We present multi-wavelength studies of the radio galaxy 3C 120 and the blazar CTA 102 during unprecedented gamma-ray flares for both sources. In both studies the analysis of gamma-ray data has been compared with a series of 43 GHz VLBA images from the VLBA-BU-BLAZAR program, providing the necessary spatial resolution to probe the parsec scale jet evolution during the high energy events. To extend the radio dataset for 3C 120 we also used 15 GHz VLBA data from the MOJAVE sample. These two objects which represent very different classes of AGN, have similar properties during the gamma-ray events. The gamma-ray flares are associated with the passage of a new superluminal component through the mm VLBI core, but not all ejections of new components lead to gamma-ray events. In both sources gamma-ray events occurred only when the new components are moving in a direction closer to our line of sight. We locate the g-ray dissipation zone a short distance from the radio core but outside of the broad line region, suggesting synchrotron self-Compton scattering as the probable mechanism for the gamma-ray production.

Context. Over the past few years, on several occasions, large, continuous rotations of the electric vector position angle (EVPA) of linearly polarized optical emission from blazars have been reported. These events are often coincident with high energy gamma-ray flares and they have attracted considerable attention, since they could allow us to probe the magnetic field structure in the gamma-ray emitting region of the jet. The flat-spectrum radio quasar 3C 279 is one of the most prominent examples showing this behaviour. Aims. Our goal is to study the observed EVPA rotations and to distinguish between a stochastic and a deterministic origin of the polarization variability. Methods. We have combined multiple data sets of R-band photometry and optical polarimetry measurements of 3C 279, yielding exceptionally well-sampled flux density and polarization curves that cover a period of 2008-2012. Several large EVPA rotations are identified in the data. We introduce a quantitative measure for the EVPA curve smoothness, which is then used to test a set of simple random walk polarization variability models against the data. Results. 3C 279 shows different polarization variation characteristics during an optical low-flux state and a flaring state. The polarization variation during the flaring state, especially the smooth similar to 360 degrees rotation of the EVPA in mid-2011, is not consistent with the tested stochastic processes. Conclusions. We conclude that, during the two different optical flux states, two different processes govern polarization variation, which is possibly a stochastic process during the low-brightness state and a deterministic process during the flaring activity.

Quelques centaines de galaxies dans l'Univers présentent des structures extraordinaires, d'une taille parfois gigantesque (jusqu'à plusieurs fois le rayon de la galaxie parente). Ces structures, visibles essentiellement dans le domaine des longueurs d'onde radio, donc appelées radiosources, sont caractérisées dans les deux tiers des cas par des ``jets'' extrêmement bien collimatés. Ils proviennent du noyau de ces galaxies (dites actives) et déversent leur énergie dans des régions diffuses appelées ``lobes''. L'étude des jets extragalactiques a une portée très large puiqu'ils constituent des sondes naturelles à la fois du coeur des galaxies actives et des milieux interstellaire et intergalactique. Ils sont sans aucun doute le signe d'étapes violentes dans l'évolution des galaxies (interaction, coalescence, trous noirs,...).<br /><br />La plus grosse partie de mes recherches s'est attachée à déterminer l'origine du rayonnement dans les jets extragalactiques. J'ai abordé cette étude sous de multiples facettes (observations optiques et radio, modélisation et théorie) parce que je suis convaincu que seule une vision globale de ces objets autorise une approche suffisamment précise de leur physique. Prises individuellement, les différentes composantes des radiosources (lobes, jets, jets VLBI, noyaux actifs,...) ne sont que très peu contraintes par les observations. La dissymétrie (ou unilatéralité) des jets peut être considérée comme le thème fédérateur de mes travaux. Expliquer pourquoi un jet rayonne alors que son opposé n'est pas visible est à mon avis un objectif ultime: lorsque nous l'aurons atteint, nous aurons alors vraiment compris la physique des jets extragalactiques. <br /><br />Ce mémoire s'articule autour de l'idée centrale que les jets sont non-relativistes et qu'une population de particules ultra-relativistes (électrons-positrons) supplémentaires explique le rayonnement synchrotron. C'est à partir d'une synthèse d'observations et de différents travaux théoriques que j'ai construit un modèle expliquant l'unilatéralité des jets à grande échelle. Il reste encore qualitatif, mais il englobe plus généralement les propriétés des radiosources. L'interprétation usuelle de la dissymétrie des jets fait appel uniquement à un effet Doppler relativiste. J'ai pu démontrer au contraire que ces dissymétries sont essentiellement intrinsèques, c'est-à-dire que les propriétés du rayonnement d'un jet dépendent des propriétés locales de son environnement. Toutes mes recherches se situent dans ce cadre et ont pour objectif d'appuyer le modèle par des observations et de le quantifier par des approches théoriques.<br /><br />Un complément indispensable à l'étude des radiosources à grande <br />échelle, est la compréhension de ce qui se passe à plus petite<br />échelle, jusque vers la zone de formation des jets. Une partie de <br />mes travaux en cours a pour objectif d'établir un lien entre les calculs théoriques de la machine infernale centrale et les structures à grandes échelles, par l'intermédiaire d'observations et de modélisations de la région la plus petite observable par des techniques d'interférométrie intercontinentale. Mon espoir est d'aboutir à une modélisation globale et entièrement cohérente à toutes les échelles d'une radiosource extragalactique.

