TESS shines light on the origin of the ambiguous nuclear transient ASASSN-18el

We analyse high-cadence data from the Transiting Exoplanet Survey Satellite (TESS) of the ambiguous nuclear transient (ANT) ASASSN-18el. The optical changing-look phenomenon in ASASSN-18el has been argued to be due to either a drastic change in the accretion rate of the existing active galactic nucleus (AGN) or the result of a tidal disruption event (TDE). Throughout the TESS observations, short-time-scale stochastic variability is seen, consistent with an AGN. We are able to fit the TESS light curve with a damped-random-walk (DRW) model and recover a rest-frame variability amplitude of (supermassive) over cap sigma = 0.93 +/- 0.02 mJy and a rest-frame time-scale of tau(DRW) = 20(-6)(+15) d. We find that the estimated tDRW for ASASSN-18el is broadly consistent with an apparent relationship between the DRW time-scale and central supermassive black hole mass. The large-amplitude stochastic variability of ASASSN-18el, particularly during late stages of the flare, suggests that the origin of this ANT is likely due to extreme AGN activity rather than a TDE.

Automated summary

We analyzed data from TESS on the ambiguous nuclear transient ASASSN-18el and found that the optical changing-look phenomenon is likely due to extreme AGN activity rather than a TDE. The short-time-scale stochastic variability observed is consistent with an AGN, and fitting the TESS light curve with a DRW model provides a rest-frame variability amplitude and time-scale. The estimated tDRW for ASASSN-18el is broadly consistent with a relationship between DRW time-scale and central supermassive black hole mass.