Physics
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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 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 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 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.Item Experimental Atomic Spectroscopy of Iron Group Elements for Astrophysics(2021) Ward, Jacob Wolfgang; Nave, Gillian; Rolston, Steve; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)The quality of modern astrophysical spectra has made it clear that there is a lack of sufficiently accurate and robust laboratory atomic reference data sets. Particularly for spectra of the iron-group elements, the growing demand for critically evaluated sets of comprehensive atomic data is a direct result of advancing stellar astrophysics models and fundamental physics problems probing beyond the standard model. My thesis reports on my critical evaluation of the Ni V spectrum and the recent laboratory measurements I have conducted to improve the state of available reference data for astrophysical applications that rely on observations of Ni V. Additionally, I report my laboratory measurements of Fe II branching fraction values in the UV/VUV. Using high-resolution grating spectroscopy at the National Institute of Standards and Technology, I have carried out an analysis of quadruply ionized iron and nickel (Fe V & Ni V) in the vacuum ultraviolet (VUV) region by both recording new spectra and critically evaluating previously published data sets. My analysis has resulted in highly accurate wavelengths, presented with calculated oscillator strengths, for roughly 1500 Ni V lines, 200 of which have uncertainties that are almost an order of magnitude lower than in previous publications. Additionally, I present over 300 Ni V energy levels derived from my evaluated wavelengths. This section of my thesis focuses on the large improvements made in the analysis of Ni V, but my work also strongly supports the previous evaluations of Fe V by another author. With the extreme accuracy requirements of modern astrophysics problems, confirming the wavelength scale and uncertainty evaluation of previous Fe V data sets is still significant. In addition to the above work, my thesis also presents measurements of singly ionized iron (Fe II) branching fractions (BFs) in the VUV using high-resolution Fourier-transform spectroscopy. BFs are essential values for interpreting complex astrophysical spectra, but are notoriously difficult to measure in the VUV; for this reason, VUV BFs of Fe II have only been reported by one other research group for just seven levels. My thesis reports accurate BFs for 11 Fe II levels, involving approximately 100 spectral lines (16 in the VUV), which roughly doubles the amount of reported Fe II BFs in VUV.Item PROBING FUNDAMENTAL PHYSICS WITH GRAVITATIONAL WAVES FROM INSPIRALING BINARY SYSTEMS(2021) Sennett, Noah; Buonanno, Alessandra; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)The mergers of black holes and/or neutron stars in binary systems produce the most extreme gravitational environments in the local universe. The first direct detections of gravitational waves by Advanced LIGO and Virgo provide unprecedented observational access to the highly dynamical, strong-curvature regime of gravity. These measurements allow us to test Einstein’s theory of General Relativity in this extreme regime. This thesis examines how the gravitational-wave signal produced during the inspiral—the earliest phase of a binary’s coalescence—can better inform our understanding of the fundamental nature of gravity. My work addressing this topic is comprised of two major components. First, I examine the behavior of binary black-hole and neutron-star systems in various possible extensions of General Relativity, constructing analytic models of their orbital motion and gravitational waveform—their gravitational-wave signature—during their inspiral. The majority of alternative theories I consider modify General Relativity by introducing a new scalar component of gravity. In many of these theories, standard perturbative techniques are used to model the inspiral of binary systems. However, I also examine in depth the non-perturbative phenomenon of scalarization for which such methods fail. I show that this phenomenon occurs due to a second-order phase transition in the strong-gravity regime and develop an analytic framework to model the effect across a range of alternative theories of gravity. The other component of this thesis is the development of a statistical infrastructure suitable for testing General Relativity using gravitational-wave observations. I adopt a more flexible and modular approach than existing alternatives, allowing this infrastructure to be immediately applied with a wide range of waveform models. In work done in conjunction with the LIGO Scientific and Virgo Collaborations, I use this statistical framework to place bounds on phenomenological deviations from General Relativity using the binary black-hole and neutron-star events detected during LIGO’s first and second observing runs—no evidence for deviations from Relativity is found. These two research directions outlined above are complementary; the type of statistical inference discussed here requires models for the gravitational-wave signal produced by inspiraling systems that allow for deviations from General Relativity, and the analytic models I construct