Physical characterization of X-ray supersoft active galactic nuclei

The X-ray spectra of active galactic nuclei (AGN) are thought to be produced in a hot coronal region where relativistic electron inverse-Compton scatter the low-energy (optical/UV) photons emitted by the cold accretion disc around a supermassive black hole (SMBH). The slope of the primary X-ray continuum, usually described through a simple power law, is therefore determined by the physical properties of the corona, in particular its temperature and optical depth. Both observational and theoretical arguments suggest a correlation between the X-ray photon index and the accretion rate, whereby systems with higher accretion rates have steeper X-ray spectra as their corona is more efficiently cooled. This topic has generally been addressed by identifying highly accreting objects via multiwavelength diagnostics and then studying their X-ray properties. The aim of this master thesis is to follow the opposite route, starting from the selection of a robust sample of AGN with steep X-ray spectra (Γ > 2.5) based on XMM-Newton archival observations, and then considering ancillary multiwavelength data to investigate whether X-ray supersoft AGN are preferentially found among highly accreting systems. The results will have important implications on the formation of SMBHs in the early Universe and on the X-ray properties of AGN populations at high redshift.