Investigating active galactic nuclei with X-ray polarimetry: the present and the future

Polarimetry is a crucial tool for investigating the physical and geometrical properties of the emission components in Active Galactic Nuclei (AGN). The successful launch of the Imaging X-ray Polarimetry Explorer (IXPE) on December 9, 2021, has extended the previously limited scope of polarimetry into the X-ray domain, enabling the first X-ray polarimetric studies of extragalactic sources. The origin of X-ray emission in AGN is attributed to relativistic jets, hot coronae, and reflected emission from accretion disk and circumnuclear components. However, the detailed physical and geometrical properties of each emission component remained yet largely unknown. In this work, leveraging on the first opportunity to study AGN through X-ray polarimetry provided by IXPE, the X-ray polarization properties of various classes of AGN were measured and analyzed using different methods. Additionally, spectral analyses of simultaneous observations from other X-ray telescopes, such as XMMNewton, NuSTAR, and Swift, were conducted to examine the intrinsic physical properties of the sources, including flux and spectral characteristics. Moreover, multiwavelength polarimetry campaigns were conducted, providing crucial insights into the physical and geometrical properties of AGN. Over two and a half years of operations, IXPE has uncovered evidence of energystratified shock acceleration as the particle acceleration mechanism and captured the strong evidence of helical magnetic field structure in relativistic jets of blazars. IXPE results also provided evidence supporting the leptonic acceleration scenario to explain the origin of high-energy emission. Furthermore, studies of the hot corona in Seyfert galaxies found that X-ray polarization favor radially extended slab and wedge geometries over the widely accepted vertically extended lamppost and conical geometries. IXPE also confirmed the presence of reflection emission from the dusty torus in obscured Seyfert galaxies and constrained its opening angle. In addition to the remarkable discoveries on AGN, IXPE has also highlighted the limitations of the current photoelectric polarimeters. One of the major challenges is the sensitivity limitations related to dead time, which restricts the increase in the effective area of the telescope to capture more photons per unit time. To address this limitation and expand our understanding of X-ray polarimetry, a new Monte Carlo simulation based on Geant4 and a new track reconstruction algorithm were developed, leveraging the application of a new generation of multipurpose Application Specific Integrated Circuit (ASIC) like Timepix3, which offers both imaging and timing functionalities. Based on the three-dimensional advancements achieved through the new detector, its prospective performance was tested and evaluated. As a result, significant enhancement was observed with the new threedimensional track reconstruction algorithm, and the expected response of the new detector system was derived. This new simulation tool for the next-generation X-ray polarimeter will be fine-tuned and refined under various conditions, playing a crucial role in the design of future X-ray polarimetry missions that aim to expand our exploration into broader and deeper regions of the universe.