The quest for the first detection of a galaxy cluster in the high energy gamma-ray regime is ongoing, and even though clusters are observed in several other wave-bands, there is still no firm detection in gamma-rays. To complement the observational efforts we estimate the gamma-ray contributions from both annihilating dark matter and cosmic-ray (CR) proton as well as CR electron induced emission. Using high-resolution simulations of galaxy clusters, we find a universal concave shaped CR proton spectrum independent of the simulated galaxy cluster. Specifically, the gamma-ray spectra from decaying neutral pions, which are produced by CR protons, dominate the cluster emission. Furthermore, based on our derived flux and luminosity functions, we identify the galaxy clusters with the brightest galaxy clusters in gamma-rays. While this emission is challenging to detect using the Fermi satellite, major observations with Cherenkov telescopes in the near future may put important constraints on the CR physics in clusters. To extend these predictions, we use a dark matter model that fits the recent electron and positron data from Fermi, PAMELA, and H.E.S.S. with remarkable precision, and make predictions about the expected gamma-ray flux from nearby clusters. In order to remain consistent with the EGRET upper limit on the gamma-ray emission from Virgo, we constrain the minimum mass of substructures for cold dark matter halos. In addition, we find comparable levels of gamma-ray emission from CR interactions and dark matter annihilations without Sommerfeld enhancement. / At the time of the doctoral defense, the following papers were unpublished and had a status as follows: Paper 2: Accepted.

The BAT Slew Survey (BATSS) is the first wide-field survey of the hard X-ray sky (15–150 keV) with a slewing coded aperture imaging telescope. Its fine time resolution, high sensitivity and large sky coverage make it particularly well-suited for detections of transient sources with variability timescales in the \(\sim 1 sec–1 hour\) range, such as Gamma-Ray Bursts (GRBs), flaring stars and Blazars. As implemented, BATSS observations are found to be consistently more sensitive than their BAT pointing-mode counterparts, by an average of 20% over the 10 sec–3 ksec exposure range, due to intrinsic systematic differences between them. The survey’s motivation, development and implementation are presented, including a description of the software and hardware infrastructure that made this effort possible. The analysis of BATSS science data concentrates on the results of the 4.8-year BATSS GRB survey, beginning with the discovery of GRB 070326 during its preliminary testing phase. A total of nineteen (19) GRBs were detected exclusively in BATSS slews over this period, making it the largest contribution to the Swift GRB catalog from all ground-based analysis. The timing and spectral properties of prompt emission from BATSS GRBs reveal their consistency with Swift long GRBs (L-GRBs), though with instances of GRBs with unusually soft spectra or X-Ray Flashes (XRFs), GRBs near the faint end of the fluence distribution accessible to Swift-BAT, and a probable short GRB with extended emission, all uncommon traits within the general Swift GRB population. In addition, the BATSS overall detection rate of 0.49 GRBs/day of instrument time is a significant increase (45%) above the BAT pointing detection rate. This result was confirmed by a GRB detection simulation model, which further showed the increased sky coverage of slews to be the dominant effect in enhancing GRB detection probabilities. A review of lessons learned is included, with specific proposals to broaden both the number and range of astrophysical sources found in future enhancements. The BATSS survey results provide solid empirical evidence in support of an all-slewing hard X-ray survey mission, a prospect that may be realized with the launch of the proposed MIRAX-HXI mission in 2017. / Physics

