Multi-wavelength polarimetric studies of relativistic jets in active galactic nuclei

This Thesis is focussed on the study of relativistic jets, commonly present in multiple astrophysical sites, from active galactic nuclei (AGN), to micro- quasars or gamma-ray bursts (GRBs). In the case of AGN, huge amounts of energy across the whole electromagnetic spectrum are released as a conse- quence of the accretion of material onto a supermassive back hole (SMBH) lurking at their centers. The accretion leads to the formation of a pair of very powerful and highly collimated jets extending far beyond the size of the host galaxy. We analyzed the correlation between the multi-wavelength emission and the radio jet in three powerful AGN, the radio galaxies 3C 120 and M 87, and the quasar CTA 102. The main goal of this Thesis is to obtain a better understanding of the jet dynamics and the role played by the magnetic field, and to determine what are the sites and mechanisms for the production of the γ-ray emission observed in these sources. We have performed multi-wavelength studies of the radio galaxy 3C 120 and the blazar CTA 102 during unprecedented γ-ray flares for both sources. The NASA satellite Fermi registered in September-October 2012 a bright γ-ray flare in CTA 102, and between December 2012 and October 2014 a prolonged γ-ray activity in the radio galaxy 3C 120. In both studies, to determine where the γ-ray emission is produced, the analysis of Fermi data has been compared with a detailed study of the morphology and evolution of the parsec scale jet through a series of extremely-high angular resolution Very Long Baseline Array (VLBA) images at 43 GHz from the Boston University blazar monitoring program, in which our research group is actively participating. In the case of 3C 120 we have also collected 15 GHz VLBA data from the MOJAVE monitoring program, extending our study of the radio jet from June 2008 to May 2014. For the study of CTA 102 a total of 80 VLBA images at 43 GHz have been analyzed and compared with observations across the whole electromagnetic spectrum between June 2004 and June 2014. These include observations at millimeter, near-infrared, and optical bands from observatories across the world, as well as ultraviolet and X-ray data from the Swift satellite. Our multi-wavelength observations of 3C 120 and CTA 102 have revealed very similar properties during γ-ray events in both sources, despite repre- senting very different classes of AGN. We found that in both sources the γ-ray flares are associated with the passage of knew superluminal components through the millimeter VLBI core, corresponding to the bright emission at the upstream end of the jet. But not all ejections lead to γ-ray emission; in fact bright superluminal components have been observed crossing the millimeter VLBI core without having a counterpart at γ-ray energies. We have found that in both sources γ-ray flares occurred only when the new component is moving in a direction closer to our line of sight. We located the γ-ray dissipation zone a short distance downstream of the radio core but outside of the broad line region, suggesting synchrotron self-Compton scattering as the probable mechanism for the γ-ray production. In addition, during the multi-wavelength outburst observed in CTA 102 the optical polarized emission displayed intra-day variability and a clear clockwise rotation of the polarization vectors, which we associated with the path followed by the component as it moves along helical magnetic field lines. These results have been reported in two papers published in high-impact peer-reviewed journals (Casadio et al., 2015a,b) that we present here in their original form. The location of γ-ray activity close to the radio core and far from the black hole in both 3C 120 and CTA 102 implies that we need a mechanism to reaccelerate particle in situ; this leads to consider the core as a recollimation shock. Hence, it is important to understand the physics of recollimation shocks and what could be the related observational evidences. For this purpose we have performed polarimetric studies of the jets in the radio galaxies 3C 120 and M 87 aimed to understand the nature of the peculiar emitting regions known as C80 and HST-1, located hundreds of parsecs from the core of the jet. In Agudo, Gómez, Casadio, et al. (2012) we analyzed polarimetric VLBA observations at 5, 8, 12, and 15 GHz of the jet in 3C 120 revealing that the stationary component C80 corresponds to the peak emission of a larger and more extended emission structure in arc, downstream of which other moving components are observed. The agreement between our observations and numerical simulations led us to conclude that the emitting region C80 corresponds in fact to a recollimation shock located ∼190 pc from the core of the jet. Interestingly, our observations of the peculiar structure HST-1 in the radio- galaxy M 87 revealed a similar structure to that observed in C80 of 3C 120; in Giroletti et al. (2012) we found that HST-1 corresponds to an extended emission structure in which new components appear to be released from the stationary upstream end of HST-1. It has been suggested that very high-energy emission has been originated in the HST-1 region. However, our new VLBA and JVLA observations of M 87 confirmed that HST-1 was in a low energy state between 2011 and 2013, ruling out its implication in the high energy flare in M 87 in March 2012, as we pointed out also in Hada et al. (2014).