Date of Award


Document Type


Degree Name

Doctor of Philosophy (PhD)


Physics and Astronomy

Committee Chair/Advisor

Dr Marco Ajello

Committee Member

Dr. Dieter Hartmann

Committee Member

Dr. Stefano Marchesi

Committee Member

Dr. Jonathan Zrake

Committee Member

Dr. Catalina Marinescu


Active Galactic Nuclei (AGN) are among the Universe's most energetic and powerful objects, fueled by an accreting supermassive black hole (SMBH) at the host galaxy's center, surrounded by a toroidal structure of dusty gas. Ultraviolet photons arising from the accretion disk get up-scattered to X-rays via inverse Compton scattering by hot electrons close to the accretion disk. Being produced in the very center of the AGN, X-ray photons are powerful messengers that probe the physics of the accretion system and the matter in the surroundings. The torus, formerly considered homogeneous, appears to be a more complex structure of clouds with different densities. These clouds play a pivotal role in reprocessing incoming X-ray radiation through photoelectric absorption (quantified by the hydrogen column density, NH) and Compton-scattering, yielding distinct absorption and reprocessing features evident in the X-ray spectra of Seyfert 2 galaxies. Thus, the modeling of the X-ray spectra of obscured AGN (NH>1022 cm-2) is a precious tool to unveil the state of the circumnuclear matter, which plays an important role in connecting the innermost region of the AGN with the host galaxy.

In this thesis work, I present the analysis and study done on all available X-ray archival data of a pool of local (z

Two primary projects are detailed herein: the first is centered on characterizing the obscuring material of the Compton-thick AGN (NH>1.5 1024cm-2), NGC 7479 (Chapter 2, Pizzetti et al. 2022). In contrast, the second expands the analysis to encompass a broader set of 13 obscured AGN, aiming to delineate their torus properties and variability profiles (Chapter 3, Pizzetti et al. submitted). Employing a comprehensive approach, all archival observations from Chandra, XMM-Newton, Suzaku, and NuSTAR are analyzed and modeled using three distinct torus models, borus02, MYTorus, and UXCLUMPY, enabling the characterization of key torus properties and NH variations across epochs. To complement the analysis, additional sources from Torres-Alba et al. 2023 and Sengupta et al. in prep. are integrated to provide insights into the variability profiles of the entire AGN sample. Classification of the sources as NH Variable, Non-variable in NH, or Undetermined is based on the extent of variability and the ability to disentangle intrinsic flux and NH variability. The study revealed 37% of the AGN to be variable in NH at a 90% confidence level. However, no discernible difference between geometrical and intrinsic properties is found among the three variability classes, suggesting no intrinsic differences between the variable a non-variable sources. 48% of the analyzed sources require the addition of a Compton-thick reflector in the fitting, the so-called inner ring. Among these, 30% exhibit recorded 22 GHz megamaser emission, suggesting a potential connection between the inner reflector and the megamaser disk.

The manuscript is structured as follows: Chapter 1 introduces the historical context surrounding the discovery of the first active galaxies, providing an overview of the primary AGN types and their key physical sub-structures, with a particular emphasis on Seyfert galaxies. A brief overview of the X-ray telescopes employed in this work is also given. Chapters 2 and 3 present the two papers reporting the work done as part of my doctoral studies. Chapter 4 reports the main conclusions achieved from the two projects, offering a view of the work from a broader scientific standpoint. Appendix A presents a tree-diagram of the AGN unification model, while Appendix B briefly reports the relevant radiative processes.

Author ORCID Identifier




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