SIMBAD references

We investigate the use of optical variability to identify and study active galactic nuclei (AGNs) in the GOODS-South field. A sample of 22 mid-IR 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 AGNs 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.