In this thesis I present our recent work on gamma-ray searches for dark matter with the Fermi Large Area Telescope (Fermi-LAT). We have targeted dwarf spheroidal galaxies since they are very dark matter dominated systems, and we have developed a novel joint likelihood method to combine the observations of a set of targets. In the first iteration of the joint likelihood analysis, 10 dwarf spheroidal galaxies are targeted and 2 years of Fermi-LAT data is analyzed. The resulting upper limits on the dark matter annihilation cross-section range from about 10−26 cm3 s−1 for dark matter masses of 5 GeV to about 5 × 10−23 cm3 s−1 for dark matter masses of 1 TeV, depending on the annihilation channel. For the first time, dark matter models with a cross section above the canonical thermal relic cross section (∼ 3 × 10−26 cm3 s−1) are strongly disfavored by a gamma-ray experiment. In the second iteration we include 15 dwarf spheroidal galaxies in the combined analysis, employ 4 years of data and an improved calculation of the dark matter density. The obtained upper limits range from about 10−26 cm3 s−1 for dark matter masses of 2 GeV to about 10−21 cm3 s−1 for dark matter masses of 10 TeV, depending on the annihilation channel. I briefly describe some of the evidence for dark matter, the Fermi-LAT instrument and public data releases, dwarf spheroidal galaxies, likelihood analysis, and results from analyses of Fermi-LAT data. I also document some of the tests made to verify the method and to compare different analysis setups.

Galaxy clusters are the most massive bound systems known in the Universe and are believed to have formed through large scale structure formation. They host relativistic cosmic-ray (CR) populations and are gravitationally bound by large amounts of Dark Matter (DM), both providing conditions in which high-energy gamma rays may be produced either via CR interactions with the intracluster medium or through the annihilation or decay of DM particles. Prior to the launch of the Fermi satellite, predictions were optimistic that these sources would be established as γ-ray-bright objects by observations through its prime instrument, the Large Area Telescope (LAT). Yet, despite numerous efforts, even a single firm cluster detection is still pending. This thesis presents a number of studies based on data taken by the LAT over its now seven year mission aiming to discover these γ rays. Using a joint likelihood technique, we study the γ-ray spectra of a sample of nearby clusters searching for a CR-induced signal due to hadronic interactions in the intracluster medium. While we find excesses in some individual targets, we attribute none to the cluster. Hence, we constrain the maximum injection efficiency of hadrons being accelerated in structure formation shocks and the fraction of CR-to-thermal pressure. We also perform a refined search targeting the Coma cluster specifically due to its large variety of existing observations in other wavebands. In the latter case we find weak indications of an excess which however falls below the detection threshold. Because the cluster emission we consider is inherently extended, we need to take into account the imperfect modeling of the foreground emission, which may be particularly difficult such as is the case with the Virgo cluster. Here, we assess the systematics associated with the foreground uncertainties and derive limits based on an improved background model of the region. For the first time we derive limits on the γ-ray flux from CR and DM-interactions in which we take into account the dynamical state of the system. For DM we also include the contribution from substructure. The DM domain is further explored by searching for line-like features as they arise from the annihilation of DM into two photons in a large sample of clusters, including Virgo and Coma. Finding no evidence for γ-ray lines, we derive limits on the DM annihilation cross section that are roughly a factor 10 (100) above that derived from observations of the galactic center assuming an optimistic (conservative) scenario regarding the boost due to DM substructure. / <p>At the time of the doctoral defense, the following papers were unpublished and had a status as follows: Paper 3: Submitted. Paper 4: Submitted.</p>