Galaxy clusters are the largest known gravitationally bound objects, their study is important for both an intrinsic understanding of their systems and an investigation of the large scale structure of the universe. The multi- component nature of galaxy clusters offers multiple observable signals across the electromagnetic spectrum. At X-ray wavelengths, galaxy clusters are simply identified as X-ray luminous, spatially extended, and extragalactic sources. X-ray observations offer the most powerful technique for constructing cluster catalogues. The main advantages of the X-ray cluster surveys are their excellent purity and completeness and the X-ray observables are tightly correlated with mass, which is indeed the most fundamental parameter of clusters. In my thesis I have conducted the 2XMMi/SDSS galaxy cluster survey, which is a serendipitous search for galaxy clusters based on the X-ray extended sources in the XMM-Newton Serendipitous Source Catalogue (2XMMi-DR3). The main aims of the survey are to identify new X-ray galaxy clusters, investigate their X-ray scaling relations, identify distant cluster candidates, and study the correlation of the X-ray and optical properties. The survey is constrained to those extended sources that are in the footprint of the Sloan Digital Sky Survey (SDSS) in order to be able to identify the optical counterparts as well as to measure their redshifts that are mandatory to measure their physical properties. The overlap area be- tween the XMM-Newton fields and the SDSS-DR7 imaging, the latest SDSS data release at the starting of the survey, is 210 deg^2. The survey comprises 1180 X-ray cluster candidates with at least 80 background-subtracted photon counts, which passed the quality control process. To measure the optical redshifts of the X-ray cluster candidates, I used three procedures; (i) cross-matching these candidates with the recent and largest optically selected cluster catalogues in the literature, which yielded the photometric redshifts of about a quarter of the X-ray cluster candidates. (ii) I developed a finding algorithm to search for overdensities of galaxies at the positions of the X-ray cluster candidates in the photometric redshift space and to measure their redshifts from the SDSS-DR8 data, which provided the photometric redshifts of 530 groups/clusters. (iii) I developed an algorithm to identify the cluster candidates associated with spectroscopically targeted Luminous Red Galaxies (LRGs) in the SDSS-DR9 and to measure the cluster spectroscopic redshift, which provided 324 groups and clusters with spectroscopic confirmation based on spectroscopic redshift of at least one LRG. In total, the optically confirmed cluster sample comprises 574 groups and clusters with redshifts (0.03 ≤ z ≤ 0.77), which is the largest X-ray selected cluster catalogue to date based on observations from the current X-ray observatories (XMM-Newton, Chandra, Suzaku, and Swift/XRT). Among the cluster sample, about 75 percent are newly X-ray discovered groups/clusters and 40 percent are new systems to the literature. To determine the X-ray properties of the optically confirmed cluster sample, I reduced and analysed their X-ray data in an automated way following the standard pipelines of processing the XMM-Newton data. In this analysis, I extracted the cluster spectra from EPIC(PN, MOS1, MOS2) images within an optimal aperture chosen to maximise the signal-to-noise ratio. The spectral fitting procedure provided the X-ray temperatures kT (0.5 - 7.5 keV) for 345 systems that have good quality X-ray data. For all the optically confirmed cluster sample, I measured the physical properties L500 (0.5 x 10^42 – 1.2 x 10^45 erg s-1 ) and M500 (1.1 x 10^13 – 4.9 x 10^14 M⊙) from an iterative procedure using published scaling relations. The present X-ray detected groups and clusters are in the low and intermediate luminosity regimes apart from few luminous systems, thanks to the XMM-Newton sensitivity and the available XMM-Newton deep fields The optically confirmed cluster sample with measurements of redshift and X-ray properties can be used for various astrophysical applications. As a first application, I investigated the LX - T relation for the first time based on a large cluster sample of 345 systems with X-ray spectroscopic parameters drawn from a single survey. The current sample includes groups and clusters with wide ranges of redshifts, temperatures, and luminosities. The slope of the relation is consistent with the