Large and variable polarization is an inherent property of a majority of blazars. Systematic rotations of the polarization vector have been claimed for several blazars. In some cases, however, the reality of these rotations may be questionable. We suggest an additional method for the verification of the reality of polarization position angle rotations based on the correlation of the normalized Stokes parameters.

Στόχος της παρούσης εργασίας είναι η διερεύνηση των μεταβολών της ροής της οπτικής ακτινοβολίας στα φίλτρα B, V, R στη διάρκεια μιας νύχτας του ενεργού γαλαξία S5 1803 + 784 με ανάλυση παρατηρήσεων που ελήφθησαν με το τηλεσκόπιο των1.3m του αστεροσκοπείου του Πανεπιστημίου Κρήτης (από τον Δρ. Παν. Μπούμη) με τη μέθοδο της συγκριτικής φωτομετρίας. Στο πρώτο κεφάλαιο περιγράφονται τα γενικά χαρακτηριστικά των κανονικών γαλαξιών και του δικού μας γαλαξία, ενώ στο δεύτερο τα γενικά χαρακτηριστικά των ενεργών γαλαξιών ή ενεργών γαλαξιακών πυρήνων (AGN) - επειδή τα έντονα ενεργητικά φαινόμενα επικεντρώνονται στην κεντρική περιοχή του πυρήνα. Αναφέρεται η διάκρισή τους σε κατηγορίες με βάση τα παρατηρησιακά χαρακτηριστικά και εξετάζονται οι βασικές ιδέες που οδηγούν στο επικρατέστερο μοντέλο ενοποίησης, σχετικά με την ενεργειακή μηχανή τους καθώς και η εξέλιξη και τροποποίηση του με βάση τα νέα πειραματικά δεδομένα. Το τρίτο κεφάλαιο επικεντρώνεται στα ιδιαίτερα χαρακτηριστικά των blazars στα οποία ανήκει ο S5 1803 + 784. Στο τέταρτο κεφάλαιο περιγράφονται συνοπτικά οι βασικές αρχές της φωτομετρίας αλλά και οι βασικές παράμετροι μιας CCD κάμερας. Στο τέταρτο κεφάλαιο γίνεται μια βιβλιογραφική παρουσίαση του S5 1803 +784 σε όλα τα μέρη του φάσματος και περιγράφονται αναλυτικά οι παρατηρήσεις, τα αρχικά στάδια επεξεργασίας των αστρονομικών εικόνων και τα αναλυτικά στάδια της φωτομετρίας του γαλαξία με τις απαραίτητες διορθώσεις. Τέλος στο έκτο κεφάλαιο κατασκευάζονται οι καμπύλες μεταβολής της ροής και σχολιάζονται τα αποτελέσματα Ακολουθεί στο παράρτημα, η περιγραφή των προγραμμάτων και των διαδικασιών που χρησιμοποιήθηκαν τόσο στην ανάλυση όσο και στην φωτομετρία μέσω των αστρονομικών πακέτων IRAF και MIDAS , ως μικρός οδηγός για τους επόμενους ενδιαφερόμενους. / The main aim of this dissertation entitled “Variabillity in the optical band of blazar S51803+784” is the investigation of flux microvariability through observations carried out with the 1.3 m , Ritchey-Chretien telescope at Skinakas Observatory in Crete, Greece (by Associate Researcher P. Boomis of the Institute of Astronomy and Astrophysics of the National Observatory in Athens) in 2001 under the supervision of Lecturer Mrs E-P. Christopoulou (Physics Dept. University of Patras, Greece). This work consists of six relatively distinct chapters. The first chapter summarizes the main characteristics of normal galaxies whereas the second one describes the various classes of Active Galactic Nuclei which emerge from multiwavelength studies and the unified model proposed to explain the energy source behind their taxonomy. The last section of this part focuses on the observational evidence of supermassive black holes. The third chapter focuses on the special characteristics of the class named Blazars, the proposed theoretical models for the formation of jets and the observed microvariability thought the EM spectrum. The fourth chapter describes the main philosophy of photometry, the comparison of differential and absolute photometry and the selection of photometric standard stars. The last part briefly presents the charge-coupled devices (CCDs) which are used to record astronomical images and their main properties. The fifth chapter deals with the current observations of the Blazar S51803+784 and the standard image processing (bias substraction and flat fielding using twilight –sky exposures) applied to all frames before the performance of aperture photometry techniques which is described analytically in the rest part. The topics covered include methods for image center determination, routines for estimating the sky background and the techniques of both aperture and point-spread function fitting photometry. Procedures for transforming instrumental magnitudes to those on a standard system are also discussed. The results based on the data reduction of the photometric observations of the object are presented in the last chapter together with the discussion and conclusions. Finally in the Appendix there is a mini-guide for the beginner in order to be familiar with the format of the astronomical files, and the various astronomical packages and procedures being used for the above reduction.

