Das IceCube Neutrino Observatory ist das größte Neutrinoteleskop der Welt und befindet sich am Südpol. IceCube hat im Jahr 2013 einen isotropen Fluss astrophysikalischer Neutrinos entdeckt, aber ihre Herkunft bleibt größtenteils unbekannt. Das Ziel dieser Doktorarbeit ist es, nach den Quellen der energiereichsten Neutrinoereignisse zu suchen, die von IceCube detektiert wurden. Diese sogenannten ‘Neutrino Alert’- Ereignisse sind höchstwahrscheinlich astrophysikalischen Ursprungs. Diese Arbeit präsentiert die Ergebnisse der ersten IceCube Analyse, die nach Korrelationen zwischen einem Katalog von ‘Neutrino Alert’-Ereignisse und potenziellen Quellen sucht. Zwei Quellenklassen werden untersucht: Gammastrahlen-Blazare aus dem Fermi-LAT 4LAC-DR2-Katalog und radiohelle Aktive Galaktische Kerne aus dem RFC 2022a-Katalog. Beide Klassen wurden in vielen theoretischen Modellen als Neutrinoquellen vorgeschlagen. Sowohl aufgrund der extremen Beschleunigung, die in ihnen stattfindet, als auch der hohen Dichte ihrer umgebenden Materie. Um den Beitrag jeder Quelle im Katalog zum gesamten Ergebnis zu quantifizieren, wird ein Gewicht angewendet. Dieses Gewicht hängt vom Modell des Neutrinoproduktionsmechanismus ab. Für den Fermi-LAT 4LAC-DR2-Katalog werden zwei Gewichtungsschemata betrachtet: der normalisierte 10-Jahres-Durchschnittsenergiefluss in Gammastrahlen und der normalisierte Energiefluss im 1-Monats-Zeitintervall, das mit der Ankunftszeit des Neutrinos zusammenfällt. Für den RFC 2022a-Katalog werden die Quellen nach der Flussdichte gewichtet, die über VLBI-Bilder bei 8,6 GHz integriert ist. Es wurde keine signifikante Neutrinoemission in einer der drei Katalogsuchen festgestellt. Dieses Ergebnis ist mit einem kleinen Anteil von Quellen (< 1%), die Neutrinos hoher Energie emittieren, vereinbar. Als Teil dieser Doktorarbeit wird die Auswirkung von systematischen Unsicherheiten auf die Winkelrekon- struktion von ‘Neutrino Alert’-Ereignissen untersucht. Zwei Datensätze simulierter Ereignisse wurden erstellt, die eine Methode verwenden, die sicherstellt, dass alle Ereignisse innerhalb jeder definierten Kategorie ähnlich sind. Diese Ereignisse werden dann mit dem Algorithmus rekonstruiert, der für ‘Neutrino Alert’-Ereignisse angewendet wird. Die Ergebnisse dieser Studien zeigen, dass eine robusterer Umgang mit den systematischen Unsicherheiten erforderlich ist, da die Unsicherheitskonturen, die mit der aktuellen Methode berechnet werden, nicht den erwarteten Vertrauensniveaus für einzelne Ereignisse entsprechen. Darüber hinaus ermöglicht diese Arbeit zum ersten Mal einen direkten Vergleich mit anderen Rekonstruktionsalgorithmen, die eine bessere Winkelauslösung bei einem Bruchteil der Rechenkosten erreichen. / The IceCube Neutrino Observatory is the largest neutrino telescope in the world, located at the South Pole. IceCube discovered an all-sky isotropic flux of astrophysical neutrinos in 2013, but their origin remains mostly unknown. The goal of this thesis is to identify the sources of the most energetic neutrino events detected by IceCube. These so-called neutrino alert events are very likely to be of astrophysical origin. This work is the first analysis performed in IceCube that searches for correlations between a catalog of neutrino alert events and potential sources. Two source classes are considered, gamma-ray blazars from the Fermi-LAT 4LAC-DR2 catalog and radio-bright Active Galactic Nuclei from the RFC 2022a catalog. Both classes have been suggested as neutrino emitters in many theoretical models due to the extreme acceleration that happens inside and the high density of their surrounding matter. To quantify the contribution of each source in the catalog to the final result, a weight is applied. This weight depends on the model of the neutrino production mechanism. For the Fermi-LAT 4LAC-DR2 catalog, two weighting schemes are considered: the normalized 10-year average energy flux in gamma rays, and the normalized energy flux in the 1-month time bin that coincides with the neutrino arrival time. For the RFC 2022a catalog, the sources are weighted by the flux density integrated over VLBI