X-ray Variability Of Active Galactic Nuclei In The 7 Ms Chandra Deep Field-South

We systematically analyze X-ray variability of active galactic nuclei (AGNs) in the 7 Ms Chandra Deep Field-South (CDF-S) survey. Our sample contains 148 sources with reliable 0.5-7 keV variability measurements, which have an X-ray luminosity range of 1041 - 1045 erg s-1 and a redshift range of 0-5. On the longest timescale(≈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) mod-els to fit the anti-correlation between normalized excess variance (σnxv2) and luminosity,and obtain a best-fit power law indexp β of- 1.161-0.05+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 de-pendence of σnxv2 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 =1.16-0.05+0.05, which is larger than the frequently-assumed value of β=1.We then investigate the evolution of the AGN σnxv2,. The limited sample sizes lead to no reliable conclusion. Only hints of an overall decreasing trend and a suggestive peak are found. We also use 3-month-bin light curves to search for the transient events in the 7 Ms CDF-S 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...