Gamma-Ray Probe of the QSO’s Obscured Evolution

The obscured phase of QSOs, as well as their accretion history, can be best followed by observing QSOs bright in >10 MeV gamma-rays. By analysing the resonant absorption troughs in spectral energy distribution of flaring QSOs one can measure the (baryonic) absorbing column and baryonic content of the QSO host galaxy,while the flare strength will give information on the accretion rate of the QSO powering supermassive black hole. By measuring the baryonic absorbing column for QSOs at different redshifts one can follow the early obscured evolution of AGN at redshifts up to z~6