images at 8.6 GHz. There is no significant neutrino emission in any of the three searches. This result is compatible with a small fraction of sources (< 1%) emitting neutrino alert events. As part of this thesis, the impact of systematic uncertainties on the angular reconstruction of neutrino alert events is studied. Two datasets of simulated events were created using a method that ensures the similarity of all events within each defined category. These events are then reconstructed with the algorithm that is applied to neutrino alert events. The results from these studies prove that a more robust treatment of the systematic uncertainties is needed, since the uncertainty contours calculated with the current method do not correspond to the expected confidence levels for individual events. Furthermore, this work enables a direct comparison to other reconstruction algorithms for the first time, revealing that these alternative methods achieve better angular resolution at a fraction of the computational cost.

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.

Cosmic rays reach Earth from beyond the Milky Way and with energies up to 10^20 eV.The responsible accelerators have to date not been discovered. However, multi-messenger astronomy can shed light on the question, based on the principle that protons and nuclei accelerated in dense and energetic environments would also produce gamma rays and neutrinos. Such environments may be found in "blazars", which are therefore cosmic ray accelerator candidates. Their gamma-ray emission has been observed to increase, sometimes by orders of magnitude, during "flares" as observed in light curves taken by the Large Angle Telescope on the Fermi satellite. When the latter was launched in 2008, the IceCube Neutrino Observatory had also started taking data, detecting the Cherenkov light from high-energy neutrino interactions in the glacier ice under the geographic South Pole. These two experiments have enabled multi-messenger searches for neutrinos in time correlation with the gamma-ray emission from blazars. This work builds on this principle and extends it by "stacking" the signal from multiple blazar flares. Thus, their individually undetectable neutrino emission could still be discoverable. One first analysis focused on the blazar TXS 0506+056, whose flare in 2017 coincided with arrival of the neutrino IceCube-170922A. Extending into a lower energy range than the alert, the search found no additional excess neutrinos associated with the flare. A second analysis used 179 bright and variable blazars. They were divided in two specific blazar classes and weighted relatively to each other, with two weighting schemes motivated physically using the observed gamma-ray luminosity and a third, generic weighting to cover unconsidered scenarios. No significant neutrino excess was found in the unblinded likelihood fits for any of the source catalogues and weighting schemes. Their combined trial-corrected p-value was p=(79.1 +/- 0.3)%. The limits derived from this analysis are also discussed and its relation with other searches considered. Since that was the first "blazar flare stacking", this work also proposes further improvements to the analysis which will help advance the search for cosmic ray accelerators. / Les rayons cosmiques proviennent d'au-delà de la Voie lactée et atteignent la Terre avec des énergies pouvant aller jusqu'à 10^20 eV. Les objets qui accélèrent ces rayons cosmiques n'ont toujours pas été découverts. Toutefois, l'astronomie multimessager peut apporter un élément de réponse à cette question, en supposant que les protons et les noyaux accélérés dans des environnements denses et énergétiques pourraient également produire des rayons gamma et des neutrinos. Les "blazars" sont de possibles candidats pour les accélérateurs de rayons cosmiques. Une augmentation de leurs émissions de rayons gamma, parfois de plusieurs ordres de grandeur, a été observée lors de phénomènes qu'on appelle "éruption", comme le montrent les courbes de lumière prises par le télescope spatial Fermi-LAT. Lorsque ce dernier a été lancé en 2008, l'observatoire de neutrinos