Particle acceleration and emission in high-energy BL Lacs

This thesis has been conducted for my PhD in Physics at the Università degli Studi di Genova, Scuola di Scienze Matematiche, Fisiche e Naturali, in collaboration with the Istituto Nazionale di Astrofisica (INAF) - Osservatorio di Brera e Merate, under the supervision of Dr. Fabrizio Tavecchio and Dr. Silvano Tosi. This thesis focuses on two open questions in high-energy blazar astrophysics that recently have attracted the interest of the scientific community: the modeling of the extreme TeV BL Lacs and the multiwavelength polarimetry of high-energy BL Lacs. In the first three chapters, the thesis explores the basics of relativistic jets in AGNs. Chapter 1 introduces AGNs, focusing on blazars and introducing the two open questions tackled by the thesis. Chapter 2 examines the main nonthermal mechanisms responsible for blazar emission, specifically focusing on the leptonic scenario, in which the blazar radiative output is explained by synchrotron and inverse Compton emission. Chapter 3 examines particle acceleration, covering two key mechanisms: shocks and turbulence. It begins with an analysis on a fluid scale, exploring their hydrodynamical behavior, and then transitions to kinetic scales to investigate acceleration dynamics. Chapter 4 introduces an alternative model for extreme TeV BL Lacs. Conventional single-shock acceleration models fail to explain their SEDs. We propose a two-step acceleration model: electrons are first accelerated by a shock and further energized via downstream turbulence. The model is first adjusted manually on the data of 1ES 0229+200, then it is applied to other extreme TeV BL Lacs using an MCMC sampler, which permits a deeper exploration of the parameter space. Chapter 5 investigates the polarization in high-energy BL Lac objects using a post-processing code based on the Lagrangian particle approach. Observations reveal that X-ray polarization exceeds optical and radio counterparts. Starting from fluid simulations of parsec-scale jet, the post-processing code models the emission, integrating recent PIC findings for shock acceleration. Results successfully reproduce polarization chromaticity observed in blazars, aligning with IXPE and multi-wavelength campaigns.