Deepest View of AGN X-Ray Variability with the 7 Ms&IT Chandra&IT Deep Field-South Survey

We systematically analyze the X-ray variability of active galactic nuclei (AGNs) in the 7 Ms Chandra Deep Field-South survey. On the longest timescale (approximate to 17 years), we find only a weak (if any) dependence of X-ray variability amplitudes on energy bands or obscuration. We use four different power spectral density (PSD) models to fit the anticorrelation between normalized excess variance (sigma(2)(nxv)) and luminosity, and obtain a best-fit power-law index beta = 1.16(-0.05)(+0.05) for the low-frequency part of the AGN PSD. We also divide the whole light curves into four epochs in order to inspect the dependence of sigma(2)(nxv) on these timescales, finding an overall increasing trend. The analysis of these shorter light curves also infers a beta of similar to 1.3 that is consistent with the above-derived beta, which is larger than the frequently assumed value of beta = 1. We then investigate the evolution of sigma(2)(nxv). No definitive conclusion is reached because of limited source statistics, but if present, the observed trend goes in the direction of decreasing AGN variability at fixed luminosity toward high redshifts. We also search for transient events and find six notable candidate events with our considered criteria. Two of them may be a new type of fast transient events, one of which is reported here for the first time. We therefore estimate a rate of fast outbursts <(N) over dot > = 1.0(-0.7)(+1.1) x 10(-3) galaxy(-1) yr(-1) and a tidal disruption event (TDE) rate <(N) over dot(TDE)> = 8.6(-4.9)(+8.5) x 10(-5) galaxy(-1) yr(-1) assuming the other four long outbursts to be TDEs.