In this report, we aim to assess the sensitivity and 5$\sigma$ discovery potential of IceCube, the largest neutrino observatory on Earth, and compare it with prior findings. Our thesis will focus on a point source analysis, exploring the energy and declination dependencies, with particular emphasis on high-energy neutrinos. The primary objective is to establish the feasibility of detecting 5$\sigma$ evidence supporting the hypothesis that blazars serve as sources of neutrinos in the Southern sky, as suggested in a recent publication. Our findings indicate a substantial improvement in both discovery potential and sensitivity for the Southern sky in recent years. Furthermore, we highlight the increasing significance of investigating the origins of high-energy neutrinos in the Southern sky.

No description available.

Dédié à l'étude du ciel en rayons gamma, le satellite Fermi comporte à son bord le Large Area Telescope (LAT), sensible au rayonnement gamma de 20 MeV à 300 GeV. Les données recueillies par le LAT depuis son lancement en 2008 ont permis de multiplier par 10 le nombre de noyaux actifs de galaxie (NAG) détectés dans le domaine du GeV. Les rayons gamma observés dans les NAGs proviennent de processus énergétiques faisant intervenir des particules chargées de très haute énergie. Ces particules sont confinées dans un jet de plasma magnétisé qui prend sa source dans une région proche du trou noir supermassif habitant la zone centrale de la galaxie hôte. Ce jet s’éloigne à des vitesses aussi élevées que 0.9999c, formant dans de nombreux cas des lobes radio sur des échelles du kiloparsec voire du mégaparsec. Les NAGs dont le jet fait un angle faible avec la ligne de visée sont appelés blazars. La combinaison de cette très faible inclinaison du jet par rapport à la ligne de visée et de vitesses d’éjection relativistes donne lieu à des effets relativistes : mouvements apparents superluminiques, amplification de la luminosité et modification des échelles de temps. Les blazars sont caractérisés par une extrême variabilité à toutes les longueurs d’onde, sur des échelles de temps allant de quelques minutes à plusieurs mois. Une étude temporelle et spectrale du plus brillant d'entre ceux détectés par le LAT, 3C 454.3, a été réalisée afin de contraindre les modèles d'émission. Une nouvelle méthode de génération de courbes de lumière à échantillonnage adaptatif est également proposée dans cette thèse. Celle-ci permet d'extraire le maximum d'information des données du LAT quel que soit l'état de flux de la source. / The Fermi Gamma-ray Space Telescope was launched on 2008 June 11, carrying the Large Area Telescope(LAT), sensitive to gamma-rays in the 20 MeV – 300 GeV energy range. The data collected since then allowed to multiply by a factor of 10 the number of Active Galactic Nuclei (AGN) detected in the GeV range. Gamma-rays observed in AGNs come from energetic precesses bringing into play very high energy charged particles. These particles are confined in a magnetized plasma jet rising in a region close to the supermassive black hole in the center of the host galaxy. This jet moves away with velocities as high as 0.9999c, forming in many cases radio lobes on kiloparsec or even megaparsec scales. Among the AGNs, those whose jet inclination angle to the line of sight is small are called blazars. The combination of this small inclination angle with relativistic ejection speeds leds to relativistic effects : apparent superluminal motions, amplification of the luminosity and modification of the time scales. Blazars are characterized by extreme variability at all wavelengths, on time scales from a few minutes to several months. A temporal and spectral study of the most luminous of those detected by the LAT, 3C 454.3, was done so as to constrain emission models. A new method for generating adaptive-binning lightcurves is also suggested in this thesis. It allows to extract the maximum of information from the LAT data whatever the flux state of the source.

