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This thesis presents theoretical results on the tidal disruption of stars by supermassive black holes (SMBHs). The multiwavelength ares produced by tidal disruption events (TDEs) have supernova-like luminosities, and associated relativistic jets can be visible to cosmological distances. TDEs probe the demography of quiescent SMBHs, and are natural laboratories for jet launching mechanisms and super-Eddington accretion. The first chapter broadly surveys TDE physics. The second and third chapters estimate the TDE rate following gravitational wave (GW) recoil of a SMBH (after a SMBH binary merger). Immediately after GW recoil, the TDE rate increases, sometimes to \(~10^{-1}\) TDEs per year. This "burst" of TDE flares can provide an electromagnetic counterpart to low frequency GW signals, localizing sources and measuring cosmological parameters. Millions of years later, recoiled SMBHs wandering through their host galaxies will produce spatially offset TDEs at a rate which is likely detectable with the LSST. In the fourth chapter, we show that standard estimates for \(\Delta\epsilon\), the energy spread of TDE debris, are wrong, sometimes by orders of magnitude. Correcting this error reduces the observability of many TDEs. We introduce a new analytic model for tidal disruption, calculate \(\Delta\epsilon\)'s dependence on stellar spin, estimate general relativistic corrections to \(\Delta\epsilon\), and quantify the GW signal generated from tidal compression. The fifth chapter presents hydrodynamical simulations of TDE debris circularization, focusing on eccentric, rather than parabolic, orbits. General relativistic precession drives debris circularization, in contrast to past simulations using smaller black holes. In the sixth chapter, we show that TDE light curves can constrain or measure SMBH spins, as Lense-Thirring torques produce quasiperiodic variability in disk emission. Precession of a relativistic jet could also measure SMBH spin, and we apply our model to the relativistic Swift 1644+57 TDE. The seventh chapter considers the disruption of neutron stars (NSs) by stellar mass black holes (BHs) or other NSs. Jet precession in associated short-hard gamma ray bursts is uniquely possible for NS-BH (not NS-NS) mergers. We quantify typical precession amplitudes and periods, and calculate their time evolution. If disk viscosities are relatively low, electromagnetic observations alone could distinguish NS-BH from NS-NS mergers. / Astronomy

京都大学 / 0048 / 新制・課程博士 / 博士(理学) / 甲第20179号 / 理博第4264号 / 新制||理||1613(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)准教授 上田 佳宏, 教授 嶺重 慎, 教授 長田 哲也 / 学位規則第4条第1項該当 / Doctor of Science / Kyoto University / DFAM

Diese Dissertation untersucht den Ursprung der hochenergetischen Neutrinos, welche das IceCube-Observatorium am Südpol seit einer Dekade detektiert. Sie stellt das junge Feld der Neutrino-Astronomie vor und beschreibt das systematische Follow-Up-Programm für hochenergetische Neutrinos, das wir seit vier Jahren mit der Zwicky Transient Facility (ZTF) durchführen, einem optischen Teleskop. Ein wesentliches Resultat ist die Identifikation des astrophysikalischen Objekts AT2019fdr als mögliche Quelle eines hochenergetischen Neurinos. Dieses Objekt wird im Detail untersucht; so werden Daten quer durch das elektromagnetische Spektrum zusammengetragen und analysiert. Basierend auf diesen Beobachtungen und einer Modellierung der Lichtkurve komme ich zu dem Schluss, dass AT2019fdr ein sogenanntes Tidal Disruption Event (TDE) darstellt, wenn auch ein ungewöhnliches. Die Wahrscheinlichkeit, dass eine solche Assoziation nur Zufall ist, liegt bei 0.034 %, wenn man eine weitere TDE-Neutrino-Assoziation mit einberechnet. Weiterhin diskutiere ich das Infrarot-Staubecho von diesem Objekt im Zusammenhang mit zwei weiteren Assoziationen von möglichen TDEs mit hochenergetischen Neutrinos, die ebenfalls ein solches Staubecho aufweisen. Diese Studie wird begleitet von der Erstellung des ZTF nuclear sample, der ersten systematischen Sammlung innerhalb des ZTF-Datensatzes von solchen astrophysikalischen Ereignissen, die sich nahe dem Nukleus ihrer Wirtsgalaxie ereignen. Eines der Ziele dieser Untersuchung war es, die Zahl der TDEs zu vergrößern. Ich bewerkstellige dies durch die Entwicklung eines Algorithmus mit Verfahren des maschinellen Lernens zur photometrischen Typisierung astrophysikalischer Ereignisse. Dieser Klassifikator wird mit einem Datensatz nahegelegener astrophysikalischer Ereignisse trainiert, der zusätzlich künstlich verrauschter und lichtärmer gemacht wird, um dem nuclear sample mehr zu entsprechen. In Anwendung dieses Klassifikators auf das nuclear sample finde ich 27 neue TDE-Kandidaten. Die Identifikation von TDE-Kandidaten mittels ihres Infrarot-Staubechos ist ebenfalls erfolgreich und resultiert in 16 bisher nicht publizierten TDE-Kandidaten. / This thesis is concerned with the origin of high-energy neutrinos detected by the IceCube Observatory at the South Pole since a decade. It summarizes the young field of neutrino astronomy and details the systematic high-energy neutrino follow-up program we have been conducting with the Zwicky Transient Facility (ZTF), an optical survey telescope, for the past four years. One major finding is the establishment of the astrophysical transient AT2019fdr as counterpart to a high-energy neutrino. This transient is studied in detail, aided by the collection and reduction of data across the electromagnetic spectrum. Based on these observations and modeling of the light curve I conclude that this event constitutes a Tidal Disruption Event (TDE), albeit an unusual one. The chance coincidence of such an association is 0.034 % when including another previous association. Furthermore, I discuss the infrared dust echo from this transient in the context of two further possible associations of candidate TDEs with high-energy neutrinos, which also display such a dust echo. This study is appended by the creation of the ZTF nuclear sample, the first systematic sample of transient events found near the cores of their host galaxies within the ZTF survey. One goal of this sample is to enlarge the number of TDEs found so far. This is achieved by the development of a machine-learning based photometric typing algorithm. That classifier is trained on a survey of bright ZTF transients, including augmentation of that sample to account for the fainter nature of the nuclear sample. When applying the classifier to the nuclear sample, an additional 27 new candidate TDEs are found. Furthermore, the identification of candidate TDEs via their infrared dust-echo signal is also successful, resulting in 16 previously unpublished TDE candidates.

