京都大学 / 新制・課程博士 / 博士(理学) / 甲第24409号 / 理博第4908号 / 新制||理||1701(附属図書館) / 京都大学大学院理学研究科物理学・宇宙物理学専攻 / (主査)教授 柴田 大, 教授 井岡 邦仁, 教授 橋本 幸士 / 学位規則第4条第1項該当 / Doctor of Science / Kyoto University / DFAM

京都大学 / 新制・課程博士 / 博士(エネルギー科学) / 甲第24254号 / エネ博第452号 / 新制||エネ||85(附属図書館) / 京都大学大学院エネルギー科学研究科エネルギー応用科学専攻 / (主査)教授 大垣 英明, 教授 白井 康之, 教授 宮内 雄平 / 学位規則第4条第1項該当 / Doctor of Energy Science / Kyoto University / DFAM

This thesis comprises linear polarization measurements on gamma rays emitted from the previously observed 1.984, 2.572, 2.969 and 3.059 MeV levels of ²²Na using a Ge(Li) Compton polarimeter. Consistency with previous measurements on parameters characterizing these levels was first checked before assigning J^π = 2⁺ for the 1.984 MeV level and determining that both 2.969 MeV and 3.059 MeV levels have positive parity. Investigation of the 2.572 MeV level produced inconsistency with some previous work which had indicated a 2⁻ assignment. However, except for some pickup reaction work. the polarization measurement is consistent with all former measurements and indicates a 2⁺ assignment. / Thesis / Master of Science (MSc)

This work describes a study of the gamma rays and beta particles emitted during the decay of the 33-second fission product Krypton-90. Procedures for the analysis of gamma, gamma-gamma coincidence, beta and beta-gamma coincidence experiments are discussed. The application of these analyses to the short-lived Krypton-90 activity yields the prominent features of this decay. The total energy release of this decay was found to be 4.56 ± 0.02MeV, in agreement with beta systematics. A level scheme for the daughter, Rubidium-90, is proposed. / Thesis / Doctor of Philosophy (PhD)

The astrophysical origins of the rapid neutron-capture process (r-process), which gives rise to roughly half of the elements heavier than iron, has remained a mystery for almost 70 years. The likely violent events, which seed the r-process abundances in our solar system and galaxy, remain uncertain to this day. This is in part due to nuclear physics uncertainties associated with the r-process itself, but mainly due to uncertainties in astrophysics modeling. The discovery of the radioactively-powered kilonova emission from the neutron star merger event GW170817 confirmed the violent deaths of neutron stars as one key site of the r-process in the universe. However, other evidence appears to favor an additional r-process channel that more promptly follows star formation in the universe, such as core-collapse supernovae (CCSNe), i.e. the brilliant births of neutron stars. The two viable sites for the r-process are (1) core-collapse supernovae (CCSNe), which are explosions of massive stars at the end of their lives and (2) compact object mergers, which are violent collisions of stellar remnants formed at the endpoints of stellar evolution. Chapters 2 and 3 of this dissertation present general relativistic magnetohydrodynamic simulations of one potential r-process site associated with CCSNe: the neutrino-driven wind. These outflows are launched from the hot proto-neutron star (PNS) remnant by neutrino-heating above their surfaces, within seconds after the collapse of a massive star. However, previous work has shown that spherically symmetric winds from non-rotating PNS fail to achieve the requisite conditions for a robust r-process. Chapter 2 explores for the first time the combined effects of rapid rotation and strong gravity of the PNS on the wind properties. Chapter 3 explores the impact of a dynamically strong ordered magnetic field on the properties of non-rotating PNS winds. The wind in both cases is simulated in a controlled environment rather than as a part of a self-consistent global CCSNe simulation, to assess the viability of r-process nucleosynthesis as a function of PNS properties (neutrino energies/luminosities, rotation rate, magnetization). We find that rapid rotation allows for outflows that are ~10% more neutron-rich in the equatorial region, where the mass loss rate is roughly an order of magnitude higher than that of otherwise equivalent non-rotating models. The birth of very rapidly spinning neutron stars may thus be a site for the production of light r-process nuclei (38 < Z < 47). For PNSs with sufficiently strong magnetic fields (such that magnetic pressure exceeds gas pressure above the PNS surface), we find that equatorial outflows are trapped by the magnetic field in a region near the surface, and therefore receive additional neutrino heating relative to a freely-expanding unmagnetized wind. This allows a modest fraction of the wind material to achieves entropies high enough to synthesize 2nd peak r-process elements via an alpha-rich freeze-out mechanism. The final chapter explores the interplay between the r-process and the dynamics of compact object merger ejecta. Gravitational wave observatories are expected to detect several additional binary neutron star (BNS) and black hole-neutron star (BHNS) mergers in current and future observing runs, some of which may be accompanied by electromagnetic counterparts such as kilonovae. However, distinguishing more distant BNS from BHNS mergers based on their associated gamma-ray bursts (GRB), has proven tricky. This chapter presents a calculation of the effects of r-process heating on the dynamics of tidal ejecta from BNS and BHNS mergers. In particular we explore whether late-time fall-back of weakly bound debris created during the merger to the central black hole remnant, can explain the temporally extended X-ray emission observed following several merger GRB on timescales of several seconds to minutes. As a result of the different impact that r-process heating has depending on the composition of the ejecta and the mass of the black hole, a method to differentiate BHNS from BNS mergers, based on their extended X-ray emission, is proposed.

