ASASSN-14ko is a Periodic Nuclear Transient in ESO 253-G003

Kavli Affiliate: Michael M. Fausnaugh

| First 5 Authors: Anna V. Payne, Benjamin J. Shappee, Jason T. Hinkle, Patrick J. Vallely, Christopher S. Kochanek

| Summary:

We present the discovery that ASASSN-14ko is a periodically flaring AGN 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 six years of V- and g-band
ASAS-SN observations reveal that ASASSN-14ko has nuclear flares occurring at
regular intervals. The seventeen observed outbursts show evidence of a
decreasing period over time, with a mean period of $P_0 = 114.2 pm 0.4$ days
and a period derivative of $dot{P} = -0.0017pm0.0003$. The most recent
outburst in May 2020, which took place as predicted, exhibited spectroscopic
changes during the rise and a 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. TESS observed an outburst during Sectors
4-6, revealing a rise time of $5.60 pm 0.05$ 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
2020-09-7.4$ pm $1.1 and UT 2020-12-26.5$ pm $1.4.

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