Accretion on all time scales from TDEs to QSOs

Supermassive black holes are amongst the most energetic and interesting astrophysical objects in our known Universe. Their study, started almost a century ago, shed light on several aspects of their nature, both through theoretical and observational means. However many others remain mysteries. One of the most interesting feature these objects exhibit is the wide variety in the time scales of their accretion: supermassive black hole can be found accreting material regularly, keeping a steady-state across centuries, or being in a non-accreting state for as much long; or they can suddenly burst, emitting in days the energy hundreds of galaxies emit in years, along with all the intermediate states. This thesis addresses the matter of accretion on different time scales mainly by focusing on two phenomena: tidal disruption events and quasi-stellar objects. Both phenomena indicates supermassive black holes accreting at time scales which differ by several orders of magnitudes. This thesis is, as the entire supermassive black hole research field is, a multi-technique work, coupling theoretical analysis, numerical simulations, observational efforts and catalog searches in order to investigate these most interesting titanic monsters.