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We investigate the use of optical variability to identify and study ActiveGalactic Nuclei (AGN) in the GOODS-South field. A sample of 22 mid-infraredpower law sources and 102 X-ray sources with optical counterparts in the HSTACS images were selected. Each object is classified with a variabilitysignificance value related to the standard deviation of its magnitude in fiveepochs separated by 45-day intervals. The variability significance is comparedto 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 sourceswith soft X-ray band ratios. For the mid-IR AGN candidates which havemultiwavelength SEDs, we find optical variability for 64% of those classifiedwith SEDs like Broad Line AGNs. While mostly unobscured AGN appear to have themost significant optical variability, some of the more obscured AGNs are alsoobserved as variables. In particular, we find two mid-IR power law-selected AGNcandidates without X-ray emission that display optical variability, confirmingtheir AGN nature.

Optical Variability of Infrared Power Law-Selected Galaxies & X-ray Sources in the GOODS-South Field