Black hole jets and accretion discs are the most extreme objects in modern astrophysics whilst dark energy is undoubtedly the most mysterious. This thesis focuses on understanding these three topics. The majority of this thesis is dedicated to investigating the structure and properties of black hole jets by modelling their emission. I develop an inhomogeneous jet model with a magnetically dominated parabolic accelerating base, transitioning to a slowly decelerating conical jet, with a geometry set by radio observations of M87. This model is able to reproduce the simultaneous multiwavelength spectra of all 38 Fermi blazars with redshifts in unprecendented detail across all wavelengths. I constrain the synchrotron bright region of the jet to occur outside the BLR and dusty torus for FSRQs using the optically thick to thin synchrotron break. At these large distances their inverse-Compton emission originates from scattering CMB photons. I find an approximately linear relation between the jet power and the transition region radius where the jet first comes into equipartition, transitions from parabolic to conical and stops accelerating. The decreasing magnetic field strength and increasing bulk Lorentz factor with jet power are the physical reasons behind the blazar sequence. I calculate the conditions for instability in a thin accretion disc with an α parameter which depends on the magnetic Prandtl number, as suggested by MHD simulations. The global behaviour of the instability induces cyclic flaring in the inner regions of the disc, for parameters appropriate for X-ray binary systems, thereby offering a potential solution to a long standing problem. Finally, I calculate the effect of an interacting quintessence model of dark energy on cosmological observables. I find that a scalar-tensor type interaction in the dark sector results in an observable increase in the matter power spectrum and integrated Sachs-Wolfe effect at horizon scales.

Through the work on X-ray and gamma-ray data of AGN I contributed significantly to the progress in the unification of AGN since I finished my PhD in 2000. <p> The study of the evolutionary behaviour of X-ray selected blazars (Beckmann & Wolter 2001; Beckmann et al. 2002, 2003b; Beckmann 2003) shows that their evolution is not as strongly negative as indicated by previous studies. The overall luminosity function is consistent with no evolution in the 0.1−2.4 keV band as seen by ROSAT/PSPC. There is still a difference compared to the luminosity function of FSRQ and LBL, which seem to show a positive evolution, indicating that they have been more luminous and/or numerous at cosmological distances. We indicated a scenario in order to explain this discrepancy, in which the high luminous FSRQ develop into the fainter LBL and finally into the BL Lac objects with high frequency peaks in their spectral energy distribution but overall low bolometric luminosity. <p> Studying the variability pattern of hard X-ray selected Seyfert galaxies, we actually found differences between type 1 and type 2 objects, in the sense that type 2 seemed to be more variable (Beckmann et al. 2007a). This breaking of the unified model is caused by the different average luminosity of the absorbed and unabsorbed sources, as discussed in Sect. 4.7.3. This can be explained by a larger inner disk radius when the AGN core is most active (the so-called receding disc model). <p> The work on the sample characteristics of hard X-ray detected AGN also led to the proof that the average intrinsic spectra of type 1 and type 2 objects are the same when reflection processes are taken into account (Beckmann et al. 2009d). This also explains why in the past Seyfert 2 objects were seen to have harder X-ray spectra than Seyfert 1, as the stronger reflection hump in the type 2 objects makes the spectra appear to be flatter, although the underlying continuum is the same. <p> Further strong evidence for the unification scheme comes from the observation of a fundamental plane which connects type 1 and type 2 objects smoothly (Beckmann et al. 2009d). In addition, in the case