Active galactic nuclei (AGN), the most luminous persistent objects in the sky, are powered by accretion discs onto supermassive black holes in the centers of galaxies. The study of their X-ray emission via spectroscopy gives us information on key physical parameters, such as the temperature of the hot corona of electrons located in the innermost region of the accretion flow. However, the geometry of the hot corona is currently poorly known, and so it is its physical origin. Furthermore, the large-scale environment of AGN is rich of gas and dust, thought to be distributed in a toroidal structure surrounding the accretion disc and reflecting the primary X-rays from the AGN. X-ray polarimetry, being very sensitive to geometry, allows us to directly constrain the shape of the hot corona and the geometry of the reflecting torus. This is now possible thanks to the Imaging X-ray Polarimetry Explorer (IXPE), a NASA/ASI mission operative since December 2021. In this talk, I will review the first IXPE results on radio-quiet AGN, which provide a significant advancement in the physical understanding of these objects.
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