2017ApJ...849..127Z

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 (≃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 (σ_nxv²) and luminosity, and obtain a best-fit power-law index β=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 σ_nxv² on these timescales, finding an overall increasing trend. The analysis of these shorter light curves also infers a β of ∼1.3 that is consistent with the above-derived β, which is larger than the frequently assumed value of β=1. We then investigate the evolution of σ_nxv². 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 <{dot}N>=1.0_–0.7^+1.1×10^–3 galaxy^–1 yr^–1 and a tidal disruption event (TDE) rate <{dot}N_TDE>=8.6_–4.9^+8.5×10^–5 galaxy^–1 yr^–1 assuming the other four long outbursts to be TDEs.