• Multi-wavelength
• blazar
• AGN
• variability

Active Galactic Nuclei (AGN) are extragalactic objects producing huge amounts of energy (sometimes exhibiting apparent luminosities $10^4$ times greater than those of typical galaxies), spanning a huge range of frequencies (from radio to gamma-rays), in very small volumes ($\ll 1 \, pc^3$, where $1\, pc \simeq 3.086 \times 10^{16}\, m$) and flux variability. Blazars are a specific family of AGN, which exhibit extreme flux variability at all frequencies, mainly in gamma-ray band, both in amplitude and timescales. This class of objects is defined as radio-loud because they are characterized by a ratio between flux at $5\, Ghz$ and flux in optical B band ($\sim 400\, -\,500 \, nm$) bigger than 10. They present a compact core in which a supermassive black hole ($M_{BH} \ge 10^6M_\odot$ ) is thought to reside, accreting matter from the surrounding circumnuclear interstellar medium. Blazars feature narrow jets of relativistic particles perpendicular to their accretion disks planes and oriented at small angle to the observer. Blazars are subdivided into two main classes, basically distinguished according to their optical spectra: Flat Spectrum Radio Quasars (FSRQ) exhibit strong emission lines whereas BL Lac objects
(so name after their prototype BL Lacertae) are characterized by weakness of even absence
of emission lines in their optical spectra.
The subject of my thesis is the study of multiwavelength variability of gamma-ray-loud blazars, with a main focus on their gamma-ray emission, corresponding to the frequency band where most blazars have their emission peak. This study was performed by applying a variety of statistical methods to 20 among the brightest gamma-ray blazars selected in the third Fermi-LAT catalog to bring out relevant features for physical modelling about the emission mechanisms which produces the light curves observed.
The work is divided between phenomenological presentation and interpretation. In the first part, The work is divided between phenomenological presentation and interpretation. In the first part, I present the reduction and analysis of gamma-ray data for the blazars selected among the brightest in the 3rd Fermi Catalog. I show the gamma-ray light curves and describe the procedures I used to validate my analysis by comparison with the official products of the Fermi-LAT team. This first part includes also a multiwavelength comparison with radio, optical and X-ray databases. The second part presents an interpretation of the results based on a statistical approach where I developed algorithms or used existing tools for variability characterization. This part includes an estimate of (quasi-)periodicity based on commonly used estimators (Lomb-Scargle) and an analysis of possible biases and limitations resulting from its application.