Extreme Accretion Events: TDEs and Changing-Look AGN

TL;DR

This review discusses extreme accretion phenomena like X-ray tidal disruption events and changing-look AGN, highlighting their properties, discovery history, and implications for understanding accretion physics under extreme conditions.

Contribution

It provides a comprehensive overview of TDEs and changing-look AGN, introduces the concept of frozen-look AGN, and synthesizes recent observational findings and their significance.

Findings

TDEs exhibit luminous X-ray flares with soft spectra.

Changing-look AGN show significant optical emission line variability.

Frozen-look AGN demonstrate constant emission lines despite continuum changes.

Abstract

We present a review of the topics of X-ray stellar tidal disruption events (TDEs) and changing-look active galactic nuclei (AGN). Stars approaching a supermassive black hole (SMBH) can be tidally disrupted and accreted. TDEs were first discovered in the X-ray regime and appear as luminous, giant-amplitude flares from inactive galaxies. The early X-ray observations with ROSAT also established the extreme X-ray spectral softness of these events with temperatures of order 50-100 eV that continues to be seen in the majority of more recently identified events. While the majority of X-ray TDEs has been identified from {\it inactive} galaxies and some showed the highest amplitudes of variability recorded from galaxy cores (amplitudes exceeding factors of 1000--6000), a small fraction of {\it active} galactic nuclei (AGN) has been found to be highly variable as well. In AGN, this so-called changing-look phenomenon often comes with a strong change in the optical broad emission lines, leading to Seyfert-type changes between class 1 and class 2. These two forms of activity represent the extremes of variability among active and quiescent galaxies, and have opened up a new window on understanding accretion physics under extreme conditions. Finally, we introduce the term ``frozen-look AGN'' to describe systems that show constant line emission despite strong/dramatic changes in the observed ionizing continuum. These systems are best explained by strong changes of absorption along our line-of-sight.