IceCube avait également commencé à prendre des données, détectant la lumière Tcherenkov provenant d'interactions de neutrinos à haute énergie dans la glace qui se trouve sous le Pôle Sud géographique. Ces deux expériences ont permis de mener à bien des recherches multi-messagers de neutrinos en corrélation temporelle avec l'émission de rayons gamma des blazars. Ce principe est le point de départ de cette thèse, qui va plus loin en employant la méthode du "stacking", qui consiste à combiner les signaux provenant de plusieurs éruptions de blazars. Ainsi, leurs émissions individuelles de neutrinos, habituellement indétectables, pourraient être découvertes après combinaison. Une première analyse s'est concentrée sur le blazar TXS 0506+056, dont l'éruption en 2017 a coïncidée avec l'arrivée de l'évènement IceCube-170922A. En considérant une gamme d'énergie inférieure à celle de l'alerte 170922-A, pas d’autres neutrino excédentaire n’a été associé à l'éruption. Une deuxième analyse est basée sur 179 blazars lumineux et variables. Ces blazars ont été répartis en deux classes spécifiques, et chacun d'entre eux a reçu un poids relatif. Trois schémas de pondération ont été considérés :les deux premiers étant motivés par des observations, le troisième étant plus générique. Aucun excès significatif de neutrinos n'a été observé après avoir effectué des ajustements par maximum de vraisemblance sur les données non masquées, pour les différents catalogues de sources et schémas de pondération. Leur valeur-p combinée est de p=(79.1 +/- 0.3)%. Les limites dérivées de cette analyse sont discutées ainsi que leur rapport avec les résultats d'autres recherches. Puisqu'il s'agit du premier stacking d'éruptions de blazars, nous suggérons également des améliorations à apporter à l'analyse afin de permettre la poursuivre de la recherche d'accélérateurs de rayons cosmiques. / Kosmische straling afkomstig van buiten de Melkweg bereikt de Aarde met energieën tot wel 10^20 eV. De astrofysische bronnen waarin deze deeltjes worden versneld zijn tot op heden nog niet ontdekt. De multi-boodschapperastronomie kan een nieuw licht werpen op de oorsprong van kosmische straling, aangezien protonen en atoomkernen die worden versneld in een dichte en energetische omgeving ook gammastralen en neutrino’s produceren. „Blazars” zijn mogelijke kandidaat-versnellers. Observaties van blazars, gemaakt met de ruimtetelescoop Fermi-LAT ,tonen aan dat hun gammastraling tijdens zogenaamde „flakkers” toeneemt. Rond de tijd dat deze werd gelanceerd, begon het IceCube Neutrino Observatorium ook gegevens te verzamelen. Deze laatste detecteert hoog-energetische neutrino’s aan de hand van het Cherenkovlicht dat geproduceerd wordt tijdens hun interacties met de ijskap bij de geografische zuidpool. Deze twee experimenten hebben het mogelijk gemaakt om een multibooschapperzoektocht te verrichten naar neutrino’s van blazars die een tijdscorrelatie hebben met diens flakkers van gammastraling. Dit is het uitgangspunt van dit proefschrift, waarbij er ook een zogenaamde „stapelmethode” wordt toegepast. Op deze manier kan de neutrino-emissie van indivuele blazarflakkers, die afzonderlijk te zwak is om te detecteren, gecombineerd worden en mogelijks toch worden ontdekt. Een eerste analyse legt de focus op de blazar TXS 0506+056, waarvan een flakker in 2017 samenviel met de aankomst van het neutrino IceCube 170922-A. In een relatief lager energiebereik wordt er geen surplus aan neutrino’s gevonden gecorreleerd met de flakker. In een tweede analyse maken we gebruik van de stapelmethode om neutrino’s te zoeken afkomstig van 179 heldere en variabale blazars. Deze worden onderverdeeld in twee specifieke klassen en krijgen elks een zeker gewicht in de stapelanalyse. Hiervoor worden twee wegingsschema’s gebruikt die gemotiveerd zijn door de geobserveerde gammastraling, alsook een derde generieke weging. Ook hierwordt er geen significant neutrinosignaal geobserveerd. De gecombineerde p waarde is p=(79.1 +/- 0.3)%. Hieruit worden limieten afgeleid, en worden de verbanden met andere zoekacties besproken. Aangezien dit werk de eerste analyse omvat naar neutrino’s afkomstig van blazarflakkers gebruik makende van een stapelmethode, worden er in dit werk ook verdere verbeteringen van de analyse voorgesteld. Deze zullen als een startpunt dienen voor toekomstige zoektochten naar de nog onbekende bronnen van kosmische straling. / Doctorat en Sciences / info:eu-repo/semantics/nonPublished