Das Fermi Large Area Telescope (LAT) ist ein satellitengestütztes Gammastrahlungs-Teleskop zur Messung von Gammastrahlung im Energiebereich zwischen ∼ 30 MeV und mehreren hundert GeV. Der Nachweis extraterrestrischer Gammastrahlung in diesem Energiebereich erlaubt Rückschlüsse auf die astrophysikalischen Quellen der Gammastrahlung und Beschleunigungsmechanismen kosmischer Strahlung mit Energien zwischen 1 GeV und ∼ 10 TeV. Die Beobachtung von Quellen während Phasen vorübergehend erhöhter Gammastrahlungslüsse (‘Flares’) ermöglicht dabei eine besonders empfindliche Untersuchung der Produktionsmechanismen kosmischer Strahlung in den Quellen, da die Eigenschaften kürzlich beschleunigter Teilchen unmittelbar studiert werden können. Der Hauptteil dieser Dissertation stellt das neueste Verzeichnis zeitlich variabler Gammastrahlungsquellen über 100 MeV vor, den zweiten ‘Fermi All-sky Variability Analysis’ Katalog (2FAV). Der 2FAV Katalog enthält über 4500 Flares, welche in den ersten 7.4 Jahren der LAT Datennahme auf einem hohen Vertrauensniveau gemessen und an 518 verschiedenen Himmelspositionen beobachtet wurden. 441 dieser Quellpositionen im 2FAV können Aktiven Galaktischen Kernen (AGN) zugeordnet werden. Die verbleibenden 77 Quellpositionen besitzen keine sichere Entsprechung in anderen Verzeichnissen von Gammastrahlungs- oder Blazarquellen und stellen möglicherweise neue Gammastrahlungsquellen dar. Bei der Untersuchung der Spektren der 2FAV Flares, welche sogenannten ‘Flat-Spectrum’ Radioquasaren (FSRQ) - eine Unterklasse der AGN - zugeordnet werden können, wurde durchweg ein härteres Gammastrahlungsspektrum während Phasen erhöhter Gammastrahlungsemission beobachtet. Zudem wurde eine Untergrenze in der Verteilung der spektralen Exponenten, Γ ≳ 1.5, in der Stichprobe der untersuchten Flares festgestellt. Unter der Annahme eines einfachen leptonischen Modells und dass die Verteilung beschleunigter Teilchen im Inertialsystem der Quelle isotrop ist, folgt daraus, dass die Energiespektren der kosmischen Strahlung, welche die Gammastrahlungs-Flares verursachen, mit dN/dE ∝ E −2 oder stärker abfallen. Eine andere Möglichkeit, die Beschleunigungsmechanismen kosmischer Strahlung zu untersuchen, ist die Messung der Polarisation der begleitenden Gammastrahlung. Der letzte Teil dieser Dissertation enthält eine vorläufige Studie zur Messbarkeit linearer Polarisation astrophysikalischer Gammastrahlung mit dem LAT-Instrument. Bei Konversion hochenergetischer Photonen in Elektron-Positron-Paare verursacht eine lineare Polarisation der Gammastrahlung eine Modulation des Azimutwinkels der Ebenen, in denen die Elektron-Positron-Paare erzeugt wurden. Obwohl der LAT ursprünglich nicht als Polarimeter konzipiert wurde, ermöglicht das Instrument eine Messung dieser Modulation für niederenergetische Primärteilchen (≲ 200 MeV), welche in den Silikonschichten des Detektors konvertieren. Eine Auswahl solcher Ereignisse, selektiert durch Algorithmen überwachten maschinelles Lernens (‘supervised machine learning’), wird verwendet um die statistischen und systematischen Messunsicherheiten abzuschätzen, denen eine Messung unterworfen ist. Werden allein statistische Unsicherheiten berücksichtigt, ist der LAT in der Lage, einen Polarisationsgrad von 30−50% der Gammastrahlungsflüsse vom Vela-Pulsar und vom Krebs-Pulsarwindnebels nach einer Beobachtungszeit von zehn Jahren auf einem Vertrauensniveau von 5σ nachzuweisen. Werden zusätzlich systematische Unsicherheiten berücksichtigt, wird abgeschätzt, dass ein Polarisationsgrad von ∼ 46% auf einem Vertrauensniveau von 5σ nachweisbar ist, indem eine Auswahl von AGN als unpolarisierte Testprobe verwendet und mit umfangreichen Monte-Carlo Simulationen verglichen wird. Die Analyse in dieser Dissertation berücksichtigt zum ersten Mal sämtliche Aspekte einer polarization-sensitiven Ereignissrekonstruktion und dienst damit als Grundlage für zukünftige Nachweisversuche der Polarisation astrophysikalischer Gammastrahlung mit dem LAT. / The Fermi Large Area Telescope (LAT) is a space-based pair-conversion telescope sensitive to gamma rays with energies from ∼ 30 MeV to several hundreds of GeV. Observing gamma rays in this energy range, we gain information on the sources and acceleration mechanisms of cosmic rays (CRs) of energies from ∼ 1 GeV to ∼ 10 TeV. Studying the emission of gamma-ray sources during periods of enhanced activity (flares) provides a sensitive probe of the production mechanisms of CRs, as it makes it possible to investigate the distributions of the freshly-accelerated particles. The main part of this work presents the latest catalog of variable gamma-ray sources above 100 MeV, the second Fermi All-sky Variability Analysis catalog (2FAV). The 2FAV catalog contains more than 4500 flares detected at high significance in the first 7.4 years of LAT observations and the 518 sources from which these flares originate. Probable counterparts, mostly active galactic nuclei (AGN), have been found for 441 sources in the 2FAV. The remaining 77 sources have no reliable counterparts in other gamma-ray or blazar catalogs; they are potentially new gamma-ray sources. Studying the spectra of the 2FAV flares, we observe a harder-when-brighter behavior for the entire sample of flares associated with lat spectrum radio quasars, a class of AGN. We also observe a minimum value in the distribution of the photon index Γ of the flares, corresponding to Γ ≳ 1.5. In a simple leptonic scenario, assuming that the distribution of accelerated particles is isotropic in the source reference frame, this limit on the spectral hardness implies that the spectra of the accelerated particles responsible for the lare is never harder than dN/dE ∝ E −2. Another way to probe the environments where CRs are accelerated is by measuring the polarization of the gamma rays. A preliminary study of the LAT sensitivity to linear polarization of astrophysical gamma-rays is presented in the last part of this thesis. In the pair-production regime, linear polarization of the gamma-rays induces a modulation in the azimuthal angle of the planes where the electron-positron pairs are produced. Although not designed as a polarimeter, the LAT has the potential to measure this modulation using low energy (≲ 200 MeV) events converting in the silicon detector layers of its tracker. A selection of these events using supervised machine learning algorithms is presented and used as a basis to estimate the statistical and systematic uncertainties afecting the measurement. Considering only statistical uncertainties, a degree of polarization of ∼ 30−50% could be detected at the 5σ confidence level for the Vela pulsar and the Crab pulsar-wind nebula after 10 years of observation. By including systematic uncertainties, a 5σ sensitivity limit corresponding to a polarization degree of ∼ 46% is estimated, using a stack of AGN as an unpolarized test source and comparing the data with detailed Monte Carlo simulations. This analysis addresses for the first time all the steps of a real measurement and can therefore provide the basis for future measurements of gamma-ray polarization with the LAT.

