Dissertations / Theses on the topic 'Active Galactic Nuclei – Blazars – Gamma-ray blazars'

A multiwavelength investigation is conducted for nineteen blazar-type active galactic nuclei. Studies of variability timescales and flux duty cycles are performed at x- and gamma-rays for each source, with the relationship between flux and spectral index also being probed at gamma wavelengths. The correlation between these two energy ranges is also investigated, by utilising the Discrete Correlation Function with both one and ten day binning. The sources were chosen for their availability over a range of different x- and gamma-ray data sources: observations utilised include 0.2 - 150 keV x-ray data from the Swift mission and 200 MeV- 300 GeV gamma-ray data from the Fermi mission. Daily-binned Fermi data is used to calculate the smallest rise and decay e-folding times in gamma for each source. The results range from 0.4 to 21 days, corresponding to limits on the size of the gamma emission region ranging from Rd^-1 = 4.39 x 10^12 to Rd^-1 = 5.14 x 10^14 m. Flux duty cycles for fourteen sources are created from Fermi data, with six displaying structure at high fluxes that indicate flaring states have occured. Five of these six sources also display clear flares in their light curves, confirming these results. The relationship between the flux and the spectral index shows eight of nineteen sources exhibit harder-when-brighter behaviour. Four of these eight have been previously confirmed to display such behaviour. Results from the Discrete Correlation Function show a correlation at a time lag of ~ 600 days for H 1426+428, of uncertain origin. Gaussian functions are fitted to possible near-zero peaks in 3C 66A, 3C 454.3 and Mrk 421, which could be indicative of a synchrotron self-Compton component to the emission of these objects.

This thesis is concerned with very high energy γ-ray observations of blazars observable from the southern hemisphere. The data presented were obtained using the recently deployed University of Durham Mark 6 high sensitivity Čerenkov telescope. Chapters 1 and 2 are introductory; the subjects of γ-ray astronomy, extensive air showers, Čerenkov light production, the development of the atmospheric Čerenkov technique and the current status of TeV astronomy are discussed. Chapter 3 introduces the telescopes operated by the University of Durham. The Mark 6 telescope, designed to have a low threshold energy and a high resolution imaging system, is discussed in detail. Chapter 4 presents the calibration and analysis techniques routinely applied to data obtained with the Mark 6 telescope. The chapter concludes with a set of moment parameter selections designed to reject a significant fraction of the cosmic ray cascades, while retaining the majority of γ-rays cascades. These selections have resulted in a 5σ detection of PSR 1706-44. Chapter 5 discusses active galactic nuclei and in particular blazars. Topics included are recent high energy observations; the differences between the radio/X-ray selected BL Lacs and flat spectrum radio quasars; the infrared background; high energy flux variability and γ-ray production mechanisms within these objects. The chapter concludes with a list of possible very high energy γ-ray emitting blazars observable with the Mark 6 Čerenkov telescope. Chapter 6 presents the results from four of these very high-energy γ-ray blazar candidates; PKS 0548-322, PKS 1514-24, PKS 2005-489 and PKS 2155-304. There is no evidence for γ-ray emission from these sources, either in the form of a steady flux or variable activity. Three sigma integral flux upper limits above 300 GeV are produced for these objects and the implications of these observations are discussed. The conclusions are presented in Chapter 7 as well as ideas for future work.

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

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.

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.

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).

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.

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).

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.

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.

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).

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).
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).

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.

