Active galactic nuclei flicker: an observational estimate of the duration of black hole growth phases of ∼105yr

We present an observational constraint for the typical active galactic nucleus (AGN) phase lifetime. The argument is based on the time lag between an AGN central engine switching on and becoming visible in X-rays, and the time the AGN then requires to photoionize a large fraction of the host galaxy. Based on the typical light travel time across massive galaxies, and the observed fraction of X-ray-selected AGN without AGN-photoionized narrow lines, we estimate that the AGN phase typically lasts ∼105 yr. This lifetime is short compared to the total growth time of 107–109 yr estimated from e.g. the Soltan argument and implies that black holes grow via many such short bursts and that AGN therefore ‘flicker’ on and off. We discuss some consequences of this flickering behaviour for AGN feedback and the analogy of X-ray binaries and AGN lifecycles.