Program: GS-2021B-Q-208

The South Pole Telescope has recently opened up the window onto the mm-wave transient sky. In a 9 month survey of 1500 square degrees with a daily observing cadence, we discovered two transient sources of extragalactic origin. While these are consistent with being low-luminosity jet-dominated active galactic nuclei, the amplitude of the variability and the flares are unprecedented. We request 6 hours of GMOS-S to obtain optical spectroscopy of the two sources to identify the most likely optical counterpart to the mm-wave events, obtain a spectroscopic redshift with OII to determine their luminosity, and test for the presence of an AGN through observations of MgII. These followup observations will provide the crucial guidance to categorize and understand future transient events discovered in the SPT-3G survey.