published ones of nearby clusters with higher temperatures and luminosities. The derived relation is still much steeper than that predicted by self-similar evolution. I also investigated the evolution of the slope and the scatter of the LX - T relation with the cluster redshift. After excluding the low luminosity groups, I found no significant changes of the slope and the intrinsic scatter of the relation with redshift when dividing the sample into three redshift bins. When including the low luminosity groups in the low redshift subsample, I found its LX - T relation becomes after than the relation of the intermediate and high redshift subsamples. As a second application of the optically confirmed cluster sample from our ongoing survey, I investigated the correlation between the cluster X-ray and the optical parameters that have been determined in a homogenous way. Firstly, I investigated the correlations between the BCG properties (absolute magnitude and optical luminosity) and the cluster global proper- ties (redshift and mass). Secondly, I computed the richness and the optical luminosity within R500 of a nearby subsample (z ≤ 0.42, with a complete membership detection from the SDSS data) with measured X-ray temperatures from our survey. The relation between the estimated optical luminosity and richness is also presented. Finally, the correlation between the cluster optical properties (richness and luminosity) and the cluster global properties (X-ray luminosity, temperature, mass) are investigated. / Im Rahmen dieser Arbeit habe ich die 2XMMi/SDSS Galaxienhaufendurchmusterung erstellt (2XMMi/SDSS galaxy cluster survey), eine Suche nach Galaxienhaufen welche auf der Detektion ausgedehnter Röntgenquellen im XMM-Newton Quellenkatalog (2XMMi-DR3) basiert. Die Hauptziele dieser Suche sind die Identifizierung bisher unbekannter röntgenheller Galaxienhaufen, die Erforschung ihrer Beziehungen zwischen Röntgenleuchtkraft und Temperatur (X-ray scaling relation), eine Entdeckung von möglichen weit entfernten Galaxienhaufen und die Beziehung zwischen Eigenschaften im Optischen und Röntgenbereich. Die Durchmusterung ist für alle Quellen der Himmelsregionen ausgelegt, die vom Sloan Digital Sky Survey (SDSS) erfasst werden. Das Ziel besteht darin, ihre optischen Gegenstücke zu finden und deren Rotverschiebungen zu bestimmen. Die gemeinsamen Himmelsareale zwischen XMM-Newton und dem Bildmaterial vom SDSS-DR7 umfassen 210 deg^2. Meine Durchmusterung enthält 1180 mögliche Galaxienhaufen mit wenigstens 80 vom Hintergrund bereinigten Photonen im Röntgenbereich, die einer Qualitätskontrolle erfolgreich standgehalten haben. Um die Rotverschiebungen der möglichen Galaxienhaufen im optischen Bereich zu bestimmen nutzte ich drei Vorgehensweisen: (i) Ein Abgleich jener Kandidaten mit den neuesten und umfangreichsten Katalogen optisch ausgewählter Galaxienhaufen, die in der Literatur verfügbar sind. (ii) Ich entwickelte einen Algorithmus, um Rotverschiebungen der optischen Gegenstücke aus Daten vom SDSS-DR8 zu ermitteln, welches zu photometrischen Rotverschiebungen von 530 Galaxiengruppen-/haufen führte. (iii) Ein weiterer von mir entwickelter Algorithmus nutzte die spektroskopischen Rotverschiebung von roten leuchtkräftigen Galaxien (LRGs) in den Daten des SDSS-DR9 und ergab 324 Gruppen und Haufen. Zusammengefasst enthält diese Probe 574 auch im optischen nachgewiesener Galaxiengruppen und -haufen mit bekannten Rotverschiebungen (0.03 ≤ z ≤ 0.77) - der zur Zeit umfangreichste Katalog von im Röntgenbereich ausgewählten Galaxienhaufen basierend auf aktuellen Röntgenbeobachtungen. Unter jenen Haufen waren ca. 75% im Röntgenbereich nicht bekannt und 40% fanden in der bisherigen Literatur noch keine Erwähnung. Um die Röntgeneigenschaften der im Optischen bestätigten Haufen zu bestimmen, war eine automatische Reduktion und Analyse der Röntgendaten unverzichtbar. Die Prozedur, welche Modelle an die Röntgenspektren anpasste, ergab Temperaturen kT von 0.5 – 7.5 keV für 345 Kandidaten. Für alle Haufen, die auch im optischen auffindbar waren, bestimmte ich die physikalischen Eigenschaften L500 (0.5 x 10^42 – 1.2 x 10^45 erg s^-1) und M500 (1.1 x 10^13 – 4.9 x 10^14 M⊙). Die Probe optisch bestätigter Galaxienhaufen