In this thesis I use X-ray observations to study the cores and extended structures of radio-loud AGN, to determine their structure, accretion properties and the impact they have on their surroundings. I use new Chandra data and archival XMM-Newton observations ofMarkarian 6 to look for evidence of emission from shocked gas around the external radio bubbles, using spatially resolved regions in Chandra and spectral analysis of the XMM data. The results show that the bubbles in Mrk 6 are indeed driving a shock into the halo of the host galaxy, with a Mach number of 3.9. I also find that the spectrum of the AGN has a variable absorbing column, which changes from 8 × 1021 atoms cm−2 to 3 × 1023 atoms cm−2 on short timescales (2-6 years). This is probably caused by a clump of gas close to the central AGN, passing in front of us at the moment of the observation. Using new and archival Chandra observations of the Circinus galaxy, I match them to pre-existing radio, infrared and optical data to study the kpc-scale emission. As for Mrk 6, I find that the radio bubbles in Circinus are driving a shock into the interstellar medium of the host galaxy, with Mach numbers M 2.7–3.6 and M 2.8–5.3 for the W and E shells respectively. Comparing the results with those we previously obtained for Centaurus A, NGC 3801 and Mrk 6, I show that the total energy in the lobes (thermal+kinetic) scales approximately with the radio power of the parent AGN. The spatial coincidence between the X-ray and edge-brightened radio emission in Circinus resembles the morphology of some SNR shocks, a parallel that has been expected for AGN, but has never been observed before. I investigate what underlying mechanisms both types of systems may have in common, arguing that, in Circinus, the edge-brightening in the shells may be accounted for by a B field enhancement caused by shock compression, but do not preclude some local particle acceleration. I also carry out a systematic study of the X-ray emission from the cores in the 0.02 < z < 0.7 2Jy sample, using Chandra and XMM-Newton observations. I combine the results with the mid-IR, optical emission line and radio luminosities, and compare them with those of the 3CRR sources, to show that the low-excitation objects in our sample show all the signs of radiatively inefficient accretion. I study the effect of the jet-related emission on the various luminosities, confirming that it is the main source of soft X-ray emission for our sources. I also find strong correlations between the accretion-related luminosities, and identify several sources whose optical classification is incompatible with their accretion properties. I derive the bolometric and jet kinetic luminosities for the sample and find a difference in the total Eddington rate between the low and high-excitation populations, with the former peaking at 1 per cent and the latter at 20 per cent Eddington. There is, however, an overlap between the two, indicating that a simple Eddington switch may not be possible. The apparent independence of jet kinetic power and radiative luminosity in the highexcitation population in our plots allows us to test the hypothesis in which jet production and radiatively efficient accretion are in fact independent processes that can coexist in high-excitation objects.

