Speaker
Description
Active galactic nuclei (AGN) can exhibit drastic changes that differ significantly from the normal stochastic variability, such as changing-look AGN and rapid-turn on events. Transients also occur in quiescent supermassive black hole (SMBH). When it tidally disrupts a passing star, about half of the debris can be accreted to produce a luminous flare in a tidal disruption event (TDE). It is non-trivial to observationally distinguish TDEs from AGN transients. In particular, transient surveys have discovered ``ambiguous nuclear transients (ANTs)'' that can be classified as neither TDEs nor AGN. Unveiling the nature of these ANTs is critical for understanding of the extreme accretion episodes in SMBHs. I will present an extensive X-ray study with Swift, Chandra and NICER over 173 days for the ambiguous nuclear transient AT2019pev that showed features of both TDEs and AGN. Most X-ray properties of this transient more closely resemble an AGN. However, the X-ray spectra show a "harder-when-brighter" trend before the X-ray lightcurve peak, while the trend is inverted after the peak. This change could indicate a transition of accretion state, which is not commonly observed for both TDEs and AGN.