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.

Dans le domaine de l’astrophysique des hautes énergies, de nombreuses questions restent à ce jour sans réponse et, parmi elles se trouve l’origine des rayons cosmiques Galactiques. La première preuve observationnelle de ces particules accélérées a été apportée au sein d’un vestige de supernova il y a seulement vingt ans. Depuis, nous savons que les vestiges de supernova, les pulsars et leurs nébuleuses accélèrent efficacement des particules mais de nombreuses interrogations subsistent encore. Les preuves directes concernant l’accélération de protons (constituant 90% du rayonnement cosmique) sont rares et de nombreuses sources nouvellement détectées en gamma sont de nature inconnue. Les rayonnements produits au sein des accélérateurs Galactiques fournissent d’importants éléments de réponse quant à la nature des particules accélérées. En particulier, alors que les domaines de la radio et des rayons X ne tracent que les électrons accélérés, les rayons gamma peuvent inférer la présence d’électrons et également de protons (et noyaux en général) mais l’émission s’avère le plus souvent difficile à interpréter.Les mesures des rayons gamma de très hautes énergies dépendent de notre connaissance de l’atmosphère terrestre, dans lequel ils se propagent avant d’être détectés par les télescopes Tcherenkov au sol tels que le réseau H.E.S.S. La partie technique de cette thèse concerne l’étude de l’impact des profils d’atmosphère sur les données H.E.S.S. Grâce à des simulations et des analyses prenant en compte les caractéristiques propres à chaque prise de données, nous étudions l’impact des profils d’atmosphère mesurés sur les fonctions de réponse de l’instrument et sur la reconstruction spectrale.Le premier objectif scientifique de cette thèse est de comprendre la nature de l’émission gamma au sein de deux vestiges de supernova (G326.3-1.8 et RX J1713.7-3946) par le biais d’analyses spectro-morphologiques détaillées. L’analyse de G326.3-1.8, avec les données du Fermi-LAT, a mené à deux résultats importants: une nouvelle preuve d’accélération de protons et la première séparation morphologique et spectrale de deux composantes imbriquées en gamma. L’analyse de RX J1713.7-3946, avec les données H.E.S.S. et les outils d'analyse Ctools, a confirmé une extension plus importante en gamma qu’en rayons X mais dont l’origine reste encore incertaine.La deuxième partie de cette thèse entreprend de discuter la nature des sources Galactiques non-associées au TeV. Pour ce faire, nous présentons un code générique visant à rechercher des contreparties multi-longueurs d’onde sur ces sources au TeV, et permettant de poser des contraintes sur des paramètres physiques tels que le champ magnétique moyen et l’indice spectral en radio. En appliquant ce code sur cinq sources non-identifiées du relevé du plan Galactique de H.E.S.S., nous apportons des arguments quant à leur origine. En particulier, nous étudions deux sources, dont l’émission au TeV provient probablement de multiples contributions, soulevant ainsi l’importance des données multi-longueurs d’onde pour comprendre la nature de l’émission en gamma. / In high energy astrophysics, several questions are still open and amongst them is the origin of Galactic cosmic rays. The first observational evidence of accelerated particles has only been revealed twenty years ago. Since then, supernova remnants, pulsars and their nebulae are known to efficiently accelerate particles but several questions still hold. In particular, evidence of accelerated protons (which consist on 90% of the cosmic-ray spectrum) is still elusive and several gamma-ray sources have unknown origin. Non-thermal emissions produced in Galactic accelerators provide insights about the nature of the accelerated particles. In particular, while radio and X-ray observations indicate the presence of accelerated electrons, gamma rays can be produced by both electrons and protons (or nuclei in general) but it may be difficult to assess the origin of the emission.The technical part of this thesis concerns the study of the impact on the reconstructed H.E.S.S. data when using atmospheric profiles measured with a lidar instead of a standard atmospheric model currently used. Very high energy gamma rays propagate into the atmosphere before reaching Cherenkov Telescopes and thus, the accuracy of our measurements depends on our understanding of the atmospheric composition. Using run-wise simulations based on lidar data, we study the impact on the instrument response functions and we analyse data to quantify the effect on the spectral reconstruction.The first scientific goal of this thesis is to understand the gamma-ray emission from two supernova remnants (G326.3-1.8 et RX J1713-3946) through detailed spectro-morphological analyses. The analysis of the composite supernova remnant G326.3-1.8, with Fermi-LAT data, has led to two major results: a new evidence of accelerated protons and the first morphological and spectral separation in gamma rays of two nested components. The study of RX J1713-3946, with H.E.S.S. data and using the Ctools package, confirms a significant gamma-ray extension beyond the X-ray emitting shell but its origin remains unclear.The second part of this thesis aims to constrain the nature of the unidentified TeV sources revealed in the H.E.S.S. Galactic Plane Survey. We thus present a generic code, based on a multi-wavelength approach, to find counterparts and estimate physical parameters like the radio spectral index and the mean magnetic field. We apply this code on five unidentified TeV sources and we put constraints on their nature. In particular, we present two high-confusion cases, for which the TeV emission is probably due to the contribution from different components, emphasizing the importance of multi-wavelength data to understand the origin of the gamma-ray emission.

