On the X-ray properties of heavily obscured AGN in the backyard

One of the main goals of extragalactic astrophysics is to achieve complete knowledge of the processes which produce the observed emission from AGN. AGN classification in the X-rays divides these sources into two types: unobscured and obscured. Their emission explain most of the CXB emission. The fraction of resolved CXB at its peak is only about 30%. From models of the CXB, it is possible to have indications on the nature of sources which are ‘missing’ in the current census of CXB ‘constituents’: CT-AGN must be responsible for ∼35% of the peak CXB emission. Observations show that the fraction of CT-AGN is 10-15%; in order to fill this gap, a census of obscured AGN is needed. X-ray observations provide a characterization of the inner regions of the AGN. In this perspective, we present the X-ray spectral analysis of two low-redshift Compton-thick AGN candidates, NGC 3081 and ESO 565-G019, belonging to a larger sample of 57 candidate CT-AGN. We select these two AGN because of the high photon statistics, given by proprietary XMM-Newton and simultaneous NuSTAR data, and the presence of Chandra data. We have analyzed the two spectra using simple phenomenological spectral models and physically motivated models (MYTorus and borus02), in order to better characterize the intrinsic physical and geometric properties of the obscuring torus, as well as its nature. The main goal of this thesis is to measure the column density of the two sources under investigation, as well as the other spectral properties, to find out if they belong to the Compton-thick regime or they are Compton-thin AGN. We found that both AGN have an obscuring structure which is characterized, on average, by a Compton-thick material. Finally, it is important to underline how this type of application is also preparatory to future planned missions like Athena (∼2030) and the AXIS probe (mission under study) and can lead us to a better understanding of the properties of the torus and of the CXB components.