L'étude des mécanismes d'émission et de variabilité des blazars du TeV est l'objet d'intenses recherches depuis de nombreuses années. Les modèles courants d'émission une zone homogène utilisés posent de nombreux problèmes, notamment à cause des grands facteurs de Lorentz qu'ils requièrent et qui sont en contradiction avec les contraintes dérivées des observations radio des jets. Dans cette thèse je décris une approche multizone inhomogène dépendante du temps, dans le cadre du modèle du two-flow. Je calcule l'émission d'un jet complet, dans lequel se propagent des paires électron-positron relativistes distribuées en pileup. L'évolution et l'émission du plasma de paires est calculée en tenant compte d'un terme de chauffage turbulent, du refroidissement radiatif, ainsi que d'un terme de production de paires par photo-annihilation. Appliqué à PKS 2155−304, ce modèle permet de reproduire le spectre complet, ainsi que la variabilité simultanée multi longueur d'onde, avec un faible facteur de Lorentz. La variabilité est expliquée par l'instabilité de la création de paires dans le jet. La valeur du facteur de Lorentz est néanmoins trop élevée pour être en accord avec les observations radio et la statistique de détection de ces objet. Je montre à la fin de ma thèse, comment, en tenant compte de l'ouverture géométrique des jets, peut on réconcilier de grands facteur de Lorentz avec l'absence de mouvement superluminique observé en radio, ainsi que la relative abondance de ce type de source.

Recent research has lead to the possible discovery of a new class of gamma-ray emitting ac- tive galactic nuclei (AGN). These objects – the very radio-loud narrow-line Seyfert 1 galaxies (NLSy1s)– demonstrate observational features suggesting that they are similar to blazars. One of the key characteristics of blazars is the presence of high-amplitude optical microvariability. While this phenomenon has been investigated in individual objects, no study of the intra-night variability of radio-loud NLSy1s as a class has previously been available. This dissertation presents a sys- tematic search for optical variability in a sample of 33 radio-loud NLSy1s. It was found that 26 objects demonstrated microvariations. However, only 9 objects did so with duty cycles comparable to blazars, and only 7 of these 9 objects — J0706+3901, J0849+5108, J0948+0022, J1246+0238, PKS 1502+036, J1644+2619, and IRAS 20181-2244 — demonstrated microvariability at compa- rable amplitudes. Two objects stand out as exceptional sources. J0849+5108 was found to have a duty cycle of ~90% and was observed to undergo an enormous 4-magnitude optical flare in a two-month time span. The object has not been reported to have undergone such an event since 1975. The second object, J0948+0022, is the class prototype. High cadence data indicates that J0948+0022 has a remarkably rapid doubling time scale of ~40 minutes, and it was seen to vary by over 0.9 magnitudes within an individual night. Attempts to correlate microvariability to radio loudness, gamma-ray loudness, and other parameters were largely unsuccessful. However, it was found that only radio-loud NLSy1s that were detected at gamma-ray energies demonstrated microvariability at blazar-like duty cycles. Additionally, an analysis of the frequency of microvariations at various amplitudes suggests that the sample of radio-loud NLSy1s presented in this study share a parent population identical to low energy peaked BL Lac-type (LBL) blazars. This is in agreement with the work of astronomers such as Abdo et al. 2009, who have created spectral energy distributions for a few radio-loud NLSy1s and found them to resemble those of LBLs. Blazar-like variability was found in multiple objects with radio loudnesses of log(R) < 2, suggesting that even moderately radio-loud NLSy1s may be blazar-like objects.