of the Seyfert 1.9 galaxy MCG-05-23-016 I showed that the spectral energy distribution of this source and its accretion rate is similar to that of a Galactic binary (Beckmann et al. 2008a). <p> Throughout the studies I have shown that the intrinsic spectral shape appears to be very stable on weeks to year time scale (Beckmann et al. 2004d, 2005b, 2007b, 2008a). This implies that the overall geometry of the AGN over these time scales did not change dramatically. The variations in intensity can then be explained in two ways: either the amount of material emitting the hard X-rays varies, or the amount of plasma visible to the observer varied, e.g. through different orientation of the disk with respect to the observer. In an upcoming paper we will show though, that NGC 4151 indeed also shows different spectral states, similar to the low-hard versus high-soft spectra in Galactic black hole binaries (Lubinski et al. 2010). A similar result seems to emerge from our INTEGRAL studies on NGC 2110 (Beckmann & Do Cao 2011). For INTEGRAL's AO-8 I have submitted a proposal in order to study spectral states in the Seyfert 2 galaxy NGC 2992, which seems to show a state change over the past 5 years as seen in Swift/BAT longterm monitoring. <p> The work on the luminosity function of AGN at hardest X-rays (Beckmann et al. 2006d) had a large impact on our understanding of the cosmic X-ray background. As this was the first study of its kind, it showed for the first time that indeed the fraction of highly obscured Compton thick AGN is much lower than expected before the launch of INTEGRAL and Swift. The X-ray luminosity function we revealed is indeed not consistent with the source population seen by INTEGRAL (Beckmann et al. 2006a, 2009d; Sazonov et al. 2007) and Swift (Tueller et al. 2008) being the only contributors to the cosmic hard X-ray background. Thus other sources outside the parameter space observable by these missions have to contribute significantly to the cosmic X-ray background. Our work on the luminosity function triggered several other studies on this issue. The subsequent derived luminosity functions by other groups (Sazonov et al. 2007; Tueller et al. 2008; Paltani et al. 2008) are consistent with our findings. <p> This also gave rise to an increased interest in the exact shape of the Cosmic X-ray background around its peak at 30 keV, triggering several attempts to a new measurement. Background studies were presented based on a Earth-occultation observation by INTEGRAL (Churazov et al. 2007, 2008; Türler et al. 2010) and by Swift (Ajello et al. 2008). <p> The understanding of the emission processes in AGN requires knowledge over a wide range of the spectral energy distribution (SED). In studies using CGRO/EGRET and Fermi/LAT data I derived the SED for blazars and non-blazars towards the gamma-ray range (Beckmann 2003; Beckmann et al. 2004b, 2010b). The work on the LAT data not only presented the gamma-ray detection of five gamma-ray blazars (QSO B0836+710, RX J1111.5+3452, H 1426+428, RX J1924.8-2914, and PKS 2149-306) for the first time, but also showed the potential in the combination of INTEGRAL and Fermi data. In the case of Cen A I derived the total energy output of the inverse Compton component based on the combined LAT, ISGRI, and JEM-X data, showing evidence for a spectral break at several hundred keV (Beckmann et al. 2010b). <br> In addition I successfully showed that gamma-ray blazars can be predicted through the study of their synchrotron branch at energies below 2 keV (Beckmann 2003 and this work). <p> Contributions of mine to research in fields other than AGN include the study of INTEGRAL detected gamma-ray bursts (e.g. Beckmann et al. 2003a, 2004a, 2008b, 2009a). Here and in collaboration with other colleagues I showed the potential of INTEGRAL data on GRB research. In the field of Galactic X-ray binaries I published one of the first Swift results on a newly discovered highly absorbed HMXB, IGR J16283-4838 (Beckmann et al. 2005a, 2006b). I also contributed significantly to analysis of many other Galactic sources, as shown in Section 4.6.1.