<p dir="ltr">Neutron star mergers generate powerful gravitational waves and various types of electromagnetic signals, including gamma-ray bursts (GRB), kilonovae, and their afterglows. Observing and modeling these signals help us understand the physical processes of the merger events. Radiation from mergers can also serve as probes to study nuclear physics and cosmology. In this report, I focus on two types of signals: the GRB afterglow and the kilonova. GRB afterglows are non-thermal radiation produced by the interaction of relativistic jets and circumburst material, where the jets are launched perpendicular to the merger plane. Kilonovae are the thermal radiation emitted from the hot materials ejected during the merger. Besides the modeling of these objects, I also investigate their application in multimessenger astrophysics, especially the constraint on the expansion rate of the Universe. </p><p dir="ltr">First, I developed a GRB afterglow model to account for the off-axis observation of a structured jet. Using a jet structure derived from a three-dimensional general relativistic magnetohydrodynamic simulation, we performed a joint analysis of the multimessenger data of the neutron star merger event GW170817, including the gravitational wave data and GRB afterglow data in the radio band. We have tightly constrained the observing angle of GW170817 and broken the degeneracy between the inclination angle and luminosity distance measured in gravitational waves. With a better constrained distance, we improved the standard siren measurement of the Hubble constant to $H_0 = 69.5\pm 4\ \mathrm{km\ s^{-1}\ Mpc^{-1}}$. The error bar has been reduced by a factor of 2. This work demonstrates that the modeling of off-axis GRB afterglow can significantly improve the standard siren method, provided that we have a reliable jet structure.</p><p dir="ltr">Second, I upgrade the GRB afterglow model in the first work, extending it to the late time where lateral spreading of the GRB jet becomes important. In this model, the ultra-relativistic blastwave is approximated by an infinitely thin two-dimensional surface. With this approximation, the hydrodynamic equations can be analytically integrated over the radius. Further assuming axial symmetry, the three-dimensional hydrodynamic simulation can be reduced to one dimension, which significantly increases the computational efficiency. We have compared our method to full numerical simulations and existing GRB afterglow modeling tools. The comparison shows good agreement and verifies our approach. Compared to these tools, our model has better flexibility and is applicable in a broader context. This method has been developed into a numerical code, \texttt{jetsimpy}, which we have provided to the community. It will serve as a powerful tool in the era of multimessenger astrophysics.</p><p dir="ltr">Finally, I investigate the possibility of long-lived massive neutron stars as neutron star merger remnants. A long-lived massive neutron star can inject a significant amount of energy into the merger ejecta, boosting the luminosity of kilonova by several orders of magnitude. However, this type of event has not yet been observed in optical sky surveys. We developed a boosted kilonova model with a detailed calculation of the photoionization process to better describe the efficiency of energy injection from spin down power to the ejecta. Our study found that boosted kilonovae, if commonly occurring, they should have already been observed given the accumulated time in sky surveys. As a result, the absence of detection implies that long-lived massive neutron stars as neutron star merger remnants are likely to be rare in the Universe.</p>

[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.

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.

