Clues on the physics of AGN through X-ray spectral, temporal and polarimetric analysis

X-ray emission of active galactic nuclei (AGN) can be considered as a powerful probe of the close environment of supermassive black holes (SMBHs), at the centres of these objects. The X-ray properties of AGN are studied in this dissertation, addressing several open questions in AGN physics: - the properties of the X-ray corona in AGN (its nature and geometry) , - the presence of a population of thermal non-relativistic electrons, - probing general relativity in the strong-field regime, addressing especially the possibility of estimating accurately the SMBH spin in AGN, through the identification of relativistic reflection features in their X-ray spectra and polarization signal, - the nature of variability in X-ray light curves, and the identification of the different physical components contributing to it. In this dissertation, I shed light on these topics by - Analysing the X-ray spectra and light curves of individual sources, using simultaneous XMM-Newton and NuSTAR observations. These simultaneous observations can provide the highest signal-to-noise data in the ~0.3-79 keV band which can be achieved with the current X-ray observatories. By covering the broad X-ray band, XMM-Newton (in the 0.3-10 keV) and NuSTAR (in the 3-79 keV) allow us to determine with a high precision the various spectral and physical properties of low- and high-redshift AGNs, as well as to identify the various spectral components responsible of the emission in these sources, and study their temporal evolution. - Simulating high S/N spectra of local AGN, using the instrumental responses of XMM-Newton and NuSTAR, and assuming a generic model for the X-ray emission in AGN, in order to test the reliability of spin measurements which can be achieved with current instruments. - Predicting the theoretical spectral and polarimetric signatures of X-ray eclipses, by broad-line region (BLR), clouds which can be used in order to probe the signal arising from the innermost regions of the accretion disc.