Titolo della tesi: Variability in blazars from the optical band to γ-rays: a multiwavelength perspective
Active galactic nuclei (AGN) are processes powered by accreting supermassive black holes (SMBHs) at the center of galaxies. Among them, blazars are a subclass in which a powerful relativistic jet is pointing directly toward us; being detected in increasingly large numbers at all wavelengths, and in particular at energies from the X-rays to the very-high energy (VHE) γ-rays by instruments such as the Swift and Fermi-LAT satellites and the imaging atmospheric Cherenkov telescopes (IACTs), such AGN are playing an important and growing role in today’s multi-wavelength (MWL) and multi-messenger (MM) astrophysics. In addition, the only sources of high-energy extragalactic neutrinos identified so far (TXS 0506+056 and PKS 1424−418) are also blazars. Studies of blazars allow astronomers to investigate a variety of topics related to the physics of such sources, including the disk-to-jet relation, the jet physics, the AGN feedback, the evolution of the radio-emitting counterparts, the neutrino and cosmic-ray emission processes. In particular, the exact location where the observed γ-ray emission occurs is still a matter of debate, and a firm localization of it would allow to understand the formation and the physical properties of the jets. In this context, the investigation of the multi-band variability of blazars can provide information on the distances between the various emitting regions and on the physical processes at work in the components of an AGN central engine (disk, corona, jets). In this thesis work, I focused on the study of the emission from the optical to the VHE γ-rays of three blazars – namely, PG 1553+113, PKS 1413+135 and TXS 0506+056 – that exhibit peculiar morphology characteristics and variability of their light emission, with the goal of improving the current knowledge on their physics and internal structure. In detail, PG 1553+113 shows a periodic modulation of its MWL light curves (LCs) with the indication of different periodicities at different energies; PKS 1413+135 is an atypical blazar likely hosted in a non-elliptical galaxy; and the aforementioned TXS 0506+056 is one of the only two cases in which a concomitant MM emission has been detected. In my investigation, I reduced the data of such targets ranging from the optical band to the VHE γ-rays, in order to construct their MWL LCs and VHE spectral energy distributions (SEDs) and analyze them with robust methodologies that are suited for the study of time series. In doing so, I have significantly detected for the first time a variability period TX ∼ 1.4 years in the PG 1553+113 X-ray LC that is ∼35% lower than the already established γ-ray periodicity (Tγ ∼ 2.2 years), and revealed the VHE emission of PKS 1413+135 above 100 GeV with a significance level of >6σ. Such achievements are highly remarkable in the study of blazars and in general of jetted AGN, since they allow to make a step forward both in the precise modelling of the central engine structure of sources with periodic variability patterns and in increasing the number of known objects that show detectable fluxes at the most extreme energies.