This dissertation seeks to describe advancements made in computational methods for multi-messenger astrophysics (MMA) using gravitational waves GW and neutrinos during Advanced LIGO (aLIGO)’s first through third observing runs (O1-O3) and, looking forward, to describe novel computational techniques suited to the challenges of both the burgeoning MMA field and high-performance computing as a whole. The first two chapters provide an overview of MMA as it pertains to gravitational wave/high energy neutrino (GWHEN) searches, including a summary of expected astrophysical sources as well as GW, neutrino, and gamma-ray detectors used in their detection. These are followed in the third chapter by an in-depth discussion of LIGO’s timing system, particularly the diagnostic subsystem, describing both its role in MMA searches and the author’s contributions to the system itself. The fourth chapter provides a detailed description of the Low-Latency Algorithm for Multi-messenger Astrophysics (LLAMA), the GWHEN pipeline developed by the author and used in O2 and O3. Relevant past multi-messenger searches are described first, followed by the O2 and O3 analysis methods, the pipeline’s performance, scientific results, and finally, an in-depth account of the library’s structure and functionality. In particular, the author’s high-performance multi-order coordinates (MOC) HEALPix image analysis library, HPMOC, is described. HPMOC increases performance of HEALPix image manipulations by several orders of magnitude vs. naive single-resolution approaches while presenting a simple high-level interface and should prove useful for diverse future MMA searches. The performance improvements it provides for LLAMA are also covered. The final chapter of this dissertation builds on the approaches taken in developing HPMOC, presenting several novel methods for efficiently storing and analyzing large data sets, with applications to MMA and other data-intensive fields. A family of depth-first multi-resolution ordering of HEALPix images — DEPTH9, DEPTH19, and DEPTH40 — is defined, along with algorithms and use cases where it can improve on current approaches, including high-speed streaming calculations suitable for serverless compute or FPGAs. For performance-constrained analyses on HEALPix data (e.g. image analysis in multi-messenger search pipelines) using SIMD processors, breadth-first data structures can provide short-circuiting calculations in a data-parallel way on compressed data; a simple compression method is described with application to further improving LLAMA performance. A new storage scheme and associated algorithms for efficiently compressing and contracting tensors of varying sparsity is presented; these demuxed tensors (D-Tensors) have equivalent asymptotic time and space complexity to optimal representations of both dense and sparse matrices, and could be used as a universal drop-in replacement to reduce code complexity and developer effort while improving performance of existing non-optimized numerical code. Finally, the big bucket hash table (B-Table), a novel type of hash table making guarantees on data layout (vs. load factor), is described, along with optimizations it allows for (like hardware acceleration, online rebuilds, and hard realtime applications) that are not possible with existing hash table approaches. These innovations are presented in the hope that some will prove useful for improving future MMA searches and other data-intensive applications.

