  We investigate the use of optical variability to identify and study Active
Galactic Nuclei (AGN) in the GOODS-South field. A sample of 22 mid-infrared
power law sources and 102 X-ray sources with optical counterparts in the HST
ACS images were selected. Each object is classified with a variability
significance value related to the standard deviation of its magnitude in five
epochs separated by 45-day intervals. The variability significance is compared
to the optical, mid-IR, and X-ray properties of the sources. We find that 26%
of all AGN candidates (either X-ray- or mid-IR-selected) are optical variables.
The fraction of optical variables increases to 51% when considering sources
with soft X-ray band ratios. For the mid-IR AGN candidates which have
multiwavelength SEDs, we find optical variability for 64% of those classified
with SEDs like Broad Line AGNs. While mostly unobscured AGN appear to have the
most significant optical variability, some of the more obscured AGNs are also
observed as variables. In particular, we find two mid-IR power law-selected AGN
candidates without X-ray emission that display optical variability, confirming
their AGN nature.
