Scavenger hunt: Selection of obscured active galactic nuclei combining multiband optical variability and colors

Context. As wide-field optical surveys such as Vera C. Rubin Observatory’s Legacy Survey of Space and Time (LSST) begin operations, time-domain astronomy is about to face a data revolution, paving the road for new and expanded variability studies. Aims. This work leverages the complementary power of optical variability and color selection to identify active galactic nuclei (AGN), with a particular emphasis on optimizing the identification of obscured AGN, typically more challenging to distinguish from inactive galaxies based on optical variability alone. The analysis is designed to provide valuable insights in the context of performance forecasting for the LSST, albeit using a scaled-down version of the LSST dataset. Methods. We present the first combined AGN selection based on g-, r-, and i-band light curves from the VST-COSMOS survey, spanning a 3.3 yr baseline. We identify AGN candidates independently in each band using a random forest (RF) classifier trained on features primarily related to optical variability, along with six optical/infrared colors and a morphology indicator. We subsequently merge the resulting band-specific samples in order to enhance selection purity and reliability. We then focus on defining a subset of features that significantly improve the identification of obscured AGN. Results. The RF classifiers yield a consistent performance across the three bands, highlighting the critical role played by contamination. Using the combined three-band plus color selection we successfully recover $ 58^{+9}_{-8}\% $ of all AGN and $ 69^{+10}_{-8}\% $ of the known obscured AGN that have been independently confirmed in all three bands. When requiring confirmation in at least two out of the three bands, these fractions increase to $ 69^{+10}_{-8}\% $ and $ 80^{+10}_{-9}\% $, respectively. Moreover, we demonstrate that, while combining variability features with colors is crucial to improve obscured AGN selection, relying solely on color features results in a markedly higher contamination rate.