mit gemessenen Rotverschiebungen und Röntgeneigenschaften kann für viele astrophysikalische Anwendungen genutzt werden. Als eine der ersten Anwendungen betrachtete ich die Beziehung zwischen LX - T; das erste Mal für eine so grosse Anzahl von 345 Objekten. Der aktuelle Katalog enthält Gruppen und Haufen, die einen grossen Bereich in Rotverschiebung, Temperatur und Helligkeit abdecken. Der Anstieg jener Beziehung ist im Einklang mit bereits publizierten Werten für nahegelegene Galaxienhaufen von hoher Temperatur und Helligkeit. Nach dem Ausschluss leuchtschwacher Gruppen und der Einteilung der Daten in drei nach Rotverschiebung geordneter Gruppen, waren keine signifikanten Änderungen von Anstieg und intrinsischer Streuung zu beobachten. Als zweite Anwendung unserer Durchmusterung, untersuchte ich die Haufen bezüglich deren Eigenschaften im Optischen und im Röntgenbereich. Zuerst betrachtete ich den Zusammenhang zwischen den Eigenschaften (absolute Helligkeit und optische Leuchkraft) der hellsten Haufengalaxie (BCG) mit denen des Haufens als Ganzem (Rotverschiebung und Masse). Danach berechnete ich die Reichhaltigkeit der Galaxienhaufen und deren optische Leuchtkraft innerhalb von R500 für eine Stichprobe nahegelegener Haufen (z ≤ 0.42, hier sind SDSS Daten noch empfindlich genug um den Grossteil der Haufengalaxien abzubilden) mit gemessenen Röntgentemperaturen. Schlussendlich konnten dieWechselwirkungen zwischen den optischen Eigenschaften (Reichhaltigkeit und Leuchtkraft) und den globalen Eigenschaften (Röntgenleuchtkraft, Temperatur und Masse) näher untersucht werden.

<p> Core-collapse supernovae are among the most consequential astronomical events. They impact galaxy evolution, chemical enrichment of the Universe, and the creation of exotic objects (e.g., black holes and neutron stars). However, aspects of supernovae such as explosion asymmetry and progenitor mass loss are not well understood. Young, nearby supernova remnants are excellent laboratories to uniquely constrain some these fundamental properties. In this thesis, I investigate two nearby oxygen-rich supernova remnants and measure the proper motion of their ejecta to estimate their center of expansions and explosion ages. These properties are important for determining central compact object ‘kick’ velocities, guiding searches for surviving companions, and creating 3D remnant reconstructions. </p> <p><br></p> <p>I estimate the center of expansion and age of two supernova remnants, 1E0102.2-7219 (E0102) and N132D utilizing two epochs of Hubble Space Telescope imaging to measure the proper motion of their ejecta. For E0102, the proper motions show evidence for a nonhomologous expansion, which combined with spectral observations, support the idea that this remnant is expanding into an asymmetric circumstellar environment. Using the new proper-motion derived age and center of expansion, I provide a new ‘kick’ velocity estimate for E0102’s candidate neutron star. For N132D, I measure the proper motion of the ejecta both visually and using a novel computer vision procedure which identifies and measures the proper motions of the knots. I find that N132D’s ejecta are still ballistic, along with evidence of explosion asymmetry. My results represent the first proper-motion derived center of expansion and age of N132D. </p> <p><br></p> <p>Finally, I investigate diffuse interstellar bands observed towards progenitor candidates of core-collapse supernovae to test whether time variability can be a possible probe of the mass loss and surrounding environments of these systems. I find evidence of time variability in diffuse interstellar band carriers located in two of these environments. This is especially unusual as diffuse interstellar bands are normally attributed to the interstellar medium. These findings imply that the sources of these bands are closer to the stellar objects than previously thought and can provide insight into the currently unknown sources of diffuse interstellar bands. </p>