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

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Der Ursprung ultra-hochenergetischer kosmischer Strahlung (UHECRs) ist eine der wichtigsten offenen Fragen der Astrophysik. Gammastrahlenblitze (GRBs) galten als potentielle Quellen, da sie zu den energetischsten Ereignissen im Universum zählen. Konventionelle Szenarien sind jedoch durch Neutrinodaten stark eingeschränkt. Außerdem weisen Messungen der chemischen Zusammensetzung kosmischer Strahlen auf schwere Kerne hin, welche in zu dichten Strahlungsfeldern disintegrieren würden. Um dieses Dilemma zu umgehen deuten neue Studien auf versteckte Beschleuniger hin, welche schwer zu detektieren sind. In dieser Dissertation präsentieren wir neue Ansätze um nukleare Prozesse in astrophysikalischen Quellen effizient und selbstkonsistent zu berechnen. Wir quantifizieren diese Wechselwirkungen anhand der nuklearen Kaskade, welche die Disintegration schwerer Kerne in leichtere Fragmente beschreibt. Auch in umfassenden Modellen, wie sie in dieser Arbeit entwickelt werden, sind GRBs durch Neutrinodaten unter Druck. Dennoch zeigen wir, dass eine Population von GRBs niedriger Luminosität konsistent mit derzeitigen Messungen ist und zugleich auch das Spektrum und die Zusammensetzung kosmischer Strahlung über den Knöchel hinweg sowie Neutrinodaten beschreiben kann. Aus unserer Prozedur können wir zusätzlich weitere Quelleneigenschaften wie die baryonische Ladung oder die Ereignisrate bestimmen. Wir zeigen weiter, dass auch von schwarzen Löchern zerrissene Sterne mögliche Kandidaten eines gemeinsamen Ursprungs der gemessenen kosmischen Strahlung und PeV-Neutrinos sind. Sie können jedoch durch kosmogenische Neutrinos von LLGRBs abgegrenzt werden. Schließlich wenden wir unser Modell auf das Gravitationswellenereignis GW170817 an. Wir zeigen für verschiedene Jet-Szenarien, dass der erwartete Neutrinofluss weit unter der Sensitivität derzeitiger Instrumente liegt. Dennoch könnten verschmelzende Neutronensterne die kosmische Strahlung unterhalb des Knöchels erklären. / The origin of Ultra-High Energy Cosmic Rays (UHECRs) is still one of the most important open questions in astrophysics. Gamma-Ray Bursts (GRBs) were considered as potential sources as they are among the most energetic events known in the Universe. However, conventional GRB scenarios are strongly constrained by astrophysical neutrino data. In addition, cosmic ray composition measurements indicate the presence of heavy nuclei, which would disintegrate if the radiation fields in the source were too dense. In order to circumvent this dilemma, recent studies point towards hidden accelerators, which are intrinsically hard to detect. In this dissertation, we present novel approaches to efficiently and self-consistently calculate the nuclear processes in astrophysical sources. We quantify these interactions by means of the nuclear cascade, which describes the subsequent disintegration of heavy nuclei into lighter fragments. Even in sophisticated source-propagation models, as the ones developed in this thesis, conventional GRBs are in tension with neutrino data. However, we demonstrate that a population of low-luminosity GRBs is not only consistent with current constraints, but can even describe the UHECR spectrum and composition across the ankle as well as neutrino data simultaneously. From our fitting procedure we can further constrain certain source properties, such as the baryonic loading and the event rate. Furthermore, we show that stars disrupted by black holes are viable candidates for a simultaneous description of cosmic ray and PeV neutrino data too. However, they can be discriminated from LLGRBs by cosmogenic neutrinos. Finally, we apply our model to GW170817. We show for different jet scenarios that the expected neutrino flux is orders of magnitude below the sensitivity of current instruments. Nevertheless, binary neutron star mergers could in principle support cosmic rays below the ankle.