The standard model of particle physics cannot currently explain the origin of neutrino masses and anomalies that have been observed at different experiments. One solution for this is to introduce a beyond the standard model origin for these issues, which introduces a coupling between neutrinos and dark matter. Such an interaction would have implications on cosmology and would be constrained by astrophysical neutrino sources. A promising astrophysical source to probe this interaction is core-collapse supernovae as they release ~3x10^53 erg in neutrinos for each transient. However, more observations that constrain the internal physics of core-collapse supernovae are needed in order to better understand their neutrino emission. This dissertation studies two probes of internal physics that allow for a better understanding of the neutrino emission from core-collapse supernovae. The first is a novel approach to try and detect more supernova neutrinos that do not come from galactic events nor from the diffuse supernova background. This is accomplished by doing an offline timing coincidence search at neutrino detectors with a search window determined by optical observations of core-collapse supernovae. With a two-tank Hyper-Kamiokande, this allows for ~1 neutrino detection every 10 years with a confidence level of ~2.6 sigma, resulting from low nearby core-collapse rates and large background rates in the energy range of interest. The second probe of internal physics is high energy gamma-rays from the decays of unstable nuclei in proto-magnetar jets. The abundance distribution of the unstable nuclei depends directly on the neutrino emission, which controls the electron fraction, as well as properties of the proto-magnetar. We find that different proto-magnetar properties produce gamma-ray signals that are distinguishable from each other, and multiple types of observations allow for estimations of the jet and proto-magnetar properties. These gamma-ray signals are detectable for on-axis jets out to extragalactic distances, ~35 Mpc in the best case, and for off-axis jets the signal is only detectable for galactic or local galaxies depending upon the viewing angle. This dissertation also studies a phenomenological constraint on the interactions between neutrinos and dark matter. Using the neutrino emission from supernovae and the inferred dark matter distributions in Milky Way dwarf spheroidals, we constrain the amount of energy the neutrinos can inject into the dark matter sub-halos. This then allows a constraint on the interaction cross-section between neutrinos and dark matter with assumptions about the interaction kinematics. Assuming Lambda-CDM to be correct, the neutrinos cannot interact with low mass dark matter too often as it will become gravitationally unbound, changing the mass of the core we see today. For high mass dark matter, neutrinos can only inject a fraction of ~6.8x10^-6 of their energy in order to not conflict with estimates of the current shapes of the dark matter sub-halos. The constraints we obtain are sigma_nu-DM(E_nu=15 MeV, m_DM>130 GeV) ~ 3.4x10^-23 cm^2 and sigma_nu-DM(E_nu=15 MeV, m_DM <130 GeV) ~ 3.2x10^-27} (m_DM/1 GeV)^2 cm^2, which is slightly stronger than previous bounds for these energies. Consideration of baryonic feedback or host galaxy effects on the dark matter profile can strengthen this constraint. / Doctor of Philosophy / In our current understanding of the physics of the particles that govern how the universe behaves, there is no way to explain the properties we observe for the neutrino. Neutrinos were originally theorized to have zero mass, however neutrino experiments suggests otherwise. The current model of particle physics cannot explain how the neutrinos have mass, therefore an viable way to explain it is to introduce new physics that can generate the neutrino masses. A way to do this is to allow the neutrinos to interact with dark matter, which is matter that does not interact with light and is therefore invisible to the human eye. We know dark matter should exist in the universe due to the gravitational effects it has, making things like galaxies much heavier than what the stars and gas we see can explain. If neutrinos and dark matter interact, we should be able to see the effects of these interactions in the universe, and also possibly at locations where many neutrinos are produced. One such source of neutrinos in the universe are core-collapse supernovae, which are the deaths of massive stars and produce copious amounts of neutrinos. This dissertation studies signals that allow us to better understand the neutrino emission from core-collapse supernovae. One of these signals comes from summing the neutrinos we detect from many distant core-collapse supernovae. This technique uses the optical observations of the supernovae to give us a time window around which we can go through neutrino detector data to find if there are any neutrino detections that cannot be explained as coming from background events. Another method is to observe gamma-rays, high energy photons, that come from the radioactive decay of elements in jets moving near the speed of light powered by rare core-collapse supernovae. The specific gamma-rays and the overall brightness of them allows for an estimation of the properties of the neutrino emission and properties of the central engine that accelerates the jet to near the speed of light. This dissertation also studies the implications of a possible interactions in small and dim satellite galaxies of the Milky Way known as dwarf spheroidals. The shape of the dark matter that is distributed in these dwarf spheroidals can be inferred from the motion of the stars in that dwarf spheroidal, and this shape disagrees with the prevailing theory of dark matter in the universe. We take advantage of this disagreement to place an upper limit on both the mass loss that can occur in this region and the energy that past core-collapse supernovae within the dwarf spheroidals can inject into the dark matter. The mass loss bound lets us place a constraint on how often neutrinos can interact with light dark matter particles. The energy injection limit and an assumption on the energy transfer in each interaction between dark matter and neutrinos allows us to constrain how often the interaction can occur for heavy dark matter particles.