<p dir="ltr"> This article-based dissertation provides an account of two distinct classes of expansive astrophysical outflows and techniques to interpret their observations using numerical modeling. The primary purpose of this dissertation is to provide an extensive description of the research projects I undertook during my tenure as a Graduate Research Assistant, under the guidance of my advisor Prof. Paul Duffell.</p><p dir="ltr">Chapter 1 provides a brief introduction to numerical hydrodynamics and techniques of modeling expanding flows numerically. I also introduce the aforementioned classes of astrophysical outflows, namely relativistic jets from Active Galactic Nuclei (AGN), and supernova remnants (SNRs). I provide a general overview of the theoretical picture, and the general strategy used in this work to model them.</p><p dir="ltr">Chapter 2 describes my investigation on the connection of kiloparsec scale AGN jet properties to their intrinsic parameters and surroundings, based on an article published in The Astrophysical Journal. Using a suite of over 40 relativistic hydrodynamic jet models, we find that the dynamics of relativistic jets can be described in terms of only two parameters, the jet to ambient medium energy density ratio, and the jet opening angle. The former is found to strongly control the Fanaroff-Riley (FR) morphological dichotomy, which was previously thought to be tied to the magnitude of the jet luminosity. We also suggest a purely hydrodynamical origin of bright spots observed in some AGN jets. Our models were tested against and found to be consistent with the observations of the jets in M87 and Cygnus A.</p><p dir="ltr">In chapter 3, I present my moving-mesh hydrodynamics code Sprout, also described in an article published in The Astrophysical Journal Supplements. Sprout solves the equations of ideal hydrodynamics on an expanding Cartesian mesh. The expanding mesh can follow fluid outflows for several orders of magnitude with very little numerical diffusion. This allows Sprout to capture expanding flows with very high dynamic range. Sprout is thus particularly suitable for studying expanding outflows such as supernova remnants and active galactic nuclei. Relative to other moving mesh codes, the simple mesh structure in Sprout is also convenient for implementing additional physics or algorithms. I discuss many code tests that were performed to test the accuracy and performance of the numerical scheme.</p><p dir="ltr">Chapter 4 details my study of hydrodynamic instabilities in supernova remnants (SNRs) as they expand against the circumstellar medium (CSM). This is based on an article published in The Astrophysical Journal. A suite of 3D hydrodynamical SNR models, generated using my hydro code \sprout, was used to study the impact of the stellar ejecta density profile and seed anisotropies in the ejecta and the CSM on formation of turbulent structures in the SNRs. We found that most of the turbulent power in these models resides at a typical angular mode or scale that is determined by the ejecta density structure. It was also found that clumps or anisotropies in either the ejecta or CSM do not imprint upon these turbulence structures unless they are massive and form large-scale coherent structures.</p><p dir="ltr">In chapter 5, I discuss the implementation of a technique to measure anisotropies in observed SNRs just using 2D high-resolution images. This technique is calibrated using 3D hydro SNR models and synthetic images derived from them. As seen in Chapter 4, we find a similar dominant angular scale of turbulent structures dictated by the ejecta density structure. Both the 3D models and the synthetic images yield the same value of this scale, which validates the image analysis technique used in this work. As an example of how this technique can be applied to observations, we analyze observations of a known supernova remnant (Tycho's SNR) and compare with our models. Our technique picks out the angular scale of Tycho's fleece-like structures and also agrees with the small-scale power seen in Tycho.</p><p dir="ltr">PhChapter 6 summarizes the results, conclusions, and future prospects of all the research work described so far. It is followed by a bibliography, my curriculum vita, and a list of publications.</p>