Made available in DSpace on 2015-05-14T12:14:07Z (GMT). No. of bitstreams: 1 arquivototal.pdf: 9808595 bytes, checksum: 235eec9737496afa4dfbdf1feafbc21f (MD5) Previous issue date: 2013-02-04 / Coordenação de Aperfeiçoamento de Pessoal de Nível Superior - CAPES / The Dark matter (DM) problem constitutes a key question at the interface among Particle Physics, Astrophysics and Cosmology. The observational data which have been accumulated in the last years point to an existence of non baryonic amount of DM. Since the Standard Model (SM) does not provide any candidate for such non-baryonic DM, the evidence of DM is a major indication for new physics beyond the SM. We will study in this work one of the most popular DM candidates, the so called WIMPs (Weakly Interacting Massive Particles) from a direct and indirect detection perspective. In order to approach the direct and indirect dection of DM in the context of Particle Physics in a more pedagogic way, we will begin our discussion talking about a minimal extension of the SM. Later we will work on the subject in a 3-3-1 model. Next, we will study the role of WIMPs in the Big Bang Nucleosynthesis. Lastly, we will look for indirect signals of DM, by looking for gamma-ray excess observed by the NASA Satelite, called Fermi-LAT, in the Galactic Center. Through an analyses of the data events observed by Fermi-LAT and some background models, we will constrain the annihilation cross section-mass relation. As a result of this PhD 9 articles have been done. Two of them are still under the publication process. / O problema da matéria escura (ME) constitui uma questão chave na interface entre física de partículas, astrofísica e cosmologia. O acúmulo de dados observacionais nos últimos anos apontam para uma enorme quantidade de ME não bariônica. Uma vez que o Modelo Padrão (MP), não fornece um candidato para este tipo de matéria, a evidência de ME é uma forte indicação de física nova, além do MP. Estudaremos neste trabalho um dos candidatos à ME mais populares, os chamados WIMPS (partículas massivas que interagem fracamente) sob o ponto de vista de detecção direta e indireta de ME. Para que possamos abordar os meios de detecção direta e indireta de ME no contexto de Física de Partículas de forma didática, iniciaremos nossa discussão apresentando uma extensão mínima do MP. Posteriormente trataremos do assunto no contexto de um modelo 3-3-1. Adiante verificaremos qual o papel da ME no cenário da Nucleossíntese Primordial. Por ´ultimo procuraremos por sinais indiretos de ME, na busca por excessos em raios gama observados pelo satélite da NASA, chamado Fermi-LAT, no centro da nossa galáxia. Através de uma análise dos eventos observados pelo Fermi-LAT e de alguns modelos de background astrofísico iremos impor vínculos com relação à massa e seção de choque de aniquilação. Ao longo desse doutorado foram publicados 9 artigos. Um destes ainda sob o processo de publicação.