L'Univers est continûement le théâtre d'événements explosifs capables de relâcher une énorme quantité d'énergie sur des courtes échelles de temps. Ces sources transitoires comme les sursauts gamma, les supernovae ou les noyaux actifs de galaxie sont souvent associées à des objets extrêmes comme des étoiles à neutrons ou des trous noirs. De manière générale, ces sources émettent des radiations électromagnétiques dans une large bande spectrale voire sur la totalité du spectre pour les cas les plus extrêmes. Dès lors, une analyse multi-longueur d'onde est vitale pour étudier et comprendre la physique complexe de ces objets. De plus, au voisinage de ces sources, des particules (rayons cosmiques, RC) pourraient être efficacement accélérées jusqu'à des énergies très elevées dans des processus de chocs violents. L'interaction de ces RCs avec l'environnement peut conduire à la production d'un nombre significatif de neutrinos de hautes énergies. Par conséquent, l'étude des objets transitoires par le biais de l'astronomie neutrino offre la possibilité d'identifier enfin la nature des puissants accélérateurs cosmiques.Cette thèse est dédiée à l'étude de deux sources transitoires parmi les plus extrêmes dans l'Univers : les sursauts gamma (en anglais, Gamma-Ray Bursts : GRBs) détectés il y a ~ 50 ans et les sursauts radio (en anglais, Fast Radio Bursts : FRBs) fraîchement découverts il y a ~ 15 ans. Ces sources sont caractérisées par l'émission "prompte" d'un flash gamma (keV-MeV) durant de quelques ms à plusieurs secondes dans le cadre des GRBs et d'un flash intense en radio (GHz) durant quelques ms pour les FRBs. Dans le cas des GRBs une émission rémanente dite "afterglow" est observée dans une large gamme spectrale (X, visible et radio) alors que jusqu'à présent aucune autre contrepartie électromagnétique provenant d'un FRB n'a été découverte. Ces dernières années des modèles d'émission multi-longueur d'onde et multi-messager ont été développés afin d'expliquer ces 2 phénomènes. L'objectif majeur de ce travail de thèse est de tester ces modèles d'émission afin de contraindre la physique et la nature de ces deux objets. Pour cela, une analyse détaillée des propriétés physiques de l'émission afterglow des GRBs a été menée grâce à un large échantillon de données collectées ces 20 dernières années par diverses télescopes. Cette étude a permis de mettre en évidence les lacunes et les réussites du modèle GRB dit "standard" mais aussi les liens physiques subtils existant entre l'émission prompte des GRBs et leurs rémanences. Une recherche de signal neutrino en coïncidence avec les GRBs/FRBs a aussi été réalisée avec le télescope à neutrinos ANTARES. Les résultats sont décrits dans cette thèse ainsi que les contraintes apportées sur les processus d'accélération des particules durant ces phénomènes transitoires. Enfin, ce manuscrit rend compte des différents programmes d'observations innovants qui ont été engagés sur les télescopes optiques TAROT et Zadko et le télescope à neutrinos ANTARES afin de contraindre la nature des progéniteurs des GRBs/FRBs. / The Universe is continuously the scene of explosive events capable of releasing a tremendous amount of energy in short time scales. These transients like Gamma-Ray Bursts, Supernovae or Active Galactic Nuclei are often associated with extreme objects such as neutron stars or black holes. Generally, these sources emit light in a large spectral energy range and sometimes in the whole electromagnetic spectrum for the most extreme cases. Thus, a multi-wavelength analysis is crucial to study and understand the complex physical processes at work. Furthermore, in the vicinity of these sources, particles (cosmic-rays, CRs) could be efficiently accelerated up to very high energies by violent shock mecanisms. The interaction of these CRs with the surrounding environment may lead to a substantial production of high-energy neutrinos. Therefore, the study of the high-energy transient objects through neutrino astronomy offer the possibility to finally identify the nature of the powerful cosmic accelerators a hundred year after the discovery of the cosmic-rays.This thesis is dedicated to the study of two transient sources among the most extreme ones observed in the Universe: the Gamma-Ray Bursts (GRBs) detected ~ 50 years ago and the Fast Radio Bursts (FRBs) newly discovered ~ 15 years ago. These sources are characterised by the "prompt" emission of a gamma-ray flash (keV-MeV) lasting few ms up to few seconds for GRBs and an intense pulse of radio light (GHz) lasting few ms for FRBs. In the case of GRBs a late broadband afterglow emission is observed in X-rays/optical/radio domain while up to now no other electromagnetic counterpart has ever been detected in coincidence with any FRBs. These last years, many models predicting a multi-wavelength and a multi-messenger emission from these two phenomena have been developped. The main goal of this thesis work is to test these models in order to constrain the physics and the nature of the GRBs/FRBs. To do so, a detailed analysis on the physical properties of the GRB afterglow emission was made thanks to a large set of data collected these last 20 years by various facilities. The study reveals the major problems but also the successes encountered with the so-called "standard" GRB model. Subtle connections between the prompt and the afterglow emission are also discussed. In addition, a search for a neutrino signal from GRBs/FRBs was realised with the ANTARES neutrino telescope. The results are described in this thesis as well as the constraints on the particle acceleration mecanisms occuring during these transient phenomena.At last, this manuscript presents the different innovative observational programs realised in the optical domain with the TAROT and Zadko telescopes and in the astroparticle side with the ANTARES neutrino telescope in order to probe the nature of the GRBs/FRBs progenitors.