Title: Study of Cosmic-Ray Acceleration in Extragalactic Objects Author: Stanislav tefnik Department: Institute of Particle and Nuclear Physics Supervisor: RNDr. Dalibor Nosek, Dr. Abstract: This thesis deals with production processes of cosmic γ-rays in astro- physical objects and methods of their detection. Possible interactions leading to the emission of high energy γ-rays in the active galaxy Centaurus A are discussed in this context. Cherenkov Telescope Array is presented as a new experiment fo- cused on the detection of air showers initiated by cosmic γ-rays. Cherenkov light of air showers is studied in the simulations done by CORSIKA simulation tool. Method of data analysis within the framework of Cherenkov telescopes is descri- bed and performed on the dataset of the active galaxy PKS 2155-304. The results include statistical tests of γ-ray source presence and its time variability. Keywords: Cosmic rays, gamma rays, astroparticle physics, high energy astro- physics, acceleration, CTA experiment

Les noyaux actifs de galaxie (NAG) sont les objets les plus énergétiques de l'univers. Cette incroyable puissance provient de l'énergie gravitationnel de matière en rotation autour d'un trou noir super-massif siégeant au centre des galaxies. Environ 10% des NAG sont pourvus de jets relativistes émanant de l'objet central (trou noir et matière environnante) et s'étalant sur des échelles de l'ordre de la galaxie hôte. Ces jets sont observés à toutes les longueurs d'ondes, de la radio aux rayons gamma les plus énergétiques. En dépit de nombreuses études et d'instruments de plus en plus précis depuis leur découverte dans les années 1950, les NAG sont encore très mal compris et la formation, la composition et l'accélération des jets sont des questions encore pleinement ouvertes. Le modèle le plus répandu visant à reproduire l'émission des NAG, le modèle "une zone" repose souvent sur des hypothèse ad-hoc et ne parvient pas à apporter une modélisation satisfaisante.Le paradigme du "two-flow" (deux fluides) développé à l'IPAG et basé sur une idée originale de Sol et al (1989) a pour but de fournir une vision unifiée et cohérente des jets de NAG. Cette théorie repose sur une l'hypothèse principale que les jets seraient en fait composés de deux fluides co-axiaux: une colonne centrale composée d'un plasma purement leptonique (électrons/positrons) se déplaçant à des vitesses relativistes et responsable pour la grande partie de l'émission non thermique observée entourée par une enveloppe composée d'un plasma baryonique (électrons/protons), régie pas la magnéto-hydrodynamique, se déplaçant à des vitesses sous-relativistes mais transportant la majorité de l'énergie. Cette hypothèse est basée sur des indices observationnels ainsi que sur des arguments théoriques et permet d'expliquer nombre des caractéristiques des NAG.Afin d'étudier plus en profondeur le paradigme du two-flow, un modèle numérique basé sur ses concepts et produisants des observables comparables aux observations est nécessaire.Durant ma thèse, j'ai participé au développement de ce modèle, m'intéressant notamment à la diffusion Compton inverse de photons provenant de l'extérieur du jet. Ce processus, primordial dans la modélisation des NAG, est aussi central dans le paradigme du two-flow car il est à l'origine de l'accélération de la colonne via l'effet fusée Compton. Pour cela, j'ai du développer des nouvelles approximations analytiques de la diffusion Compton d'une distribution thermique de photons.En m'intéressant à l'effet fusée Compton, j'ai pu montré que dans le champ de photon thermique d'un NAG, le facteur de Lorentz d'ensemble du plasma pouvait être sujet à des variations le long du jet en fonction de la distance à l'objet central. Ces variations peuvent avoir un effet important sur l'émission observée et peuvent induire de la variabilité spatiale et temporelle. J'ai également montré que les facteurs de Lorentz terminaux obtenus étaient compatibles avec les conditions physiques attendus dans les jets et avec les observations.Le modèle complet produit des DES directement comparables aux observations. Néanmoins, le modèle est par nature erratique et il est quasiment impossible de relier directement les paramètres du modèles avec les DES produites. Malheureusement, les procédures standards d'adaptation automatique aux données (e.g. basé sur les méthodes de gradient) ne sont