Since the construction of the IceCube Neutrino Observatory was completed in 2010, many amazing discoveries have been made in the field of neutrino physics. Recently a neutrino event has been linked to an blazar-type active galactic nucleus source, bringing us one step closer to understanding the production of high-energy extragalactic neutrinos and ushering in a new era of multimessenger astronomy. This was found by linking the neutrino event to one of the Fermi Collaboration’s gamma ray sources which had a blazar counterpart. The quest to link other neutrino events to AGN (active galactic nuclei) sources through collaboration with the Fermi Large Area Telescope has turned up some interesting candidates. The fact that some of these potential sources are not blazars is curious and, although unconfirmed as neutrino sources, these objects merit further investigation due to their unusual nature.

Blazare sind aktive galaktische Kerne mit relativistischen Plasmajets, deren Symmetrieachse in Richtung Erde zeigt. Sie sind primäre Kandidaten für die Produktion von hochenergetischen Neutrinos. Diese Arbeit umfasst die Suche nach einem kumulativen Neutrinofluss von allen 862 Fermi-LAT 2LAC Blazaren und vier spektral ausgewählten Unterpopulationen. Selektierte Myonspuren aus drei Jahren IceCubedaten werden mit einer ungebinnten "Stacking"-Punktquellenanalyse untersucht. Zwei unterschiedliche Gewichtungen werden benutzt, um den unbekannten relativen Anteil jeder Quelle am Gesamtneutrinofluss der jeweiligen Population zu berücksichtigen. Neun der zehn resultierenden Tests zeigen leichte Überfluktuationen, von denen keine statistisch signifikant ist. Das Ergebnis erlaubt es, den Maximalanteil der 2LAC-Blazare zum kürzlich entdeckten astrophysikalischen TeV-PeV Neutrinofluss auf 23% einzuschränken. Diese Grenze gilt unter der Annahme des momentan favorisierten Spektralindex des astrophysikalischen Neutrinoflusses von -2.5 und bei einem Flavorverhältnis von 1:1:1 bei Erreichen der Erde. Die Ergebnisse erfordern keine rein hadronische Produktion der beobachteten Gammastrahlung und bleiben, bis auf einen Faktor zwei, für moderat härtere Spektren oder für kleinere Unterpopulationen, wie z.b. die GeV-detektierten TeVCat Quellen, gültig. Zusätzlich werden obere Flussgrenzen für generische Spektren, die einem Potenzgesetz folgen, sowie für konkrete spektrale Modelle der diffusen Neutrinoemission von Blazaren, ausgerechnet. 12 von 14 dieser Modelle können eingeschränkt oder ausgeschlossen werden. Wenn die größte Überfluktuation als physikalischer Effekt interpretiert wird, findet man einen weichen Fluss in der 5-10 TeV Region, welcher mit Gammastrahlenbeobachtungen kompatibel ist. Mehr Daten sind bereits verfügbar und erlauben es, dieses Szenario in der nahen Zukunft zu testen. / Blazars are active galactic nuclei with relativistic plasma jets whose symmetry axis is pointing towards Earth. They are a prime source candidate for the production of high-energy neutrinos. This work describes the search for a cumulative neutrino flux from all 862 Fermi-LAT 2LAC blazars and four spectrally defined sub-populations. Selected muon-track events from three years of IceCube data are analyzed with an unbinned likelihood stacking approach. Two different weighting schemes are used to account for the unknown relative flux contributions of each source. Nine of ten tests show slight overfluctuations, none of which are statistically significant. An upper flux limit is calculated constraining the maximal contribution of the 2LAC blazars to the recently discovered diffuse TeV-PeV neutrino flux to be 23% or less assuming the currently favored spectral index for the astrophysical flux of around −2.5 and an equal composition of neutrino flavors arriving at Earth. The results do not require a purely hadronic production of the observed gamma rays and remain valid for moderately harder spectra or smaller sub-populations, e.g. the TeVCat sub-sample, up to a factor of around 2. Additionally, upper limits are calculated for generic power-law spectra and for concrete spectral models of the diffuse neutrino emission of blazar populations. 12 out of 14 of these models are either constrained or excluded. If the largest overfluctuation is interpreted as a physics effect, one finds a soft flux in the 5-10 TeV region that is compatible with gamma-ray observations. Further years of data are already available which makes this scenario testable in the near future. If confirmed, blazars might become the first known extragalactic hadronic acceleration site.