De nombreuses questions liées à la physique des jets des Noyaux Actifs de Galaxies restent ouvertes. Une classe particulière d’AGN, les blazars, a un jet pointant vers la Terre. Une telle orientation du jet nous permet de sonder une riche variété de phénomènes physiques mal compris sur les écoulements relativistes. Les blazars montrent une émission non thermique, provenant du jet, qui est très variable sur tout le spectre électromagnétique, des radiofréquences aux rayons gamma du TeV. Le flux d’énergie peut augmenter d’un ordre de grandeur sur des échelles de temps aussi courtes que quelques minutes, un phénomène appelé “sursaut” (flare), et aussi longues que des mois ou même des années. Malgré la quantité croissante de données disponibles sur plusieurs longueurs d’onde (multi-wavelength, MWL), l’origine et les mécanismes physiques derrière les sursauts fréquemment observés dans les blazars ne sont toujours pas bien compris. De nombreuses tentatives ont été faites pour décrire les flares avec différents modèles d’émission, mais les propriétés détaillées de l’évolution temporelle des flux dans différentes bandes spectrales restent difficiles à reproduire. Afin d’identifier les processus physiques impliqués lors des sursauts de blazars, j’ai développé un code radiatif polyvalent, basé sur un traitement dépendant du temps de l’accélération des particules, de l’échappement et du refroidissement radiatif. Le code calcule l’évolution dans le temps de la fonction de distribution des électrons dans la zone d’émission du blazar et le spectre de l’émission Synchrotron Self-Compton (SSC) par ces électrons. J’ai appliqué le code à un sursaut multi-lambda géant du blazar Mrk 421, représentant de la classe des BL Lacertae, qui est le sursaut le plus brillant détecté jusqu’ici en provenance de cette source. Dans notre approche, nous considérons le sursaut comme une perturbation modérée de l’état de flux stationnaire et recherchons des interprétations avec un nombre minimum de paramètres libres. En conséquence, j’ai développé un nouveau scénario physique de l’activité observé pendant le sursaut, qui décrit l’ensemble des données, comprenant des spectres à l’état haut de la source dans différentes gammes d’énergie, et des courbes de lumière multi-lambda du domaine optique aux rayons gamma VHE. Dans ce scénario, le processus déclenchant le sursaut est l’accélération des particules par un processus de type Fermi du second ordre, dû à la turbulence qui emerge au voisinage de la région d’émission stationnaire du blazar. Dans cette thèse, j’ai également effectué une analyse des données du High Energy Stereoscopic System (H.E.S.S.) de deux sursauts géants du blazar 3C 279, représentant de la classe des Flat Spectrum Radio Quasars (FSRQ). Enfin, j’ai contribué à la préparation du Cherenkov Telescope Array (CTA), qui est un observatoire de rayons gamma au sol de nouvelle génération, dont l’entrée en service est prévue à partir de 2022. L’instrument, qui est actuellement en cours de développement, aura des performances considérablement améliorées par rapport aux Imaging Atmospheric Cherenkov Telescopes (IACTs) qui sont actuellement en fonctionnement, y compris une couverture spectrale sans précédent de quelques dizaines de GeV à ~300 TeV. Dans le cadre du CTA, j’ai effectué des simulations de performances optiques du Gamma-Ray Cherenkov Telescope (GCT), l’un des trois modèles proposés de télescopes de petite taille (SST) pour CTA. De plus, en utilisant les observations d’étoiles brillantes effectuées par le prototype de télescope installé sur le site de l’Observatoire de Paris à Meudon, j’ai étudié l’effet de la micro-rugosité des miroirs du télescope sur la fonction d’étalement du point (PSF) et calculé le niveau de qualité de polissage des miroirs requis pour optimiser les performances
Many questions related to the physics of jets of Active Galactic Nuclei remain open. A particular subclass of AGN, blazars, have a jet pointing towards the Earth. Such suitable orientation of the jet allows us to probe a rich variety of poorly understood physical phenomena related to relativistic outflows. Blazars show non-thermal emission, originating from the jet, which is highly variable across the entire electromagnetic spectrum, from radio frequencies to TeV gamma-rays. The energy flux can enhance by an order of magnitude on time-scales as short as minutes, a phenomenon referred to as a “flare”, and as long as months or even years. Despite the growing amount of available multi-wavelength (MWL) data, the origin and the physical mechanisms behind the frequently observed flaring events in blazars are still not well understood. Many attempts have been made to describe the flares with different emission models, but detailed properties of flux variation patterns (light curves) in different wavebands remain difficult to reproduce. In order to identify physical processes that are involved during blazar outbursts, I have developed a versatile radiative code, based on a time-dependent treatment of particle acceleration, escape and radiative cooling. The code computes time evolution of the distribution function of electrons in the blazar emitting zone and the spectrum of the Synchrotron Self-Compton (SSC) emission by these electrons. I applied the code to a giant MWL flare of the blazar Mrk 421, a representative of the BL Lacertae class, which is the brightest VHE flare ever detected from this source. In our approach, we consider the flare as a moderate perturbation of the quiescent state and search for interpretations with a minimum number of free parameters. As a result, I developed a novel physical scenario of the flaring activity that describes the data set, comprising spectra in the high state of the source in different energy ranges, and MWL light curves from the optical domain to the VHE gamma-ray band. In this scenario, the process initiating the outburst is the second-order Fermi acceleration of particles due to turbulence arising in the vicinity of the blazar stationary emission region. In this thesis, I also performed analysis of High Energy Stereoscopic System (H.E.S.S.) data of two giant flares of the blazar 3C 279, a representative of the Flat Spectrum Radio Quasars (FSRQ) class. Finally, I contributed to preparation of Cherenkov Telescope Array (CTA), which is a new-generation ground-based gamma-ray observatory, expected to start operations in 2022. The instrument, which is presently under development, will have greatly improved performance compared to currently operating Imaging Atmospheric Cherenkov Telescopes (IACTs), including unprecedented spectral coverage from a few tens of GeV to ~300 TeV. In the context of CTA, I performed simulations of the optical performance of the Gamma-Ray Cherenkov Telescope (GCT), one of the three proposed designs of Small-Size Telescopes (SST) for CTA. Also, using the observations of bright stars done by the telescope prototype installed on the site of Paris Observatory in Meudon, I studied the effect of micro-roughness of the telescope mirrors on the point spread function (PSF) and calculated the level of the mirror polishing quality required to optimize the performances

