Multi-wavelength and multi-messenger observation of blazars' jets

Relativistic jets in Active Galactic Nuclei (AGN) are among the most powerful sources of radiation in the Universe. The radiation spans the entire electromagnetic spectrum, from radio wavelengths to $\gamma$-rays. Active Galactic Nuclei are divided into different categories, and the one this thesis will focus on is blazars. Blazars are radio-loud galaxies whose jet is aligned with the line of sight. Hence, the observed emission is strongly beamed and amplified due to relativistic Doppler effects. Moreover, their Spectral Energy Distribution (SED) is entirely dominated by non-thermal jet emission. Blazar's SEDs are characterized by two humps: the first one is due to synchrotron radiation from relativistic electrons accelerated in the jet, while the second hump has more than one explanation. The first one is that the particle population in the jet is dominated by electrons and positrons that are being accelerated. This is the leptonic scenario. The second one is that protons and ions are accelerated in the jets. The acceleration of hadrons involves the production of neutrinos. For this reason, blazars are considered a possible origin for astrophysical neutrinos. Hence, the second peak of the SED can be attributed to both hadrons and leptons. Because of these two interpretations, the study of the multi-wavelength emission of blazars, the modeling of their broad-band SED, and the detection of neutrinos are still fundamental pieces to understand particle acceleration in blazars' jets. This thesis aims to explore the acceleration of leptons and hadrons in blazars' jets, employing data from various facilities, several data analysis techniques, different statistical approaches, and distinct physically motivated spectral model simulations. Chapter \ref{intro} presents the different messengers of the Universe and introduces the basic properties of AGN and blazars, which are the main focus of this thesis. Chapter \ref{instruments} shows the state of the art of the instruments, facilities, and the techniques to detect radiation from very high energy $\gamma$-rays to the radio band and neutrinos. For each instrument whose data are employed in this thesis, the corresponding data analysis methodology is summarized. Chapter \ref{op313_chapter} presents the 15-year multi-wavelength analysis of the Flat Spectrum Radio Quasar (FSRQ) OP 313. This chapter investigates the intense flaring activity exhibited by this source between November 2023 and March 2024, from $\gamma$-rays to radio frequencies. In particular, the attention is mainly focused on the \textit{Fermi} Large Area Telescope (LAT) $\gamma$-ray and radio Very Long Baseline Array (VLBA) data analysis to explore the intrinsic relationship between high-energy emission and jet kinematics in the blazar's jet, and on the SED modeling considering only leptons being accelerated. One important focus is the behavior of the new radio $43\,\mathrm{GHz}$ components found in OP 313's jet to determine the time of ejection with good accuracy and see if there is a correlation between the ejection time and the $\gamma$-ray flaring activity. Chapter \ref{CTAO_simu} shows a sensitivity study for the Cherenkov Telescope Array Observatory (CTAO) experiment. In particular, four configurations of CTAO were compared to explore which of them has the best performance during the observation of flaring activity from a sample of eight FSRQs. For each FSRQ, the highest \textit{Fermi}-LAT $\gamma$-ray flare was considered. The simulations aim to quantify the ability of various CTAO configurations to detect flares and constrain spectral parameters. Then, the simulations assess whether adding CTAO data to the \textit{Fermi}-LAT ones can help to discriminate between different spectral models linked to the leptonic distribution in the jet and the curvature of the SEDs. Chapter \ref{neutrino} finally considers blazars' jets as the origin of the neutrinos observed by IceCube. In particular, it focuses on IcuCube real-time alerts, and on the \textit{Fermi}-Large Area Telescope analysis to identify possible $\gamma$-ray activity from known and new sources. With the increase in the number of real-time alerts, a growing sample of $\gamma$-ray sources that are coincident with neutrinos is emerging. Most of them are caused by large positional uncertainties in the arrival directions of the neutrinos. Therefore, detailed multi-wavelength studies are necessary to select possible source candidates. A brief summary with the main results is shown in Chapter \ref{conclu_chapt}