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.

[pt] Conforme entramos na era de precisão da astronomia multimensageira, novas janelas se abrem para compreendermos melhor o Universo, desde a escala quântica até a escala cósmica. Em particular, o estudo de fenômenos astrofísicos de altas energias tem nos permitido acessar os ambientes mais extremos conhecidos pela humanidade, bem como obter avanços sem precedentes no domínio da física de partículas. Esta tese resume as descobertas importantes da astrofísica multi-mensageira ao longo dos anos, e, em seguida, foca a sua atenção em três tópicos relevantes que estão atualmente sendo investigados neste campo. Primeiramente, abordamos o problema da propagação de raios gama no espaço. Interações durante este processo levam à formação de cascatas eletromagnéticas que se desenvolvem ao longo de distâncias cosmológicas. Apresentaremos um código semi-analítico chamado “γ-Cascade”, que calcula os fluxos na Terra resultantes de tais cascatas. Também exploramos a possibilidade de se produzir neutrinos em cascatas ocorrendo a energias ultra-altas. Em segundo lugar, estabeleceremos uma relação multimensageira nova e original entre os fluxos medidos de neutrinos astrofísicos entre TeV–PeV e raios cósmicos ultra-energéticos. Para isso, utilizaremos nossas observações precisas de raios gama em energias abaixo de TeV, demonstrando o poder de uma análise multimensageira. Finalmente, estudaremos a evolução da composição de sabor de neutrinos produzidos em supernovas. Nosso novo método permite previsões genéricas sobre os possíveis sabores de neutrinos medidos na Terra. São levados em consideração os efeitos de matéria dentro dos ambientes densos de supernovas, enquanto permanecemos completamente agnósticos em relação ao resultado das conversões auto-induzidas de sabor em seus núcleos. / [en] As we enter the precision era of multi-messenger astronomy, new windows are opened for us to better understand the Universe, from quantum to cosmic scales. In particular, the study of high-energy astrophysical phenomena has allowed us to probe the most extreme environments known to mankind, as well as obtain unprecedented breakthroughs within the realm of particle physics. This thesis summarizes the important findings of multi-messenger astrophysics over the years, before focusing its attention to three relevant topics currently being investigated in the field. Firstly, we tackle the problem of γ-ray propagation in space. High center-of-momenta interactions during this process leads to the formation of electromagnetic cascades that develop over cosmological distances. We describe a semi-analytical code called “γ-Cascade, which calculates the fluxes at the Earth resulting from such cascades. We also explore the possibility of producing neutrinos in ultra-high-energy cascades. Secondly, we establish a new, original multi-messenger connection between the measured fluxes of TeV–PeV astrophysical neutrinos and ultra-high-energy cosmic rays. This is done by taking advantage of our precise γ-ray observations at sub-TeV energies, demonstrating the power of multi-messenger analyses. Finally, we study the evolution of the flavor composition of supernova neutrinos in a model-independent way. Our novel method allows for predictions of the neutrino flavor content measured at the Earth from supernovae, accounting for matter effects within its dense environment, while remaining completely agnostic about the outcome of self-induced flavor conversions in its core.

[pt] Em 2013, o IceCube Neutrino Observatory, localizado no Polo Sul, descobriu um fluxo de neutrinos astrofísicos com energias de PeV. Mais tarde, descobriu-se que este fluxo se estendia até pelo menos aproximadamente 10 TeV. Apesar de muitos esforços desde então, determinar as suas fontes permanece sendo um dos maiores desafios na comunidade de astrofísica. Nesta dissertação, investigamos possíveis fontes através de uma abordagem multimensageira bem motivada. Em qualquer mecanismo para a produção de neutrinos cósmicos, obrigatoriamente há produção simultânea de raios gama com energias comparáveis. Ao contrário de neutrinos, que atravessam o Universo ilesos, raios gama de altas energias sofrem interações com fótons de fundo em um processo de degradação de energia conhecido como cascata eletromagnética. Na Terra, eles contituem o fundo extragalático de raios gama difuso (EGB), medido com precisão pelo Fermi Gamma-ray Space Telescope entre GeV–TeV. Realizando uma análise conservativa, quantitativa e multimensageira, encontra-se uma tensão de maior ou aproximadamente 3delta (possivelmente chegando a aproximdamente 5 delta) entre os dados do IceCube e do EGB, apontando para a exitência de uma nova classe de aceleradores cósmicos de alta energia, como, por exemplo, fontes opacas para raios gama. / [en] In 2013, the IceCube Neutrino Observatory, located at the South Pole, discovered a flux of astrophysical neutrinos with PeV energies, later found to extend down to at least approximately 10 TeV. Despite many efforts since then, determining their sources remains one of the most daunting challenges in the astrophysics community. In this dissertation, we investigate possible sources via a well-motivated multimessenger approach. In any production mechanism of cosmic neutrinos, there must also be a simultaneous production of gamma-rays withcomparable energies. Unlike neutrinos, which travel unscathed throughout the Universe, high energy gamma-rays undergo interactions with background photons in an energy-degrading process known as electromagnetic cascade. At the Earth, they constitute the diffuse extragalactic gamma-ray background (EGB), precisely measured by the Fermi Gamma-ray Space Telescope in the GeV–TeV range. By performing a conservative, quantitative, multimessenger analysis, we find greater than a or approximately to 3 delta (possibly as large as approximately 5 delta) tension between IceCube and EGB data, pointing towards the existence of a new class of high energy cosmic accelerators, such as gamma-ray-opaque sources.