Les neutrinos sont des messagers uniques pour détecter les phénomènes violents de l’Univers. Les sources potentielles de neutrinos cosmiques sont, par exemple, les Noyaux Actifs de Galaxie (NAGs), ou les sursauts gammas. Dans le cas d’un accélérateur astrophysique de rayons cosmiques hadroniques, la production de neutrinos s’accompagne éventuellement d’émissions gammas de haute énergie. La recherche des coïncidences entre des neutrinos de haute énergie détectés avec le télescope à neutrinos ANTARES et des photons gamma captés par l’observatoire HAWC est présentée. La recherche dans une fenêtre temporelle particulière réduit le bruit de fond de manière significative dans les données du télescope à neutrinos et augmente donc le potentiel de découverte. Dans cette thèse, les résultats de la recherche de neutrinos détectés en corrélation avec un signal gamma en provenance de 2 NAG particulier, Markarian 421 et Markarian 501, sont présentés. En tant que blazars les plus proches de la Terre, ils constituent d’excellentes sources pour tester le scénario de connexion blazar-neutrinos, en particulier lors de leurs "bouffées" d’activités (flares) pour lesquelles la recherche de neutrinos en fonction du temps peut présenter une probabilité de détection plus élevée
Neutrinos are unique messengers for detecting violent phenomena in the Universe. Potential sources of cosmic neutrinos are, for example, Active Galactic Nuclei (AGNs) or Gamma-Ray Bursts (GRBs). In the case of hadronic cosmic rays acceleration, the production of neutrinos is possibly accompanied by high-energy gamma-ray emissions. The search for coincidences between high-energy neutrinos detected by the ANTARES neutrino telescope and gamma-rays detected by the HAWC gamma-ray observatory is presented. Searching in a particular time window significantly reduces the background noise in the neutrino data and thus increases the discovery potential. In this thesis, the results of a search for neutrinos detected in correlation with a gamma-ray signal from two particular AGNs, Markarian 421 and Markarian 501, are presented. As the blazars closest to Earth, they are excellent sources for testing the blazar-neutrino connection scenario, especially during the increase of their activities (flares) in which the search for neutrinos may have a higher detection probability

Blazars are a type of Active Galaxy that emit strong astrophysical jets. The association of a HE gamma-ray flare from the blazar TXS 0506+056 to the IceCube-170922A neutrino event in 2017, opened the possibility to a link between these two events. In this thesis, we will look at the HE gamma-ray behavior of TXS 0506+056 using data obtained from the Fermi-LAT by taking into account the other set of neutrino events associated with this source from 2014-2015. We will investigate whether both neutrino events present with comparable HE gamma-ray behavior by analyzing the lightcurves and the spectra for a quiet state, the 2014-2015 period, and the flare centered around the neutrino event from 2017. The results of the analysis performed in this thesis show no strong indication of a change in the gamma-ray behaviour in these potential neutrino detections.