Nach der Entdeckung eines diffusen Flusses hochenergetischer astrophysikalischer Neutrinos durch das IceCube South Pole Neutrino Observatory im Jahr 2013 sind die Quellen, die für den Großteil dieser Emission verantwortlich sind, immer noch unbekannt. Blazare, jene AGN mit einem relativistischen Jet, der auf die Erde zeigt, gelten als Hauptkandidaten für die Beschleunigung der kosmischen Strahlung und die Produktion hochenergetischer Neutrinos. Diese Doktorarbeit ist motiviert durch den Nachweis des Gammastrahlen-Blazers TXS 0506+056 in räumlicher Koinzidenz mit dem Neutrino IceCube-170922A. 
In dieser Arbeit präsentiere ich die Ergebnisse einer detaillierten Gammastrahlenanalyse des Blazars TXS 0506+056 über 9,6 Jahre Fermi-LAT-Beobachtungen. Die Quelle ist stark variabel im Gammastrahlenband zur Ankunftszeit des Neutrinos IceCube-170922A, was auf eine Neutrino-Gammastrahlen-Verbindung hindeutet. Die Quelle wird jedoch während eines zusätzlichen niederenergetischen Neutrinosignals, das 2014/15 von Ice-Cube in Archivdaten von derselben Quelle entdeckt wurde, in einem niedrigen Zustand beobachtet. In dieser Arbeit wurde ein zweiter Gammastrahlen-Blazar GB6 J1040+0617 in räumlicher und zeitlicher Übereinstimmung mit dem hochenergetischen Neutrino-Ereignis IceCube-141209A gefunden, während er eine erhöhte Aktivität im Gammastrahlen- und im optischen Band zeigt. Eine zweite Suche nach Blazar-Gegenstücken zu hochenergetischen Neutrinos erfolgt durch Echtzeit-Follow-up von mehr als 60 IceCube-Alerts mit dem Fermi-LAT. Der kräftige Blazar PKS 1502+106, der zusammen mit dem Ereignis IceCube-190730A gefunden wurde, stellt einen herausragenden wissenschaftlichen Fall dar. Schließlich wird eine Studie zum Vergleich der durchschnittlichen Gammastrahlenemission der potenziellen Neutrino-Gegenstücke mit der gesamten Stichprobe von Gammastrahlen-Blazaren vorgestellt. Die Ergebnisse zeigen mögliche Hinweise auf eine Korrelation zwischen Neutrino- und Gammastrahlen-Energieflüssen. / After the discovery of a diffuse flux of high-energy astrophysical neutrinos by the IceCube South Pole Neutrino Observatory in 2013, the sources responsible for the majority of this emission are still unknown. Blazars, those AGN with a relativistic jet pointing towards Earth, are considered prime candidates for cosmic-ray acceleration and the production of high-energy neutrinos. This thesis work is motivated by the detection of the flaring gamma-ray blazar TXS 0506+056 in spatial and temporal coincidence with the neutrino event IceCube-170922A, that represented a milestone for the new field of multi-messenger astronomy. In this thesis, I present the results of a detailed gamma-ray analysis of the blazar TXS 0506+056 over 9.6 years of Fermi-LAT observations. The source shows strong flux variability in the gamma-ray band at the arrival of IceCube-170922A, indicating a neutrino gamma-ray connection, while is observed in a lowstate during an additional lower-energy neutrino signal detected from the same source by IceCube in 2014/15 in archival data. This puzzling behaviour has motivated further studies on the blazar sources coincident with single high-energy neutrinos. In this thesis, a second gamma-ray blazar GB6 J1040+0617, is found in spatial and temporal coincidence with the high-energy neutrino event IceCube-141209A while showing enhanced activity in the gamma-ray and optical bands. A second search for blazar counterparts to high-energy neutrinos is done through realtime follow-up of more than 60 IceCube alerts with the Fermi-LAT. The powerful blazar PKS 1502+106 found coincident with the event IceCube-190730A represents an outstanding science case. Lastly, a study to compare the average gamma-ray emission of the potential neutrino counterparts to the entire sample of gamma-ray blazars is presented. The results show possible indications of correlation between neutrino and gamma-ray energy fluxes.

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