Deepest view of AGN X-ray variability with the 7 Ms Chandra Deep
  Field-South survey
  We systematically analyze X-ray variability of active galactic nuclei (AGNs)
in the 7~Ms \textit{Chandra} Deep Field-South survey. On the longest timescale
($\approx~17$ years), we find only 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 anti-correlation between
normalized excess variance ($\sigma^2_{\rm nxv}$) and luminosity, and obtain a
best-fit power law index $\beta=1.16^{+0.05}_{-0.05}$ for the low-frequency
part of AGN PSD. We also divide the whole light curves into 4 epochs in order
to inspect the dependence of $\sigma^2_{\rm nxv}$ on these timescales, finding
an overall increasing trend. The analysis of these shorter light curves also
infers a $\beta$ of $\sim 1.3$ 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 $\sigma^2_{\rm nxv}$. No definitive
conclusion is reached due to limited source statistics but, if present, the
observed trend goes in the direction of decreasing AGN variability at fixed
luminosity toward large redshifts. We also search for transient events and find
6 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 $\langle\dot{N}\rangle =
1.0^{+1.1}_{-0.7}\times 10^{-3}~\rm galaxy^{-1}~yr^{-1}$ and a tidal disruption
event~(TDE) rate $\langle\dot{N}_{\rm TDE}\rangle=8.6^{+8.5}_{-4.9}\times
10^{-5}~\rm galaxy^{-1}~yr^{-1}$ assuming the other four long outbursts to be
TDEs.