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

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.

Les expériences actuelles en astronomie gamma sont le satellite Fermi-LAT et les expériences au sol tel que H.E.S.S., VERITAS et MAGIC. La surveillance des sources d’énergie très élevées indique une physique diversifiée. Afin d’étudier la forme la plus énergétique de radiation et les phénomènes les plus violents qui se déroulent dans l’Univers, l’analyse des sources individuelles est importante. Les BL Lac, un type de galaxie active, constituent la classe de source extragalactique la plus abondante détecté dans les énergies du GeV au TeV, tandis que le nébuleuses de vent de Pulsar sont la classe la plus peuplée dans le plan galactique. Ces deux types de sources ont des émissions variables de rayons gamma.Dans cette thèse, la variabilité de l'objet BL Lac B2 1215 + 30 est étudiée avec les données du satellite Fermi-LAT. Une grande variation de flux, détectée par Fermi-LAT en février 2014, est simultanée avec un éruption très lumineux observé au TeV par l'expérience VERITAS. En collaboration avec la collaboration VERITAS, la variabilité du flux de rayons gamma a été utilisée pour établir des contraintes sur la taille de la région d'émission et sur le facteur Doppler. La variabilité à long terme, en utilisant près de neuf ans de données de Fermi-LAT de 100 MeV jusqu'à 500 GeV, a permis de détecter plusieurs flares. L'étude de la variabilité du flux indique un comportement quasi périodique avec une période de jours.Ensuite, la variabilité du flux de l’un des objets les plus étudié, la Nébuleuse du Crabe, au TeV est étudiée avec dix ans d'observation de l'expérience H.E.S.S. Le spectre de la nébuleuse du crabe est mesuré de 280 GeV jusqu'à 62 TeV. Ceci est la première mesure qui s'étend à ces très hautes énergies. Considérée comme une “chandelle standard” en astronomie gamma, la nébuleuse du crabe est une source utilisée pour l'étalonnage et l'étude des instruments. L’observation de variations du flux au GeV par le satellite Fermi-LAT a par conséquent été une découverte inattendue. Ces variations de flux au GeV ont motivé la recherche de variations de flux au TeV en utilisant les données de l'expérience H.E.S.S. La position de la nébuleuse de crabe dans l'hémisphère nord et la localisation de H.E.S.S. en Namibie rendent cette enquête complexe en raison des importantes erreurs systématiques introduites par des conditions d'observation non optimales. Le travail sur la nébuleuse du crabe montre que la prise en compte de la transparence atmosphérique pour l'étude de l'évolution du flux avec le temps résulte en une réduction des effets systématiques. Aucune variation de flux n'a été observée à des énergies supérieures à 1 TeV dans les données de H.E.S.S. I. Une autre variation de flux au GeV signalée par le Fermi-LAT en octobre 2016 par télégramme astronomique, a été étudiée avec H.E.S.S. II. Cette analyse a montré que le GeV éruption a duré pendant un mois, et le flux avec H.E.S.S. a une variance excessive de 15 %. Cela devrait être comparé à l'incertitude systématique de 20 % fréquemment citée par H.E.S.S
The current state-of-the-art experiments in gamma-ray astronomy are the Fermi-LAT in space and the ground-based H.E.S.S., VERITAS and MAGIC experiments. The monitoring of the very-high-energy gamma-ray emitting sources indicates the diverse physics taking place in astrophysical environments. To study the most energetic form of radiation and the most violent phenomena taking place in the Universe, individual source analyses are important. BL Lac objects, a subcategory of active galaxies, are the most abundant source class detected both in the GeV andTeV energies, while pulsar wind nebulae represent the most numerous identified source class in the galactic plane. Both source classes exhibit gamma-ray flux variations.In this thesis, the gamma-ray variability of the BL Lac object B2 1215+30 is presented with Fermi-LAT data. A bright flare, with 16 times the average quiescent flux, was detected in February 2014.In collaboration with the VERITAS experiment, the gamma-ray variability was investigated over five decades in energy. This work resulted in the detection of a luminous flare, seen simultaneously in GeV and TeV energies by both instruments. These results were used to set constraints on the size of the emission region and on the Doppler factor of the relativistic jet. Additionally, the long-term variability was studied using nine years of Fermi-LAT data. This brought out new flux enhancements, which characterize the long-term lightcurve from 100 MeV up to 500 GeV. Other striking characteristics are a steady linear increase of the yearly average flux, together with a hardening of the spectral index. The investigation of the light curve indicates a hint of quasi-periodic behavior with a period of around 1083±32 days.This work includes spectrum and flux variability studies for the well-studied but ever-surprising Crab Nebula at TeV energies with more than a decade of H.E.S.S. observations. The spectrum measured in this work goes from 280 GeV to 62 TeV, making this the first measurement tha textends to such very-high-energies. Considered as a standard candle for ground-based gamma-ray astronomy, the Crab Nebula is also used for calibration and instrument studies. The detection of GeV flares by the Fermi-LAT were unexpected and motivated the search of flux variations at TeVenergies with the H.E.S.S. experiment. The position of the Crab Nebula in the northern hemisphere makes this investigation challenging due to the large systematic uncertainties introduced by the non-optimal observation conditions. This work showed that the systematic uncertainties can be reduced by taking into account the atmospheric transparency. No flux variations were found at energies above 1 TeV from the H.E.S.S. I data. A flare reported by the Fermi-LAT in October 2016 was also investigated. This analysis showed the GeV flare lasting for one month, while the flux withH.E.S.S. II had an excess variance of 15 %. This should be compared to the commonly quoted 20% systematic uncertainty by H.E.S.S. experiment