pas adaptées au modèle à cause de son grand nombre de paramètres, de sa non-linéarité et du temps de calcul important. Afin de palier à ce problème, j'ai développé une procédure d'adaptation automatique basée sur les algorithmes génétiques. L'utilisation de cet outil a permis la reproduction de plusieurs DES par le modèle. J'ai également montré que le modèle était capable de reproduire les DES observées avec des facteurs de Lorentz d'ensemble relativement bas, ce qui pourrait potentiellement apporter une harmonisation entre les observations et les nécessités théoriques. / Active galactic nuclei (AGN) are the most energetic objects known in the universe. Their fantastic energy is due to efficient conversion of gravitational energy of mass accreted on super-massive black-holes at the center of galaxy into luminous energy. 10% of AGN are even more incredible as they display relativistic jets on galaxy scales. Those jets are observed at all energies, from far radio to highest gamma-rays. Despite intense study since their discovery in the 50's and more and more observations, favored by rapid progress in instrumentation, AGN are still widely misunderstood. The questions of formation, composition, and acceleration of jets are central but still a matter of debates. Models aiming at reproducing observed emission have been developed throughout the years. The most common one, the one-zone model, often relies on ad hoc hypothesis and does not provide a satisfactory answer.The two-flow paradigm developed at IPAG and based on an original idea from Sol et al (1989) aims at giving a more coherent and physical representation of AGN jets. The principal assumption is that jets are actually composed of two coaxial flows: an inner spine made of a pure pair plasma, moving at relativistic speed and responsible for the non-thermal observed emission surrounded by an external sheath, made of a baryonic MHD plasma, midly relativistic but carrying most of the power. The two-flow paradigm finds roots in observations as well as theoretical arguments and has been able to explain many AGN features.During my PhD, I studied this paradigm and contributed to the development of a numerical model based on its concepts. I have been particularly interested in the inverse Compton scattering of thermal photons, fundamental process in the modeling of AGN emission, as well as the Compton rocket effect, key to the acceleration of the spine in the two-flow paradigm.However, taking into account the inverse Compton emission, with the complete cross-section (including the Klein-Nishina regime) and the anisotropy can be very time consuming. To accomplish fast and efficient computation of the external Compton emission, I have had to formulate new analytical approximations of the scattering of a thermal distribution of photons.I have also studied the Compton rocket effect, responsible for the acceleration of the inner spine in the two-flow paradigm. I showed that the resulting bulk Lorentz factor of the flow in the complex photon field of an AGN is subject to variations along the jet as a function of the distance to the central engine. These variations can have drastic effects on the observed emission and could induce variability, both spatially and temporally.I also showed that the terminal bulk Lorentz factor obtained are compatible with physical conditions expected in jets and with observations.The complete model produce spectral energy distribution (SED) comparable to observed ones. However, the model is by nature erratic and it is difficult to make a direct link between the model parameters (input) and the SED (output). Unfortunately, standard data fitting procedures (e.g. based on gradient methods) are not adapted to the model due to its important number of parameters, its important computing time and its non-linearity. In order to circumvent this issue, I have developed a fitting tool based on genetic algorithms. The application of this algorithm allowed me to successfully fit several SED. In particular, I have also showed that the model, because based on a structured jet model, can reproduce observations with low bulk Lorentz factor, thus giving hope to match observations and theoretical requirements in this matter.