It is evident that the origin of EGRB is not well established. In this thesis I examine the unresolved discrete origin of EGRB. The contribution from normal galaxies, starburst galaxies and AGNs to the EGRB is examined. The second chapter includes the methodology used to find the contributions from different source classes. In the third chapter the contribution from normal and starburst galaxies is discussed. A methodology is developed to derive the contribution from normal and starburst galaxies to the EGRB considering all the major γ-ray production processes in these galaxies. The calculations in this thesis consider the detailed γ-ray measurements of our galaxy(Hunter etal.1997) to derive suitable scaling relations to extend the analysis beyond the Milky Way. It is assumed that all normal and starburst galaxies also have similar γ-ray spectra. A relationship is derived between the γ-ray luminosity and SFR of a normal galaxy. Infrared luminosity of a normal galaxy is used as a tracer of SFR of that galaxy (Kewley et al.2002). For starburst galaxies, the contribution depends on the relative ratio(β)of cosmic-ray enhancement per SFR w.r.t the Milky Way. To find the proportionality constants between cosmic-ray production rate and SFR of starburst galaxies, M82 has been taken as a standard. Contribution from FSRQs and BL Lacs to the EGRB is discussed in the fourth chapter. FSRQs and BL Lacs are considered as separate source classes, and their luminosity functions are constructed separately from the recent identifications of EGRET sources(Sowards-Emmerd,Romani&Michelson2003 and Sowards-Emmerd et al.2004) which almost doubled the blazers count than that used by Chiang & Mukher-jee(1998). Radio-loud AGNs with all possible jet to line-of-sight angle (SSRQs, FR IIs, FR Is) are termed here off-axis AGNs. It is considered that SSRQs and FSRQs and FR IIs are from one parent population, BL Lacs and FR Is are from another parent population. The scenario considered includes an AGN jet which slows down as it moves away from the central source. The contributions from these AGNs (relative to the FSRQs and BL Lacs contribution) are discussed in chapter five. Chapter six briefly summarised the findings from the thesis.

Des modèles de Gravité Quantique (QG) prédisent une violation de l'invariance de Lorentz (LIV), se manifestant par une dispersion de la lumière dans le vide. Si un tel effet existe, des photons d'énergies différentes émis en même temps par une source distante sont détectés sur Terre à des moments différents. Les émissions transitoires à (très) hautes énergies provenant de sources astrophysiques lointaines, comme les sursauts gamma (GRBs) et les blazars sont utilisées pour contraindre cet effet LIV. Cet ouvrage présente les études menées avec deux télescopes gamma majeurs : H.E.S.S. -- pour lequel une étude de la qualité des données étalonnées a été réalisée -- et Fermi-LAT. Les énergies et les temps d'arrivée de photons individuels ont été utilisés pour contraindre le paramètre de dispersion dans le vide ainsi que l'échelle d'énergie E_QG à laquelle des effets LIV peuvent apparaitre. La méthode de maximum de vraisemblance est décrite, avec une étude détaillée des systématiques. Une modification dans le cas de fond non négligeable est appliquée aux données de l'éruption d'un blazar observé par H.E.S.S. : les limites obtenues sur E_QG sont moins contraignantes que les meilleures limites précédentes, mais elles se trouvent à un redshift non couvert à ce jour. Quatre GRBs observés par Fermi-LAT ont aussi été analysés, en déterminant la courbe de lumière de deux manières : ajustements gaussiens et estimation par densité de noyaux. Les meilleures limites sur E_QG pour le cas linéaire/subluminal sont obtenus avec GRB090510 : E_QG,1 > 7,6 E_Planck. Des limites plus robustes, tenant compte des effets intrinsèques à la source, ont également été produites. / Some Quantum Gravity (QG) theories allow for a violation of Lorentz invariance (LIV), manifesting as a dependence on the velocity of light in vacuum on its energy. If such a dependence exists, then photons of different energies emitted together by a distant source will arrive at the Earth at different times. (Very) high energy transient emissions from distant astrophysical sources such as Gamma-ray Bursts (GRBs) and blazars can be used to search for and constrain LIV. This work presents the studies obtained with two leading Gamma-ray telescopes: H.E.S.S. -- for which a study of the quality of the calibrated data was performed -- and Fermi-LAT. The energies and arrival times of individual photons were used to constrain the vacuum dispersion parameter and the energy scale EQG at which QG effects causing LIV may arise. The maximum likelihood method is described, with detailed studies of the systematics. A modification for a non-negligible background is provided and applied to the data of an AGN flare observed by H.E.S.S.: the obtained limits on the QG energy scale are less constraining than the previous best limits obtained with blazars; yet, the new limits lie a redshift range not covered this far. Four bright and quasi background-free GRBs observed by the Fermi-LAT were also analysed, with two different template light curve determinations -- Gaussian fits and Kernel Density Estimates. The best limits on the E_QG scale for the linear/subluminal case are from the shortest burst, GRB090510: E_QG,1 > 7.6 E_Planck. More robust limits, considering the intrinsic effects possibly occurring at the source, were also derived.

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