Presentation #138.07 in the session “AGN and Quasars 1”.
We present the discovery that ASASSN-14ko is a unique and highly unusual periodically flaring AGN at the center of the galaxy ESO 253-G003. At the time of ASASSN-14ko’s discovery by the All-Sky Automated Survey for Supernovae (ASAS-SN), this transient event was classified as a supernova close to the nucleus. The subsequent six years of V- and g-band ASAS-SN observations reveal that ASASSN-14ko has nuclear flares occurring at regular intervals. The eighteen observed outbursts show a decreasing period over time, with a mean period of 114.4±0.4 days and a period derivative of -0.002±0.0003. Recent outbursts in May 2020 and September 2020, which took place as predicted, revealed a UV bright, blackbody spectral energy distribution similar to tidal disruption events (TDEs). However, X-ray flux decreased during the outbursts by an order of magnitude. In addition, TESS observed an outburst during Sectors 4-6, revealing a rise time of 5.60±0.05 days in the optical and a decline that is best fit with an exponential model. We currently favor a repeated partial TDE scenario to explain ASASSN-14ko’s periodic outbursts.