We studied the X-ray variability of sources detected in the Chandra Deep Field South ( ) , nearly all of which are low to moderate z AGN ( ) .
We find that 45 % of the sources with > 100 counts exhibit significant variability on timescales ranging from a day up to a year .
The fraction of sources found to be variable increases with observed flux , suggesting that > 90 \% of all AGNs possess intrinsic variability .
We also find that the fraction of variable sources appears to decrease with increasing intrinsic absorption ; a lack of variability in hard , absorbed AGNs could be due to an increased contribution of reflected X-rays to the total flux .
We do not detect significant spectral variability ( \Delta \Gamma > 0.2 ) in the majority ( \sim 70 \% ) of our sources .
In half of the remaining 30 \% , the hardness ratio is anti-correlated with flux , mimicking the high/soft–low/hard states of galactic sources .
The X-ray variability appears anti-correlated with the luminosity of the sources , in agreement with previous studies .
High redshift sources , however , have larger variability amplitudes than expected from extrapolations of their low-z counterparts , suggesting a possible evolution in the accretion rate and/or size of the X-ray emitting region .
Finally , we discuss some effects that may produce the observed decrease in the fraction of variable sources from z = 0.5 out to z = 2 .