Let me look it up… – revealing the nature of new variable objects from archival precovery monitoring

The future synoptic projects, including the LSST, are preceded by a number of surveys, albeit shallower, or covering smaller areas or delivering a limited number of epochs. Nevertheless, this rich information trove can help to classify the newly identified variable objects, by providing either additional wavelength coverage or precovery monitoring. As a forerunner to the coming LSST avalanche of data, I apply this approach to the Gaia Alerts, using archival data from the VVV/X and VMC ESO public surveys and from the WISE satellite. Combining the optical Gaia colors and the infrared colors from these surveys helps to unambiguously identify young stellar objects and young stars. Furthermore, some precovery light curves show brightening of objects that were considered to have undergone microlensing events at a later date, during the Gaia coverage. The extremely low probability for multiple lensing events of the same star suggests that variability of these objects must be explained by other – and repetitive – mechanisms.