We investigate the multiwavelength behaviour of the high-energy peaked BL Lac object (HBL) Mrk 421 at redshift z = 0.031 in the period 2007-2015. We use optical photometric, spectroscopic, and polarimetric data and near-infrared data obtained by 35 observatories participating in the GLAST-AGILE Support Program (GASP) of the Whole Earth Blazar Telescope (WEBT), as well as by the Steward Observatory Support of the Fermi Mission. We also employ high-energy data from the Swift (UV and X-rays) satellite to study correlations among emission in different bands.

We present Chandra and XMM-Newton X-ray, Very Large Array (VLA) radio, and optical observations of three candidate compact steep spectrum (CSS) radio galaxies. CSS sources are of a galactic scale and are presumably driving a shock through the interstellar medium (ISM) of their host galaxy. B3 1445+410 is a low-excitation emission line CSS radio galaxy with possibly a hybrid Fanaroff-Riley FRI/II (or fat double) radio morphology. The Chandra observations reveal a point-like source that is well fit with a power law consistent with the emission from a Doppler boosted core. 3C 268.3 is a CSS broad-line radio galaxy (BLRG) whose Chandra data are consistent spatially with a point source centered on the nucleus and spectrally with a double power-law model. PKS B1017-325 is a low-excitation emission line radio galaxy with a bent double radio morphology. While from our new spectroscopic redshift, PKS B1017-325 falls outside the formal definition of a CSS, the XMM-Newton observations are consistent with ISM emission with either a contribution from hot shocked gas or non-thermal jet emission. We compile selected radio and X-ray properties of the nine bona fide CSS radio galaxies with X-ray detections so far. We find that two out of the nine show X-ray spectroscopic evidence for hot shocked gas. We note that the counts in the sources are low and that the properties of the two sources with evidence for hot shocked gas are typical of the other CSS radio galaxies. We suggest that hot shocked gas may be typical of CSS radio galaxies due to their propagation through their host galaxies.

We report on long-term multiwavelength monitoring of blazar Mrk 421 by the GLAST-AGILE Support Program of the Whole Earth Blazar Telescope (GASP-WEBT) collaboration and Steward Observatory, and by the Swift and Fermi satellites. We study the source behaviour in the period 2007-2015, characterized by several extreme flares. The ratio between the optical, X-ray and gamma-ray fluxes is very variable. The gamma-ray flux variations show a fair correlation with the optical ones starting from 2012. We analyse spectropolarimetric data and find wavelength-dependence of the polarization degree (P), which is compatible with the presence of the host galaxy, and no wavelength dependence of the electric vector polarization angle (EVPA). Optical polarimetry shows a lack of simple correlation between P and flux and wide rotations of the EVPA. We build broad-band spectral energy distributions with simultaneous near-infrared and optical data from the GASP-WEBT and ultraviolet and X-ray data from the Swift satellite. They show strong variability in both flux and X-ray spectral shape and suggest a shift of the synchrotron peak up to a factor of similar to 50 in frequency. The interpretation of the flux and spectral variability is compatible with jet models including at least two emitting regions that can change their orientation with respect to the line of sight.

Blazars are active galactic nuclei characterized by ultra-luminous broad-band, non-thermal radio to gamma-ray continuum radiation, and by irregular, rapid flux variability across wavebands. They are divided into two subclasses: BL Lac objects and flat spectrum radio quasars (FSRQs). A primary method employed to probe our understanding of these objects is to study their spectral energy distributions (SEDs). Until recently, however, studies of blazar SEDs have been hindered by an insufficient number of simultaneous observations across the spectrum, a critical deficiency with such variable objects. In this thesis, I discuss the data accumulated by an international, long-term, 35-blazar monitoring program led by the Boston University blazar group. By systematically processing these data, I produce SEDs using measurements obtained on average within nine hours per epoch. Sufficient measurements exist within the data set to study the sources in differing states of activity. I propose a definition of quiescent and active states, and extract measurements for epochs during which the sources were either quiescent or active in the gamma-ray regime. For these epochs, I measure the spectral slopes and statistically analyze the relationships between slopes at the different frequency regimes. While the subclasses exhibit some distinct characteristics in the optical and gamma-ray indices when quiescent, these distinctions are significantly less pronounced when the objects are active. The spectral indices for the FSRQs steepen when active in the optical, flatten in the gamma-ray, and remain flat and stable in the X-ray. Generally, BL Lacs exhibit less pronounced changes between states than do the FSRQs.

