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

TL;DR

ASASSN-14ko is a periodically flaring active galactic nucleus with decreasing outburst intervals, likely caused by a repeated partial tidal disruption event, as observed over six years with multi-wavelength data.

Contribution

This study reports the discovery of a periodic nuclear transient with decreasing period, providing evidence for a repeated partial tidal disruption event in an active galactic nucleus.

Findings

Periodic outbursts with mean period 114.2 days

Decreasing period over time with a period derivative of -0.0017

Spectroscopic and UV observations consistent with TDE-like events

Abstract

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 $\dot{P} = -0.0017\pm0.0003$. 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.