Die Entdeckung eines hochenergetischen Flusses astrophysikalischer Neutrinos stellt einen wesentlichen physikalischen Durchbruch der letzten Jahre dar. Trotz allem ist der Ursprung dieser Neutrinos immer noch unbekannt. Die Suche nach den Quellen der hochenergetischen kosmischen Strahlung ist direkt verbunden mit der Suche nach Neutrinos, da diese in den gleichen hadronischen Prozessen erzeugt werden und eine Neutrinoquelle deshalb einen direkten Hinweis auf eine Quelle der kosmischen Strahlung darstellen würde. Viele potentielle Quellen der Neutrinos werden diskutiert, darunter Kern-Kollaps Supernovae. In dieser Arbeit werden sieben Jahre Daten des IceCube Neutrinoteleskopes mit der Richtung mehreren Hundert Kernkollaps-Supernovae auf Korrelation getestet. Die Analyse gewinnt dabei durch die gute Richtungsrekonstruktion der 700000 Muonspurdaten und der großen Datenbank optische beobachteter Supernovae. Die Sensitivität der zeitabhängigen Likelihood-Analyse wird durch die Kombination mehrere Quellen in einer einzigen Analyse gesteigert. Es wurde kein statistisch signifikantes Cluster von Neutrinos an den Positionen der Supernovae gefunden. Daraus wurden obere Grenzen für verschiedene Modelle berechnet und der Beitrag von Kernkollaps-Supernovae zum diffusen Neutrinofluss eingeschränkt. Daraus können bestimmte Typen von Supernovae als dominate Quelle der diffusen hochenergetischen astrophysikalischen Neutrinos ausgeschlossen werden. / The recent discovery of a high energy flux of astrophysical neutrinos was one of the breakthroughs of the last years. However, the origin of these neutrinos remains still unknown. Also, the search for the sources of high-energy cosmic rays is closely connected to neutrinos since neutrinos are produced in hadronic interactions, and thus the detection of a neutrino source would be a \textit{smoking gun} signature for cosmic rays. Many potential neutrino source classes have been discussed, among these are core-collapse supernovae. In this thesis, seven years of data from the IceCube neutrino observatory are tested for correlation with the direction of hundreds of core-collapse supernovae. The analysis benefits from the good angular reconstruction of the order of one degree and below of the about 700000 muon track events and an extensive database of optical observations of supernovae. Using a time-dependent likelihood method, the sensitivity of the analysis is increased by stacking the sources in a combined analysis. No significant clustering of neutrino events around the position of core-collapse supernovae is found. Upper limits of different neutrino light curve models are computed, and the contribution of core-collapse supernovae to the measured diffuse high energetic neutrino background is constrained. These limits allow excluding certain types of core-collapse supernovae as the dominant source of the observed high energetic astrophysical neutrino flux.

