<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>

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.

[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