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Gamma-ray polarization in BL Lac objets with AMEGO

Authors: André Filipe Graça Neves

Supervisors: Sonia Antón Castillo

MSc thesis, Master in Physics (2025)

Abstract: Active Galactic Nuclei (AGN) are among the most extreme non-explosive sources in the observable universe, emitting across the entire electromagnetic spectrum. Among the various types of AGN, one of the most luminous and energetic classes is the blazar, which results from an astrophysical jet aligned with our line of sight at an angle of approximately 10-15◦, leading to relativistic beaming effects.The Spectral Energy Distribution (SED) of blazars is characterized by two distinct components: a low-energy and a high-energy component, typically ranging from soft X-rays to γ-rays. The low-energy component is widely attributed to synchrotron radiation, exhibiting Polarization Degree (PD) of up to ∼ 75%. However, the origin of the high-energy component remains uncertain, with competing models suggesting either leptonic or hadronic origins. A key difference between these models is that the leptonic scenario predicts a lower PD (∼ 30-40%), whereas hadronic models predict polarization levels comparable to, or even exceeding, those of the synchrotron component.Despite multiple astrophysical missions covering frequencies from radio to UV and even soft X-rays, the hard X-ray to MeV γ-ray range remains largely unexplored. Understanding blazars in this energy range is crucial for deciphering the underlying physical mechanisms. This is where missions such as NASA’s All-sky Medium Energy Gamma-ray Observatory (AMEGO) will play a pivotal role, providing valuable insights to complete the AGN puzzle.In this study, we analyze three groups of blazars that differ in their synchrotron peak positions, which influence the energy range of their high-energy components, from X-rays to GeV γ-rays. We first model each blazar using a Synchrotron Self-Compton (SSC) leptonic framework to fit its SED and predict the expected PD. We then compare these predictions with the Minimal Detectable Polarization (MDP) achievable by AMEGO in the 100 - 3000 keV range. Our results indicate that, with 30 days of continuous observation, AMEGO can achieve MDP values below the 30−40% PD expected from the leptonic model, making it a crucial step toward understanding blazar spectra.