Zeitabhängige Multimessenger-Astronomie ist die Studie von vorübergehenden astrophysikalischen Phänomenen mithilfe verschiedener Botschafterteilchen und -wellen. Das High Energy Stereoscopic System (H.E.S.S.) ist ein bodengestütztes Teleskop-Array, das Gammastrahlen im Bereich von 30 GeV bis 100 TeV misst. In dieser Arbeit werden drei H.E.S.S.-Programme zur Nachverfolgung von vorübergehenden Phänomenen diskutiert. Zuerst wird das Nova-Programm vorgestellt, zusammen mit der Entdeckung des Ausbruchs von RS Ophiuchi (RS Oph) im Jahr 2021. RS Oph ist eine Nova, welche schon mehrmals ausgebrochen ist. Die Analyse der Nova, zeigt eine klare Detektion über die ersten fünf Nächte sowie eine marginale Detektion bis zu vier Wochen nach dem Ausbruch. RS Oph ist damit das erste galaktische vorübergehende Phänomen, das bei Energien von ~1 TeV detektiert wurde. Eine klare Variabilität des Spektrums wird gezeigt. Eine Diskussion zeigt, dass die beobachteten Gammastrahlen höchstwahrscheinlich durch beschleunigte Protonen innerhalb eines astrophysikalischen Schocks stammt. Die Ergebnisse zeigen eine zeitaufgelöster Teilchenbeschleunigung. Das zweite Programm ist das Gravitationswellen (GW)-Programm. Hier wird die Analyse von vier beobachteten Verschmelzungsereignissen von binären schwarzen Löchern vorgestellt. Es wird keine Detektion gemeldet, stattdessen werden Himmelskarten mit Höchstgrenzen produziert. Die Effektivität der GW-Nachverfolgung mit H.E.S.S. wird diskutiert. Das dritte Programm ist das Pionierprogramm für Gezeiten-Sternzerrissereignisse (TDEs), das in den letzten Jahren etabliert wurde. Herausforderungen des Programms werden diskutiert, und die H.E.S.S.-Nachverfolgung des Ereignisses AT2019uqv wird vorgestellt. Es werden keine Detektionen gemeldet, stattdessen werden obere Grenzen für AT2019uqv angegeben. Abschließend erfolgt ein Vergleich der Programme sowie ein Ausblick auf das nächste Jahrzehnt bodengestützter Gammastrahlenbeobachtungen. / Time-domain multi-messenger astronomy is the study of transient astrophysical phenomena using several messenger particles and waves. The High Energy Stereoscopic System (H.E.S.S.) is a ground-based telescope array, which measures very-high-energy gamma rays between 30 GeV and 100 TeV. Within this work, three H.E.S.S. transients follow-up programs and their results are shown. At first, the nova program is shown together with the detection of the 2021 RS Ophiuchi (RS Oph) outburst. RS Oph is a known nova with past eruptions. The analysis of the nova presented in this work shows a clear detection over the first five nights of observations, and a marginal detection two to four weeks after the eruption. RS Oph is thus the first Galactic transient phenomenon detected at ~1 TeV energies. A clear variability of the spectrum is shown. A discussion of the underlying physics concludes, that the observed gamma-ray emission most likely stems from cooled protons accelerated within an astrophysical shock. The results show time-resolved particle acceleration. The second program is the gravitational-waves (GWs) program. Here, the analysis of four observed binary black hole merger events is shown. No detection is reported, and upper limit sky maps are derived instead. The viability of GW follow-up with H.E.S.S. is discussed, and a case is made for a potential counterpart detection. The third program is the tidal disruption event (TDE) program, a pioneered program established at the emergence of the source class within the last few years. Unique challenges of the follow-up program are discussed, and the H.E.S.S. follow-up of the event AT2019uqv is shown. No detection is reported, and upper limits for AT2019uqv are given instead. The interest in follow-up observations of TDEs with gamma-ray instruments is stressed. In conclusion, a comparison of the different programs and their requirements is given, together with an outlook for the next decade of ground-based gamma-ray observations.

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.

