processes and describe our significant additions to its capabilities. Our first subject is the Kuiper belt object Arrokoth. On January 1, 2019, the New Horizons spacecraft flew within 3,550 km of Arrokoth, returning the first in-situ images of a small body in the classical Kuiper belt. Arrokoth was found to be bilobate, with a distinctive contact binary structure. We use pkdgrav to investigate the origins of Arrokoth's striking shape and find that plausible formation mechanisms are quite limited. We rule out the possibility of a direct impact between two unbound objects and put forward an alternate scenario in which two cometesimals in a close, synchronous orbit gradually spiral in toward one another before meeting in a gentle merger. We conclude by exploring implications for the formation of small Kuiper belt objects more generally. Next, we describe our work modifying pkdgrav to accommodate non-spherical particles. Prior work in granular physics has established that particle shape is an important factor governing the behavior of granular bodies like small solar system objects. Irregular particles tend to interlock with one another, inhibiting bulk motion and adding to the shear strength of a medium. We adapt pkdgrav's existing soft-sphere, discrete element contact physics model to allow for modeling of non-spherical grains. We then apply this new capability in three, small-scale proof of concept studies of spin-up, tidal disruption, and the Brazil nut effect. We find a significant difference in behavior when comparing small rubble-pile bodies composed of spherical particles and those composed of non-spherical particles. Finally, we apply our newly-developed tools to a more comprehensive investigation of particle shape in tidal disruption simulations. We construct small rubble piles from a range of differently-shaped constituents and subject them to simulated tidal encounters with the Earth. We conduct a parameter sweep across different encounter geometries and constituent shapes and conclude that particle shape is a significant contributor to tidal encounter outcomes. The role of particle resolution is also investigated.Item All-Sky Search for Very-High-Energy Emission from Primordial Black Holes and Gamma-Ray Bursts with the HAWC Observatory(2023) Engel, Kristi Lynne; Goodman, Jordan A; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Transient sources of very-high-energy gamma rays are short-lived astrophysical phenomena often associated with catastrophic events that change their brightness over relatively short timescales. The search for and study of such objects, especially in the TeV energy regime, has the possibility to shed light not only on the physics at play within the enigmatic, chaotic environments that produce such emission, but also to answer several remaining questions in fundamental physics. In this dissertation, we leverage the sensitivity and characteristics of the High-Altitude Water Cherenkov (HAWC) Observatory in pursuit of gaining insight into these areas. The HAWC Observatory, located on the side of the side of the Sierra Negra volcano in Puebla, Mexico at an altitude of 4,100 m above sea level, is an extensive-air-shower array sensitive to gamma rays from ~0.1 to >100 TeV that has been in operation since March of 2015. It has a wide field of view of ~2 sr at any one time and, combined with its large operational duty cycle (>95%), observes 2/3 of the sky every day. HAWC operates using the water-Cherenkov detection technique with 1,200 photomultiplier tubes (PMTs) in two different sizes to detect Cherenkov emission from secondary air-shower particles. Herein, we present an improved characterization for the larger of these two PMT models for inclusion within the Monte Carlo simulation of the HAWC Observatory, as well as the custom testing apparatus designed and constructed for this purpose. With HAWC's wide field of view, near-continuous uptime, and large archival dataset, it serves as an ideal observatory with which to search for transient sources of all kinds. We apply these advantages to perform searches for two types of transient sources--- Primordial Black Holes (PBHs) and Gamma-Ray Bursts (GRBs). The first of these, a search for emission signatures of evaporating PBHs, is performed on 959 days of HAWC data for remaining lifetimes of 0.2, 1, 10, and 100 s, assuming radiation development according to the Standard Emission Model. We show that previous attempts to perform searches for transient searches similar to PBHs with HAWC were oversampling at detrimental levels and improve upon that method to achieve greater statistical rigor. Finding no significant emission for any duration, we place upper limits at the 99% confidence level on the local burst rate density. For the second of these source types, we apply the low-energy improvements recently made to the HAWC data reconstruction procedure to search for very-high-energy emission within the first 0.1, 1, 10, and 100 s of emission for 93 GRBs within HAWC's field of view at their reported T0 over the first 7 years of HAWC operations. This search is performed using permutations of four different assumed redshift values and four different assumed spectral indices. Finding no significant emission for any duration under any set of assumption parameters, we place upper limits at the 95% confidence level on the intrinsic flux for all GRBs. For those GRBs with external flux models available from other gamma-ray detectors, we compare the HAWC limits to those models in order to constrain the possible emission in the TeV regime with respect to that at lower energy values. We also perform a follow-up execution of this analysis with start times shifted to match external model start times which differed from T0. Again finding no significant emission, we place upper limits at the 95% confidence level on the intrinsic flux for all parameter sets and for all external start times for those GRBs HAWC was most likely to have seen. Finally, we speculate about the future of searches for PBHs and GRBs with the next-generation wide-field-of-view instrument, the Southern Wide-field Gamma-ray Observatory (SWGO), presenting projected performance for these two types of transient sources.