are suitable for this task. In this thesis, I carry out the complete procedure of building and employing analytic models of gravitational waveforms to place constraints on specific alternative theories of gravity with observations by LIGO and Virgo.Item Temporal and spectral evolutionary features of gamma-ray bursts detected by theFermiGamma-Ray Space Telescope(2019) Tak, Donggeun; McEnery, Julie; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Gamma-ray bursts (GRBs) are the most powerful electromagnetic events in universe. GRBs are powered by either core-collapse of massive stars or binary mergers of two compact objects. These progenitor systems are believed to launch relativistic, collimated jets, which produce short, bright gamma-ray flashes (prompt emission) and long-lived, fading emission (afterglow) in the broad energy band from radio to gamma-rays. Even though the characteristics of the prompt emission and the afterglow have been vigorously studied, many details of the physics of GRBs remain uncertain. The Fermi Gamma-ray Space Telescope(Fermi) provides invaluable data for studying GRBs with the help of a very wide field of view and broad energy coverage from the hard X-ray to gamma-ray band. Fermi consists of two instruments, the Gamma-ray Burst Monitor (GBM; 8 keV–40 MeV) and the Large Area Telescope(LAT; 20 MeV– >300 GeV). In this thesis, I present dedicated analysis results on three bright GRBs: GRB 131108A, GRB 160709A, and GRB 190114C. Each of them shows its own evolution that includes the unusual and general features of GRBs. In addition, I performed two systematic studies using the full 10 year samples of LAT and GBM detected GRBs. For the first, I focused on the high-energy emission (>100 MeV) and its origin by tracking its temporal and spectral evolution. In the second, focusing on the prompt emission phase, I found an observational signature that originates in the geometry of the relativistic jet, which had been predicted but was previously unobserved.Item Simulations of Accretion Mechanisms and Observational Signatures of Black Hole Accretion Disks(2019) Smith, Megan; McKinney, Jonathan C; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Black holes have been a subject of fascination since they were first theorized about over a century ago. There are many questions about them left unanswered. One of these questions is how matter is accreted onto these objects when the plasma around them is rotating in an accretion disk. An answer to this question is likely to be found in the magnetohydrodynamic processes that occur in the plasma, which require highly sophisticated numerical simulations to explore. In this thesis, I describe an analysis of one magnetohydrodynamic instability found in these simulations as well as the observational signatures it produces, which might be recognized in observations of these systems. For the remainder of this thesis, I will discuss the formation and evolution of a formal near-peer mentoring program for women in the University of Maryland physics department. Mentoring programs have been shown to have a number of benefits for both mentors and mentees. Primary among them is an increased sense of belonging and science identity, which is linked to increased retention. Given the so-called "leaky pipeline" problem of women leaving physics, a field where they are already underrepresented, efforts to improve retention are vital and peer mentoring is one way to do this.Item SEARCH FOR GAMMA-RAY COUNTERPARTS OF GRAVITATIONAL WAVE EVENTS AND OTHER TRANSIENT SIGNALS WITH HAWC(2019) Martinez Castellanos, Israel; Goodman, Jordan A; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)In recent years we have seen major advances in multi-messenger astronomy. A milestone was achieved by identifying the electromagnetic counterpart of the gravitational wave event GW170817 detected by LIGO and Virgo. Similar efforts led to a set of neutrinos detected by IceCube to be associated with the blazar TXS 0506+056. Both demonstrate the potential of using multiple types of probes to study an astrophysical source. The High-Altitude Water Cherenkov Observatory (HAWC), located in the state of Puebla, Mexico, is a wide field instrument (~2 sr) sensitive to very-high-energy gamma rays (~0.1-100 TeV) which can operate with a large duty cycle (>95%). These characteristics make it well suited to look for transient events correlated with other astronomical messengers. In this work we present a maximum likelihood analysis framework developed to search and analyze signals in HAWC data of arbitrary timescales. We apply this method to search for very-high-energy gamma-ray counterparts of gravitational waves in short timescales (0.3-1000 s). We show that we would be able to either detect or meaningfully constrain the very-high-energy component of a gamma-ray burst within the binary neutron star merger horizon of current gravitational wave detectors if it occurs in our field of view. We did not find evidence for emission for any of the events analyzed. The source location of GW170817 was not observable by HAWC at the time of the merger. We also set flux upper bounds for TXS 0506+056 during the periods when the neutrino flares were identified. For the flare between September 2014 and March 2015 these are the only available limits at very high energy, and are consistent with the low state in high-energy gamma rays reported by the Fermi-LAT Collaboration.