Das Hauptziel dieser Arbeit ist die Charakterisierung extrem starker Quellen, die höchstwahrscheinlich die kosmische Strahlung beschleunigen. In dieser Arbeit wurden VHE-Gammastrahlenbeobachtungen mit den MAGIC-Teleskopen verwendet, um die Eigenschaften von Blazaren zu untersuchen. Um die Mechanismen zu untersuchen, die zur Breitbandemission von Blazaren führen, wird ein stationärer lepto-hadronischer Code unter Verwendung eines einfachen semianalytischen Frameworks entwickelt. Daher implementiert der Code neben den leptonischen Wechselwirkungen auch die relevanten hadronischen Wechselwirkungskanäle: Protonensynchrotronstrahlung, Photo-Meson-Wechselwirkungen, Proton-Proton-Wechselwirkungen und Paarkaskaden. Die Dissertation präsentiert die Ergebnisse derMAGIC- und Multiwellenlängen-Monitoring-Kampagne des Blazars 1ES 1959 + 650 im Jahr 2016. Im Jahr 2016 durchlief die Quelle eine äußerst aktive Phase und zeigte am 13. Juni, 14. Juni und 1. Juli 2016 drei bemerkenswert helle VHE-Gammastrahlenfackeln. Um die Breitbandspektren der Quelle während der bemerkenswerten Fackelaktivitäten zu untersuchen, wurden drei verschiedene theoretische Modelle übernommen: leptonisch, hadronisch und gemischt lepto-hadronisch. Sowohl das hadronische als auch das gemischte leptohadronische Modell ergaben während der intensiven Aktivitätsperiode Neutrinoflüsse, die unter der Empfindlichkeit der gegenwärtigen Generation von Neutrinoteleskopen liegen. Die Beobachtung eines hochenergetischen Neutrinos durch IceCube im räumlichen und zeitlichen Zusammentreffen mit einem aufflammenden Blazar mit dem Namen TXS 0506 + 056 ergab 2017 erstmals Hinweise auf Identifizierung einer extragalaktischen kosmischen Strahlenquelle. Die Modellierung der elektromagnetischen Daten und des vorhergesagten Neutrinoflusses impliziert, dass die Quelle tatsächlich ein potenzieller Neutrinostrahler und damit ein Beschleuniger für energiereiche kosmische Strahlen sein könnte. / The main aim of this thesis is to characterize extremely powerful sources that are most likely accelerating cosmic rays. Cosmic-ray sources are also believed to produce photons and neutrinos that act as direct tracers of their sources of origin. In this thesis VHE gamma-ray observations by the MAGIC telescopes were used to study the properties of blazars. To investigate the mechanisms giving rise to the broadband emission from blazars, a stationary lepto-hadronic code is developed using a simple semi-analytical framework. Hence along with the leptonic interactions, the code also implements the relevant hadronic interaction channels: proton synchrotron radiation, photo-meson interactions, proton-proton interactions and pair cascades. The thesis presents the results from theMAGIC and multi-wavelength monitoring campaign of the blazar 1ES 1959+650 during 2016. In 2016 the source underwent into an extremely active phase and exhibited three remarkably bright VHE gamma-ray flares on 13th June, 14th June and 1st July of 2016. On two of these nights, signs of rapid flux variability within sub-hour timescales was clearly resolved by the MAGIC observations. In order to investigate the broadband spectra of the source during the remarkable flaring activities, three different theoretical models were adopted: leptonic, hadronic and mixed lepto-hadronic. Both the hadronic and mixed leptohadronic models yielded neutrino fluxes during the intense activity period, that falls below the sensitivity of the current generation of neutrino telescopes. In 2017, the observation of a high-energy neutrino by IceCube in spatial and temporal coincidence with a flaring blazar named TXS 0506+056 yielded for the first time, hints towards identification of an extragalactic cosmic-ray source. The modelling of the electromagnetic data and the predicted neutrino flux implies that the source could indeed be a potential neutrino emitter and hence an accelerator of high-energy cosmic rays.