Hidden No More: Spotlight on tidal disruption events in active galactic nuclei

Tidal disruption events (TDEs) are typically discovered in previously quiescent galaxies. However, earlier studies have revealed a handful of TDEs occurring in pre-existing active galactic nuclei (AGNs). We discuss AT2019aalc, a promising TDE candidate in an AGN, and compare it to similar sources. We also explore Bowen fluorescence flares, a newly identified class of flaring supermassive black holes, as potential members of the TDE in AGN transient class. We aim to connect the observed properties of these flares with the expectations of TDE-in-AGN simulations.

💡 Research Summary

This paper investigates the phenomenon of tidal disruption events (TDEs) occurring within pre-existing active galactic nuclei (AGNs), a challenging class of transients to identify due to the complex background activity. The study centers on the analysis of AT2019aalc, a nuclear transient in a broad-line Seyfert-1 galaxy that exhibited two distinct optical flares separated by about four years. The multi-wavelength properties of AT2019aalc show a compelling mix of classic TDE signatures—such as UV brightness, very soft X-ray emission, a delayed radio flare, and an infrared dust echo—and peculiar features indicative of an AGN environment. These peculiarities include an optical decline not following the canonical t^(-5/3) law and the presence of strong, variable high-ionization coronal lines alongside a relatively narrow broad component of the Balmer lines (~3000 km/s), which is narrower than typical TDEs.

The author positions AT2019aalc within the newly identified class of Bowen Fluorescence Flares (BFFs), characterized by prominent emission from the Bowen fluorescence line complex (around 4640 Å) and frequent coronal line emission. A comparative analysis reveals striking similarities between AT2019aalc and other transients like AT2021acak, suggesting they may belong to the same BFF class. The paper then expands the discussion to the broader population of known BFFs, noting their diversity in X-ray properties and common occurrence in previously known, low-level AGNs.

A key insight is the proposal that BFFs may represent the long-theorized but observationally elusive population of TDEs happening in AGNs. The observed features in several BFFs—such as double flares, bumps in the light curve, quasi-periodic oscillations (QPOs) in optical and soft X-rays, and complex Balmer line profiles—are linked to theoretical predictions from TDE-AGN simulations. These simulations suggest that interactions between the stellar debris stream and the pre-existing AGN accretion disk can lead to phenomena like disk precession (potentially explaining QPOs and line profiles) and geometric obscuration (potentially explaining light curve dips or bumps). Alternative or complementary explanations involving pure accretion disk instabilities are also considered.

The paper concludes that systematic future monitoring of BFFs, enabled by surveys like the Vera Rubin Observatory’s LSST, will be crucial to test these models and understand whether BFFs indeed constitute the hidden population of TDE-AGNs, thereby refining TDE rates and providing insights into AGN fueling mechanisms and disk dynamics.