In this thesis, we have restudied a sample chosen by (Lusso & Risaliti, 2016) consisting of a set of 2,153 unique quasars given by two source catalogues: 3XMM-DR5 (Rosen et al., 2016) and DR7 catalogue (Shen et al., 2011). Quasars serve science in two different ways: 1) Understanding accretion physics by studying the physical mechanism occuring between the disk and cloud corona, and 2) they are good cosmological probes in distance estimation reaching redshifts ~ 7 (Mortlock et al., 2011). A physical relation has been observed between the optical-UV disk and the X-ray corona through a log-log ...