Blazars, a subclass of active galactic nuclei with powerful relativistic plasma jets, are among the most luminous and violently variable objects in the universe. They emit radiation across the entire electromagnetic spectrum, and often change in brightness over the course of hours or days. Different emission mechanisms are necessary in order to explain the observed flux in different frequency ranges. In the ultraviolet-optical- infrared regime these include components that arise from: 1) polarized synchrotron radiation emanating from a powerful parsec-scale jet flowing from near the central accreting black hole, 2) a multi-temperature accretion disk emitting thermal radia- tion, and 3) an optically thick dusty torus located several parsecs from the central engine that absorbs and re-emits, at infrared wavelengths, radiation originating in the accretion disk. The goal of this study is to determine the relative importance of these spectral components in the spectra of blazars. I use data from the Spitzer Space Telescope in order to search for the presence of the dusty torus surrounding four blazars, as well as to determine its luminosity and temperature. In two of the observed sources, 1222+216 and CTA102, I determine that the torus can be modeled as a 1200 K blackbody emitting at nearly 10 46 erg s −1 . Furthermore, I determine the relative variability of the accretion disk of a sample of blazars by using spec- tropolarimetry observations to separate the optical-UV spectrum into a polarized viiicomponent, consisting of radiation described by a power-law F ν ∝ ν −α , and an ac- cretion disk which consists of a thin disk described by the power-law F disk ∝ ν 1/3 plus a hot-spot of variable temperature. The spectra of several blazars are explained by a version of this model in which the thin disk component is held constant, while the blackbody varies on timescales of approximately years resulting with a flux of the blackbody component comparable to the power-law disk component. I find that variations in the emission from the hot-spot occurs approximately within 100 days of γ-ray variations.

Active Galactic Nuclei (AGN) are some of the most extreme objects in the universe. They output copious amounts of energy spanning the entire electromagnetic spectrum. There are many different subclasses of AGN depending on your viewing angle. Blazars, viewing down the relativistic jet, are the most variable class of AGN known. They exhibit extreme variability in all wavelengths on timescales as short as minutes. In this thesis I will consider the extreme faintness of 3C279 with respect to the long-term light curve as well as recent observations of microvariability. I am able to confirm small amplitude events using simultaneous observations from two telescopes and cross correlation analysis. Transitory quasi-periodic oscillations are observed during two of the nights with confirmed microvariability.

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.

Active Galactic Nuclei (AGN) are cores of distant protogalaxies, with a supermassive blackhole at the center surrounded by an accretion disk, and bipolar jets. Blazars, a subset of AGN, have their jets aligned with our line of sight. Emission from blazars is highly variable on all timescales and frequencies. Microvariability refers to rapid continuum variations that arise within the jet. Bhatta et al. (2013) suggest a modified Kirk, Rieger, \& Mastichiadis (1998) model (KRM) to explain microvariability. The KRM model assumes that when shock waves passes though the jet, each turbulent cell encountered produces a pulse of emission characterized by cell size, local density enhancement, and magnetic field strength. NASA's \kepler\ has monitored optical emission from four AGN. We use the modified KRM model to analyze micro-variations in these \kepler\ data. The distribution of cell sizes computed from these data is consistent with the distribution expected from a turbulent plasma.

<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>

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.