Schwerpunkt dieser Arbeit ist die Emission von hochenergetischer Gammastrahlung aus Aktiven Galaxienkernen, anhand von Beobachtungen mit den MAGIC Teleskopen. Aktive Galaxien entstehen durch die Freisetzung von Gravitationsenergie der Stellarmaterie, die in das zentrale Schwarze Loch fällt. Zwei diametral entgegengesetzte Jets strömen aus dem Kern. Falls einer von diesen Jets zum Beobachter zeigt, wird der aktive Galaxienkern als Blazar bezeichnet. In dieser Doktorarbeit wird die Analyse der MAGIC Beobachtungen von drei Blazaren präsentiert: der bisher unbekannte hochenergetische Blazar 1ES 1727+502 und die bekannten Objekte 3C 279 und PKS 1510-089. Die Quelle 1ES 1727+502 gehört zu der Unterklasse der BL Lac Objekte, die die zahlenmässig größte Klasse an extragalaktischen hochenergetische Objekten ist. Diese Entdeckung beweist, wie wichtig es ist, mehrere und verschiedene Kriterien für die Auswahl von Objekten zu benutzen. Der Blazar 1ES 1727+502 wurde aus einem X-ray Katalog ausgesucht, die Datennahme war unabhängig von dem Aktivitätsstatus der anderen Energiebänder. Die Blazare 3C 279 und PKS 1510-089 gehören zu der Unterklasse des Flat Spectrum Radio Quasars. Es gibt nur drei bekannte Flat Spectrum Radio Quasare. Die Entdeckung von Flat Spectrum Radio Quasaren ist schwierig, weil im leisen Zustand der hochenergetische Fluss niedrig ist. Während eines Flares steigt der Fluss in sehr kurzer Zeit um mehrere Größenordnungen an, deswegen ist die Beobachtung eines Flares schwer zu realisieren aber physikalisch äußerst interessant. Die MAGIC Beobachtungen sind mit Messungen aus anderen Energiebändern kombiniert. Mögliche Interpretationen des beobachteten Verhaltens werden diskutiert. Die Ergebnisse werden erläutert und mit historischen Beobachtungen verglichen. Die offenen Fragen und Probleme, die Ziel zukünftiger Studien sein sollen, werden am Schluss hervorgehoben.
This PhD thesis addresses the problem of understanding the very high energy emission from active galactic nuclei as detected with the MAGIC telescopes. Active galactic nuclei are galaxies powered by the release of gravitational energy of stellar material falling into a black hole, located in their core. Two opposed jets extend from the central region outwards, if one of the them points towards the observer, the source is called a blazar. In this thesis the analysis of MAGIC observations of three different blazars is presented: the newly discovered very high energy blazar 1ES 1727+502, and the two known objects 3C 279 and PKS 1510-089. The source 1ES 1727+502 belongs to the subclass of BL Lac objects, the most numerous class of extragalactic very high energy emitters. This source was selected for observations from an X-ray catalogue and was observed even if no high activity states were reported at lower energies. For this reason, this discovery proves the importance of using several criteria for the selection of very high energy observation targets. The other two objects, 3C 279 and PKS 1510-089, belong to the subclass of flat spectrum radio quasars. We count only three representatives of this class in the very high energy domain. Their detection is critical because during quiescent states they have low flux levels at very high energies. During flaring states, flat spectrum radio quasars exhibit flux enhancements of orders of magnitudes and short time scales. Observations during such states, difficult to catch, are interesting because extreme processes are taking place. Very high energy observations of these three objects are complemented with measurements at lower energies and interpretations of the observed behaviours are discussed. The results are compared with historical observations, highlighting open questions and problems which should be addressed by future studies.