[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.

[ES] Los neutrinos de alta energía son partículas esquivas: no tienen carga, tienen una sección transversal de interacción muy pequeña con la materia ordinaria y su masa es extremadamente pequeña. Los neutrinos son una sonda importante en el estudio del origen de los rayos cósmicos, y también, siguiendo algunos modelos de la física más allá del modelo Stardard, pueden producirse a partir de la propagación de partículas del modelo estándar producidas por la aniquilación de la materia oscura. En el último siglo, se han desarrollado muchos enfoques nuevos en la física de astropartículas, tratando de resolver los enigmas no resueltos del Universo, como el origen de los rayos cósmicos y la existencia de la materia oscura. Entre los diferentes experimentos destacan, sin duda, los telescopios de neutrinos. Los telescopios de neutrinos, consistentes en un gran volumen de un medio transparente monitorizado por sensores ópticos para detectar luz de Cherenkov, pueden detectar neutrinos de alta energía de fuentes galácticas o extragalácticas, y también pueden usarse para el estudio de las propiedades de los neutrinos. ANTARES y su sucesor KM3NeT son dos telescopios de neutrinos ubicados en el mar Mediterráneo. El telescopio ANTARES empezó a estar operativo en 2007 y ha tomado datos de forma casi continua hasta principios de 2022. KM3NeT, aprovechando la experiencia de ANTARES, pretende ser el telescopio de neutrinos más sensible de la próxima generación de detectores. Esta tesis presenta mis contribuciones en ambos detectores. En concreto, la parte técnica del trabajo se ha desarrollado en colaboración con KM3NeT. Está dedicado al estudio de los datos de los sensores de orientación instalados en los módulos de detección ópticos de KM3NeT: desde su calibración antes del despliegue en el mar hasta el análisis de sus datos in situ. Estos sensores permiten una monitorización de los movimientos de los elementos detectores en el mar. Por otro lado, en colaboración con ANTARES se ha desarrollado un análisis de física relacionado con la búsqueda de la aniquilación de la materia oscura en el Sol analizando trece años de datos. Se han obtenido nuevos límites superiores para los flujos de neutrinos y antineutrinos a partir de la aniquilación de materia oscura en el Sol, y a partir de estos, se han derivado límites superiores a la sección eficaz de dispersión de Materia Oscura - Nucleón. Estos resultados mejoran en un factor dos los resultados anteriores de ANTARES y son competitivos con respecto a otros experimentos. / [CA] Els neutrins d'alta energia són partícules esquives: no tenen càrrega, tenen una secció transversal d'interacció molt petita amb la matèria ordinària i la massa és extremadament petita. Els neutrins són una sonda important en l'estudi de l'origen dels raigs còsmics, i també, seguint alguns models de la física més enllà del Model Stardard, es poden produir a partir de la propagació de partícules del model estàndard produïdes per l'aniquilació de la matèria fosca. A l'últim segle, s'han desenvolupat molts enfocaments nous a la física d'astropartícules, tractant de resoldre els enigmes no resolts de l'Univers, com l'origen dels raigs còsmics i l'existència de la matèria fosca. Entre els diferents experiments destaquen, sens dubte, els telescopis de neutrins. Els telescopis de neutrins, consistents en un gran volum d'un medi transparent monitoritzat per sensors òptics per detectar llum de Cherenkov, poden detectar neutrins d'alta energia de fonts galàctiques o extragalàctiques, i també es poden utilitzar per a l'estudi de les propietats dels neutrins. ANTARES i el seu successor KM3NeT són dos