The TeV AGN Portfolio: extending Fermi LAT analysis into the CTA realm

The extragalactic γ-ray sky is completely dominated by active galax- ies, where by active we mean that a significant fraction of the emitted energy is not due to the standard components of a galaxy: stars, gases and interstellar dust. Every detected active galaxy seems to be powered by a compact region at their center; explaining why active galaxies are often referred as Active Galactic Nuclei (AGNs). About 1% of all galaxies are AGNs, believed to be fueled by the accretion onto a supermassive black hole, the central engine of the active galaxy. In addition, about 10% of AGNs display powerful jets of particles and radiation. The current model of AGNs is highly anisotropic and many of the observational characteristics of AGNs can be attributed to the way we are observing it and, in particular, to the orientation of the relativistic jets with respect to the observer. Among AGNs, blazars, which host a jet oriented at a small angle to the line of sight, are of particular interest for γ-ray astrophysics. The emission from these objects is dominated by relativistic beaming effects, which dramatically boosts the observed photon energies and luminosity, the reason why we expect that the observation of blazars at γ-ray energies should be the most fruitful. To confirm our guess, after the launch of the Fermi Gamma-ray Space Telescope, bearing on-board the Large Area Telescope (LAT), which provides virtually continuous observation of blazars between 20 MeV and 300 GeV, many new discoveries refined the current modeling of blazars, by providing useful insights into jets and other AGN features. On the other hand, at the same energies, other observations found puzzling results, bewildering astronomers and astrophysicists. In addition to the LAT, Imaging Atmospheric Cherenkov (IAC) telescopes (namely MAGIC, HESS and VERITAS) provided a good coverage at even higher energies (typically above 30 GeV) and the benefit of simultaneous observations was apparent just after the first broadband paper about PKS 2155−304 (Aharonian et al., 2009). More insights should be gained when the Cherenkov Telescope Array (CTA) will become operational, as it will cover an extended energy window with respect to operating IAC telescopes and will reduce the sensitivity threshold. In addition, CTA will have a huge energy overlap with the LAT, allowing for the first time a reliable way to correlate data obtained by the two detectors. In this Thesis, we present in-depth studies of LAT γ-ray observations of blazars, complemented by multifrequencies observations which are an essential tool to model their behavior. On one hand, we will discuss the characterization of a TeV blazars sample that were simultaneously observed both by Fermi and MAGIC instruments. The joint observations and the ideal coverage provided by the synergy of the two instruments naturally motivates the extrapolation of Fermi spectra to MAGIC energies, with the aim, in the near future, to extend this effort to CTA realm. On the other hand, we will discuss a flux-limited sample of bright blazars detected by Fermi in the first 3.5 years of operations. These objects, displaying extreme outbursts, make up less than 10% of the sources detected by Fermi in its second source catalog. We discuss the characteristics of the sample with respect to the entire catalog of AGNs detected by Fermi and adding some considerations with respect to previous γ-ray observation carried out by EGRET. At the end of this work, we will then focus on one of these objects, that met particular attention for being a gravitationally lensed system, PKS 1830−211.