ASASSN-14ko is a Periodic Nuclear Transient in ESO 253-G003
Holoien, Thomas W. -S.; Vallely, Patrick J.; Auchettl, Katie; Stanek, K. Z.; Kochanek, Christopher S.; Prieto, Jose L.; Shappee, Benjamin J.; Fausnaugh, Michael M.; Thompson, Todd A.; Neustadt, Jack M. M.; Schneider, Adam C.; Tonry, John L.; Armstrong, James D.; Heinze, A. N.; Monard, Berto; Lopez, Laura A.; Sheppard, Scott S.; Grupe, Dirk; Brimacombe, Joseph; Hinkle, Jason T.; Payne, Anna V.; Cacella, Paulo; Denneau, Larry; Tucker, Michael A.; Flewelling, Heather; Cornect, Robert; Weiland, Henry
United States, Australia, Denmark, Brazil, South Africa, Chile
Abstract
We present the discovery that ASASSN-14ko is a periodically flaring active galactic nucleus at the center of the galaxy ESO 253-G003. At the time of its discovery by the All-Sky Automated Survey for Supernovae (ASAS-SN), it was classified as a supernova close to the nucleus. The subsequent 6 yr of V- and g-band ASAS-SN observations revealed that ASASSN-14ko has nuclear flares occurring at regular intervals. The 17 observed outbursts show evidence of a decreasing period over time, with a mean period of P0 = 114.2 ± 0.4 days and a period derivative of $\dot{P}=-0.0017\pm 0.0003$ <!-- --> . The most recent outburst in 2020 May, which took place as predicted, exhibited spectroscopic changes during the rise and had a UV bright, blackbody spectral energy distribution similar to tidal disruption events (TDEs). The X-ray flux decreased by a factor of 4 at the beginning of the outburst and then returned to its quiescent flux after ∼8 days. The Transiting Exoplanet Survey Satellite 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 discuss several possible scenarios to explain ASASSN-14ko's periodic outbursts, but currently favor a repeated partial TDE. The next outbursts should peak in the optical on UT 2020 September 7.4±1.1 and UT 2020 December 26.5±1.4.