Das Hauptziel dieser Arbeit ist die Charakterisierung extrem starker Quellen, die höchstwahrscheinlich die kosmische Strahlung beschleunigen. In dieser Arbeit wurden VHE-Gammastrahlenbeobachtungen mit den MAGIC-Teleskopen verwendet, um die Eigenschaften von Blazaren zu untersuchen. Um die Mechanismen zu untersuchen, die zur Breitbandemission von Blazaren führen, wird ein stationärer lepto-hadronischer Code unter Verwendung eines einfachen semianalytischen Frameworks entwickelt. Daher implementiert der Code neben den leptonischen Wechselwirkungen auch die relevanten hadronischen Wechselwirkungskanäle: Protonensynchrotronstrahlung, Photo-Meson-Wechselwirkungen, Proton-Proton-Wechselwirkungen und Paarkaskaden. Die Dissertation präsentiert die Ergebnisse derMAGIC- und Multiwellenlängen-Monitoring-Kampagne des Blazars 1ES 1959 + 650 im Jahr 2016. Im Jahr 2016 durchlief die Quelle eine äußerst aktive Phase und zeigte am 13. Juni, 14. Juni und 1. Juli 2016 drei bemerkenswert helle VHE-Gammastrahlenfackeln. Um die Breitbandspektren der Quelle während der bemerkenswerten Fackelaktivitäten zu untersuchen, wurden drei verschiedene theoretische Modelle übernommen: leptonisch, hadronisch und gemischt lepto-hadronisch. Sowohl das hadronische als auch das gemischte leptohadronische Modell ergaben während der intensiven Aktivitätsperiode Neutrinoflüsse, die unter der Empfindlichkeit der gegenwärtigen Generation von Neutrinoteleskopen liegen. Die Beobachtung eines hochenergetischen Neutrinos durch IceCube im räumlichen und zeitlichen Zusammentreffen mit einem aufflammenden Blazar mit dem Namen TXS 0506 + 056 ergab 2017 erstmals Hinweise auf Identifizierung einer extragalaktischen kosmischen Strahlenquelle. Die Modellierung der elektromagnetischen Daten und des vorhergesagten Neutrinoflusses impliziert, dass die Quelle tatsächlich ein potenzieller Neutrinostrahler und damit ein Beschleuniger für energiereiche kosmische Strahlen sein könnte.
The main aim of this thesis is to characterize extremely powerful sources that are most likely accelerating cosmic rays. Cosmic-ray sources are also believed to produce photons and neutrinos that act as direct tracers of their sources of origin. In this thesis VHE gamma-ray observations by the MAGIC telescopes were used to study the properties of blazars. To investigate the mechanisms giving rise to the broadband emission from blazars, a stationary lepto-hadronic code is developed using a simple semi-analytical framework. Hence along with the leptonic interactions, the code also implements the relevant hadronic interaction channels: proton synchrotron radiation, photo-meson interactions, proton-proton interactions and pair cascades. The thesis presents the results from theMAGIC and multi-wavelength monitoring campaign of the blazar 1ES 1959+650 during 2016. In 2016 the source underwent into an extremely active phase and exhibited three remarkably bright VHE gamma-ray flares on 13th June, 14th June and 1st July of 2016. On two of these nights, signs of rapid flux variability within sub-hour timescales was clearly resolved by the MAGIC observations. In order to investigate the broadband spectra of the source during the remarkable flaring activities, three different theoretical models were adopted: leptonic, hadronic and mixed lepto-hadronic. Both the hadronic and mixed leptohadronic models yielded neutrino fluxes during the intense activity period, that falls below the sensitivity of the current generation of neutrino telescopes. In 2017, the observation of a high-energy neutrino by IceCube in spatial and temporal coincidence with a flaring blazar named TXS 0506+056 yielded for the first time, hints towards identification of an extragalactic cosmic-ray source. The modelling of the electromagnetic data and the predicted neutrino flux implies that the source could indeed be a potential neutrino emitter and hence an accelerator of high-energy cosmic rays.

