X-ray Polarization of Radio-Quiet, Unobscured Active Galactic Nuclei

Active Galactic Nuclei (AGN) represents a crucial phase in the evolution of galaxies. Their enormous emission arises from accretion processes around a Super-Massive Black Hole at the galaxy's core, shaping nearby and, possibly, distant regions. Within a few parsecs in the nuclear region, optical/UV photons from the accretion disk are Compton up-scattered towards the X-ray band within a cloud of hot electrons referred to as the \textit{corona}, resulting in the primary continuum emission (usually in the form of a power-law with a high-energy cut-off). The corona has been extensively studied in the X-rays in the last decades. If on one hand spectral and reverberation analyses are able to determine its physical properties (e.g., temperature and optical depth) and size, on the other hand they are insufficient for distinguishing between different geometric configurations (holding clues on its physical origin), which result in equally acceptable spectral fits. The launch of the NASA/ASI polarimeter Imaging X-ray Polarimetry Explorer (IXPE) on December 2021 opened a new, promising window on this subject, as different geometrical configurations lead to distinct polarization features of the primary continuum radiation (i.e., polarization degree and angle). IXPE observed various Radio-Quiet, Compton-thin AGN so far. For this type of sources, the energy work-band of IXPE (i.e., 2--8~keV) is usually dominated by coronal emission, making them perfect targets to be explored.\\ This work primarily consists in combining the capabilities of IXPE (often in synergy with various other X-ray spectral observatories like XMM-Newton and NuSTAR) with detailed modeling and simulations of the expected polarization properties performed with the Monte Carlo Radiative Transfer code for Comptonized Spectra \texttt{MONK}. IXPE data are analyzed from model-independent and model-dependent perspectives, while detailed simulations are performed in order to determine the expected polarization properties of the primary continuum from different geometrical models and physical configurations and compare them with actual data to constrain the coronal geometry. Even if, due to the combination of a relatively low flux of the targeted sources and the relatively low effective area of IXPE, no certain verdicts can be outlined, hints of a general picture can be delineated. In fact, several sources point towards a radially extended coronal geometry (i.e., slab or wedge). This result is driven by observed polarization fractions which are too high for being produced by a lamp-post or a conical corona (which are extended in the vertical direction) and by polarization directions which are found to be parallel to the accretion disk axis, a feature commonly determined by a radial extension of the corona itself. This inspires confidence for future developments of this field of research. In fact, IXPE is planned to observed more sources of this kind in the future, hopefully obtaining more unambiguous results. Also, ongoing efforts are being made to implement new capabilities within Monte Carlo codes (e.g., including reflection and absorption from outflowing structures in AGN or X-ray binaries). Also, a new polarimeter (i.e., the enhanced X-ray Timing and Polarimetry: eXTP), holding greater effective area and spectral capabilities compared to IXPE's, is expected to be launched by the Chinese Academy of Sciences in 2027, opening a window for further improving of IXPE's results.\\ Finally, within this work, current limitations in modeling and simulating various fundamental structures of AGN (e.g., outflowing, partially ionized winds), which could significantly affect the overall observed polarization, are assessed, discussing the ongoing efforts to implement them in the promising Monte Carlo Radiative Transfer code \texttt{SKIRT}.