telescopis de neutrins ubicats al mar Mediterrani. El telescopi ANTARES va començar a estar operatiu el 2007 i ha pres dades de forma gairebé contínua fins a principis del 2022. KM3NeT, aprofitant l'experiència d'ANTARES, pretén ser el telescopi de neutrins més sensible de la propera generació de detectors. Aquesta tesi presenta les meves contribucions a tots dos detectors. Concrètement, la part tècnica del treball s'ha desenvolupat en col·laboració amb KM3NeT. Està dedicat a l'estudi de les dades dels sensors d'orientació instal·lats als mòduls de detecció òptics de KM3NeT: des del calibratge abans del desplegament al mar fins a l'anàlisi de les seves dades in situ. Aquests sensors permeten una monitorització dels moviments dels elements detectors al mar. D'altra banda, en col·laboració amb ANTARES s'ha desenvolupat una anàlisi de física relacionada amb la recerca de l'aniquilació de la matèria fosca al Sol analitzant tretze anys de dades. S'han obtingut nous límits superiors per als fluxos de neutrins i antineutrins a partir de l'aniquilació de matèria fosca al Sol, i a partir d'aquests, s'han derivat límits superiors a la secció eficaç de dispersió de Materia Fosca - Nucleó. Aquests resultats milloren en un factor dos els resultats anteriors de ANTARES i són competitius respecte a altres experiments. / [EN] High energy Neutrinos are elusive particles: they are chargeless, have a very small cross section with ordinary matter and their mass is extremely small. Neutrinos are an important probe in the study of the origin of cosmic rays but also, following some models of physics Beyond the Standard Model, they can be produced from the decay of Standard Model particles produced by dark matter annihilation. In the last century, many new approaches have been developed in astroparticle physics, trying to solve the unsolved puzzles of the Universe such as the origin of Cosmic Rays and the existence of Dark Matter. Among the many experiments, neutrino telescopes certainly stand out. Neutrinos telescopes, made of large volume of a transparent medium observed by optical sensors, can detect high energy neutrinos from galactic or extra-galactic sources, and they can also be used for the study of neutrino properties. ANTARES and its successor KM3NeT are two neutrino telescopes located in the Mediterranean sea. ANTARES operations started in 2007 and it has taken data almost continuously until the beginning of 2022. KM3NeT, taking advantage from the experience of ANTARES, aims to be the most sensitive neutrino telescope in the next generation of detectors. This thesis presents my contributions to both detectors. In particular, the technical part of the work has been developed in collaboration with KM3NeT. It is devoted to the the study of data from the compasses installed in the KM3NeT detection elements: from their calibration before deployment to the analysis of their data in the sea. These compasses allow a tracking of the movements of the detector elements in the sea. In collaboration with ANTARES a physics analysis related to the search of dark matter annihilation in the Sun has been developed analyzing thirteen years of data. New upper limits for neutrino and antineutrino fluxes from dark matter annihilation in the Sun have been obtained, and from these upper limits on the Dark Matter - Nucleon scattering cross section have been obtained. These results improve previous ANTARES results by a factor of 2 and are competitive with those obtained by other experiments. / Poirè, C. (2022). Dark Matter Searches Towards the Sun with ANTARES and Positioning Studies for KM3NeT [Tesis doctoral]. Universitat Politècnica de València. https://doi.org/10.4995/Thesis/10251/188750