The subject of this thesis is the analysis and modeling of the active galactic nucleus PKS 1510-089. The aim is to present a thorough background of active galactic nuclei combined with the analysis and modeling of a specific active galactic nucleus. The results will then be  linked to previous research and theories about active galactic nuclei. The data used in the analysis were retrieved from the Fermi Gamma-ray Space Telescope. A light curve analysis that extended over 12 years provided knowledge about variability and presented four interesting flaring periods. The four periods underwent a spectral analysis, and the results showed that a log parabolic curvature could best describe all four periods. The last step before the modeling was to create spectral energy distributions for all four periods to retrieve spectral points from wavelengths other than those available from Fermi. Unfortunately, there were only sufficient data for one period. That period was later used in the modeling and resulted in a well-fitted external Compton model, which was compared, with relatively good results, with previous research.

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.

The recent discovery of a new population of Active Galactic Nuclei (AGN) – the Radio Loud Narrow Line Seyfert 1 (RL NLS1) galaxies – at g-ray energies by Fermi has prompted intense interest among researchers, as evidence mounts that these objects may in fact compose a new class of blazars. If RL NLS1s are indeed a new class of blazars, or at least analogous to them, then the detection of certain blazar-like properties would be expected. These properties include significant variability at all wavelengths across the electromagnetic spectrum and on timescales from minutes to years, significant & variable polarization in the radio and optical regimes, significant & variable high-energy emissions (especially in the g-ray regime), and a double-peaked structure of their spectral energy distributions. This dissertation seeks to characterize several of these properties for RL NLS1s as a class. These include the degree to which these objects are polarized and the variability of this property, the detection and characterization of these sources at g-ray energies with the Large Area Telescope aboard the Fermi spacecraft, and the degree to which these properties are interdependent. A photopolarimetric survey (the first of its kind for this class of objects) and g-ray monitoring program were conducted by the author in order to obtain the data necessary for this project. The measurements obtained via these observations are used to characterize this interesting class of objects with respect to a sample of blazars. In general, it was found that the polarizations of these objects fall between radio-quiet NLS1s and FSRQ-type blazars, and were most similar to HBL-type blazars. The 7 RL NLS1s in this sample that had been detected above a Test Statistic (TS) of 25 by Fermi/LAT were most similar to FSRQs, while 9 objects detected in the interval 9 < TS < 25 shared several properties with HBLs. Two RL NLS1s - J1443+4725 and J1644+2619 - are identified as high-confidence (TS > 25) g-ray sources for the first time, bringing the total number of members of this class firmly detected at g-ray energies to 8. The gamma-ray spectra of RL NLS1s are similar to FSRQs, though some have steeper spectra.