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.

Some of the most interesting objects in the Universe are Active Galactic Nuclei. In the centre of an active galaxy is a supermassive black hole that accretes matter from the surrounding galaxy. In the process, not yet fully understood, some of the matter is ejected in two jets, perpendicular to the plane of the galaxy. The energy of the particles in the jets are extremely high, sometimes over 1019 eV. The features of an active galaxy can be very different depending on from which angle it is viewed. This means that some astronomical objects that earlier seemed to be very heterogeneous might be only different manifestations of the same type of object, namely active galactic nuclei. This thesis introduces some of these different objects. The unifying theory is described. Ways of detecting the high-energy radiation and two important instruments, H.E.S.S. and Fermi-LAT are described. Three studies of the BL Lac PKS 2155-304, an active galactic nucleus that points its jet almost straight at Earth, are made using Fermi-LAT data. The conclusion of the studies is that the source is variable at least in the time scale of days and that in order to gather further information about these objects simultaneous multi-wavelength surveys have to be done.

An analysis is made on the Flat Spectrum Radio Quasar PKS 0426-380 using two sets of data. The first set of data is the Fermi-LAT data collected over the time 54682.66 − 59317.66 in Modified Julian Date within the energy range of 100 MeV to 500 GeV. The second set of data is a multi-wavelength spectral energy distribution within the approximate frequencies of <img src="http://www.diva-portal.org/cgi-bin/mimetex.cgi?%0A%0A%5Cleft%5B10%5E%7B10%7D,%2010%5E%7B27%7D%20%5Cright%5D" data-classname="equation" data-title="" />. First, the Fermi-LAT data were analysed and after modelling a lightcurve over the entire available time, a period of interest was located. The next step was to obtain a multi-wavelength spectral energy distribution of said period. Then, using JetSeT modelling, the data were analysed and a model of the Synchrotron Self-Compton, External-Compton and Synchrotron curves was fitted to the data. The final model which contained the best fit provided a set of physical parameters that described the source. These parameters were finally compared to two other Flat Spectrum Radio Quasars and conclusions regarding the properties of PKS 0426-380 were eventually drawn. A discussion comparing a related work on the same source to the results in this thesis followed. With the large differences in the constrained data between the Flat Spectrum Radio Quasars as background, three predictions concluded the thesis. These are, firstly, a cautioned approach to future searches for periodicity in AGN's. Secondly, in time, local periodicity for AGN's might be more common and interesting for future research. Finally, no certain values for the physical parameters of the AGN can be assessed and the results can only be wived as indications of the actual properties. / <p>Passed</p>

Dans le modèle cosmologique actuel, l'Univers est majoritairement composé de matière noire dont la nature est inexpliquée par le Modèle Standard de la physique des particules. L'annihilation de particules issues de nouveaux cadres théoriques, peut induire un signal de rayons gamma de très hautes énergies, observable par des expériences d'astronomie gamma. Largement dominées par la matière noire, les galaxies naines sphéroïdes sont des cibles privilégiées pour conduire cette recherche. Le réseau de télescopes H.E.S.S. discuté dans la première partie est un parfait exemple d'expérience d'imagerie atmosphérique stéréoscopique permettant de conduire la recherche de matière noire. Une nouvelle méthode de discrimination des gerbes électromagnétiques et hadroniques permettant d'améliorer la recherche de sources faibles est présentée. Elle est appliquée aux données des observations de la galaxie naine du Sagittaire et la limite supérieure sur le flux de gamma en provenance de cet objet est calculée. En orbite à bord du satellite Fermi depuis Juin 2008, le télescope à conversion de paire LAT permet de rechercher la matière noire sur l'ensemble de la voûte céleste. La sensibilité théorique à détecter un signal de matière noire est déterminée pour deux galaxies naines spéciques. Au terme de la première année d'observations, les limites supérieures sur les ux de gamma sont dérivées pour un catalogue de galaxies naines sphéroïdes. Des modèles de physique au-delà du Modèle Standard sont confrontés avec les contraintes calculées sur les sections ecaces d'annihilation en fonction de la masse des particules obtenues à partir des observations effectuées avec ces deux expériences.