TESS Shines Light on the Origin of the Ambiguous Nuclear Transient ASASSN-18el

TL;DR

This study uses TESS high-cadence data to analyze the ambiguous nuclear transient ASASSN-18el, finding variability consistent with an active galactic nucleus rather than a tidal disruption event, thus clarifying its origin.

Contribution

It demonstrates that the optical variability of ASASSN-18el is consistent with AGN activity through DRW modeling, providing evidence against a TDE origin.

Findings

Variability amplitude of 0.93 mJy detected.

DRW timescale of approximately 20 days.

Variability characteristics align with supermassive black hole mass relations.

Abstract

We analyze 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-timescale 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 $\hat{\sigma} = 0.93 \pm 0.02$ mJy and a rest-frame timescale of $\tau_{DRW} = 20^{+15}_{-6}$ days. We find that the estimated $\tau_{DRW}$ for ASASSN-18el is broadly consistent with an apparent relationship between the DRW timescale 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.