Context.M 87 is a giant elliptical galaxy located in the centre of the Virgo cluster, which harbours a supermassive black hole of mass 6.4 × 109 M⊙, whose activity is responsible for the extended (80 kpc) radio lobes that surround the galaxy. The energy generated by matter falling onto the central black hole is ejected and transferred to the intra-cluster medium via a relativistic jet and morphologically complex systems of buoyant bubbles, which rise towards the edges of the extended halo. Aims. To place constraints on past activity cycles of the active nucleus, images of M 87 were produced at low radio frequencies never explored before at these high spatial resolution and dynamic range. To disentangle different synchrotron models and place constraints on source magnetic field, age and energetics, we also performed a detailed spectral analysis of M 87 extended radio-halo. Methods. We present the first observations made with the new Low-Frequency Array (LOFAR) of M 87 at frequencies down to 20 MHz. Three observations were conducted, at 15−30 MHz, 30−77 MHz and 116−162 MHz. We used these observations together with archival data to produce a low-frequency spectral index map and to perform a spectral analysis in the wide frequency range 30 MHz–10 GHz. Results. We do not find any sign of new extended emissions; on the contrary the source appears well confined by the high pressure of the intra-cluster medium. A continuous injection of relativistic electrons is the model that best fits our data, and provides a scenario in which the lobes are still supplied by fresh relativistic particles from the active galactic nuclei. We suggest that the discrepancy between the low-frequency radio-spectral slope in the core and in the halo implies a strong adiabatic expansion of the plasma as soon as it leaves the core area. The extended halo has an equipartition magnetic field strength of ≃10 μG, which increases to ≃13 μG in the zones where the particle flows are more active. The continuous injection model for synchrotron ageing provides an age for the halo of ≃40 Myr, which in turn provides a jet kinetic power of 6−10 × 1044 erg s-1.

>Magister Scientiae - MSc / I combine 610 MHz Giant Metrewave Radio Telescope (GMRT) data, 1.4 GHz Very Large Array (VLA) data and 1.4 GHz Westerbork Synthesis Radio Telescope (WSRT) observations, encompassing a ∼ 4 square degree field (sq. deg. field) centred on the verification strip of the Spitzer First Look Survey (FLS) field (RA = 17h18m00s, Dec = 59◦30′30′′), to study radio sources down to fluxes of about 0.1 mJy. The results of an analysis of a sample of 107 multi-component radio sources obtained by cross-correlating the VLA and GMRT catalogues are shown. The spectral index analysis shows that the majority of multi-component sources are steep-spectrum sources. Nevertheless the spread in the spectral distribution is wide, with a significant number of ultrasteep,flat or inverted sources. By cross-correlating 107 multi-component radio sources with the optical catalogues of Marleau et al. (2007) and Papovich et al. (2006), 23 objects were identified and spectroscopically classified as galaxies. Some of them are classified as star-forming or star-burst galaxies, perhaps indicating that AGN and starformation activity are ongoing in the same galaxy. The measured redshifts span the range 0 < z < 1.8 and peak at z ∼ 0.2. According to their radio power (P), 6 of the identified objects are in the range of FR II sources (P1.4GHz > 1024.5W/Hz) while 17 are in the range of FR I sources (P1.4GHz < 1024.5W/Hz). Most of the sources having P1.4GHz < 1024.5W/Hz are compact and few are extended and peculiar, while all sources in the range of P1.4GHz > 1024.5W/Hz are extended. Further optical followup is recommended to allow a more complete census of the sub-mJy population and more information on AGN feedback from such sources.