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Identifying Active Galactic Nuclei (AGNs) through their X-ray emission isefficient, but necessarily biased against X-ray-faint objects. We aim tocharacterize this bias by comparing X-ray-selected AGNs to the ones identifiedthrough optical variability and mid-IR colours. We present a catalogue of AGNsselected through optical variability using all publicly available z-band HubbleSpace Telescope images in the GOODS-South field. For all objects in thecatalogue, we compute X-ray upper limits or discuss detections in the deepestavailable 7 Ms Chandra Deep Field South images and present the Spitzer/IRACmid-IR colours. For the variability study, we consider only sources observedover at least five epochs and over a time baseline of up to ten years. We adoptthe elevated median absolute deviation as a variability indicator robustagainst individual outlier measurements and identify 113 variability-selectedAGN candidates. Among these, 26 have an X-ray counterpart and lie within theconventional AGN area in the FX/Fopt diagram. The candidates with X-ray upperlimits are on average optically fainter, have higher redshifts compared to theX-ray detected ones and are consistent with low luminosity AGNs. Out of 41variable optical sources with IR detections, 13 fulfill the IR AGN colourselection criteria. Our work emphasizes the importance of optical variabilitysurveys for constructing complete samples of AGNs including the ones thatremain undetected even by the deepest X-ray and IR surveys.

Robust Identification of Active Galactic Nuclei through HST Optical Variability in GOODS-S: Comparison with the X-ray and mid-IR Selected Samples