<p>Active Galactic Nuclei (AGNs) are accreting supermassive black holes at the center of galaxies, known for rich spectral features and multi-time scale variability in their electromagnetic emission. The origin of the variability in AGN light curves can be either intrinsic, meaning related processes that take place inside the AGN system, or extrinsic, i.e., from the propagation of light towards Earth. In this dissertation, I present my work focusing on AGN variability. The first two works focus on the variability of blazars, a subclass of AGN with their relativistic jets beaming towards the observer. The first work combines 3D relativistic magnetohydrodynamics (RMHD) simulations with radiation transfer and shows the kink instability within the blazar jet can cause quasi-periodic radiation signatures within a typical period of time scales from weeks to months. The second work combines 2D Particle-in-Cell (PIC) simulations with radiation transfer and shows that isolated and merging plasmoids due to magnetic reconnection in a blazar environment could produce rich radiation and polarization signatures. The last work explores an extrinsic origin for AGN variability: a scenario in which interstellar medium (ISM) within our galaxy can refract light coming from AGNs. It suggests that plasma structures in ISM with an axisymmetric geometry can account for extreme scattering events (ESEs) in AGN observations. Future research directions include studies of the kink instability in jets that propagate in different environments and simulations of magnetic reconnection in 3D which may reveal additional particle acceleration mechanisms, which may play important role in the resulting radiation and polarization signatures. </p>

<p dir="ltr">Understanding the acceleration of particles responsible for high-energy non-thermal phenomena in astrophysical jets is a ubiquitous pursuit. A possible culprit for non-thermal particle acceleration is turbulence. Specifically in this thesis, I investigate highly magne- tized or relativistic turbulence, where the magnetic energy to enthalpy ratio of the plasma is much greater than one, as a possible high-energy accelerator inside relativistic jets. I do this through three distinct projects. </p><p dir="ltr">My first project [1] (discussed in Section 3) was built upon a recent study of relativistic turbulence from [2], which found that a non-thermal particle equilibrium can be achieved when a plasma is heated via turbulence but allowed to cool radiatively. I extrapolated these results from PIC (Particle-in-Cell) simulations to larger scales and magnetizations, allowing me to encode key microphysical results of PIC simulations into a Fokker-Planck formalism. Combining these results with a single zone model for a blazar jet, I successfully define the underlying particle distribution with the global parameters of the emission region. To test this model, I fit data from 12 sources and successfully constrain key blazar parameters such as magnetization, bulk Lorentz factor, emission region size, and distance from the central engine. </p><p dir="ltr">My second project covers the development and testing of the open-source toolkit Tleco. This code base was used to evolve the Fokker-Planck equation and solve the resultant emission in my first project. Tleco offers efficient algorithms for evolving particle distributions and solving the resultant emission. It is meant to be user-friendly and easily customizable. </p><p dir="ltr">My third project attempts to enhance our understanding of coherent structures in relativistic turbulence. I employ intermittency analysis to establish a link between statistical fluctuations within the plasma and regions of high-energy dissipation. To achieve this, we used first-principle turbulent PIC simulations across a range of magnetizations and fluctuating magnetic field values. By utilizing the statistical fluctuations to determine the fractal dimension of the structures, I then examine their filling fraction and its dependence on magnetization and the fluctuating magnetic field.</p>

Ce manuscrit de thèse présente l’étude des processus d’émission de deux types de noyaux actifs de galaxie détectés aux rayons gamma : des blazars vues au TeV et des Narrow Line Seyfert 1s (NLS1s) détectés au GeV. La distribution spectrale d’énergie des blazars peut être décrite en général par des modèles 'one-zone synchrotron self-Compton'. Ce modèle a été appliqué au blazar 1ES2322-409 qui fut premièrement détecté au TeV par l’expérience HESS. Des composantes externes comme le tore, le disque d’accrétion, la couronne X ou la 'Broad Line Region' sont nécessaires pour expliquer la radiation observée dans des NLS1 qui émettent des rayons gamma. Un modèle numérique qui considère ces champs des photons externes a été développé. Ce modèle explique l’émission observée et la transition entre des états bas et des états d’émission augmentée pour trois NLS1s vues au régime gamma : 1H0323+342, B20954+25A et PMN J0948+0022. / This thesis manuscript presents the study of the emission processes of two types of gamma-emitting active galactic nuclei: TeV-detected blazars and GeV-detected Narrow Line Seyfert 1s (NLS1s).The Spectral Energy Distribution (SED) of TeV blazars can in general be well described by simple one-zone synchrotron self-Compton models. Such model has been applied to the blazar 1ES2322-409 that was first detected at TeV by the HESS collaboration.Additional external photon fields such as the obscuring torus, the accretion disc, the X-ray corona or the broad line region are necessary to describe the observed radiation and broad-band SED of gamma-emitting NLS1s. A numerical model that takes into account emission from these external fields has been developed. The model explains the observed emission and the transition from quiescent to gamma-ray flaring states of three gamma-emitting NLS1s: 1H0323+342, B20954+25A and PMN J0948+0022.