The Unusual Triple-Flaring Transient AT 2021aeuk in the Active Galaxy SDSS J161259.83+421940.3: Evidence for a Tidal Disruption Event?

Core Concepts

AT 2021aeuk, a transient event in the active galaxy SDSS J161259.83+421940.3, has displayed three optical flares within five years, challenging traditional explanations like blazar activity or supernovae. The event's characteristics, including a long-term decay, infrared echo, and spectral features, suggest a tidal disruption event (TDE) as a possible explanation, although further observations are needed to confirm its nature.

Abstract

Bao, D. et al. (2024). Gleeok’s Fire-breathing: Triple Flares of AT 2021aeuk within Five Years from the Active Galaxy SDSS J161259.83+421940.3. arXiv preprint arXiv:2311.16726v2.

This paper investigates the nature of AT 2021aeuk, an unusual transient event characterized by three distinct optical flares occurring within a span of five years. The authors aim to determine the most likely physical mechanism responsible for these flares, considering possibilities such as blazar activity, supernovae, and tidal disruption events (TDEs).

Key Insights Distilled From

Gleeok's Fire-breathing: Triple Flares of AT 2021aeuk within Five Years from the Active Galaxy SDSS J161259.83+421940.3

by Dong-Wei Bao... at arxiv.org 11-12-2024

https://arxiv.org/pdf/2311.16726.pdf

Deeper Inquiries

How might the presence of a radio-loud AGN in the host galaxy influence the evolution and observational signatures of a TDE?

The presence of a radio-loud AGN in the host galaxy can significantly impact both the evolution and observational signatures of a TDE, leading to complexities in their detection and interpretation. Here's a breakdown of the potential influences:
Evolutionary Impacts:

Enhanced TDE Rate: Some theoretical models (e.g., Karas & ˇSubr 2007; McKernan et al. 2022) suggest that the presence of an AGN accretion disk can actually enhance the TDE rate. The gravitational perturbations from the AGN disk can disrupt the orbits of stars in the nuclear star cluster, funneling them towards the central black hole.
Disk-Jet Interactions: The relativistic jets launched by the radio-loud AGN can interact with the infalling debris stream from the disrupted star. This interaction could lead to enhanced emission across multiple wavelengths, potentially complicating the interpretation of the TDE's light curve and spectral features.
Reprocessing by AGN Structures: The AGN environment is rich in gas and dust. The intense radiation from both the AGN and the TDE can be reprocessed by these structures, leading to delayed emission features in the infrared and other wavelengths. This reprocessing can make it challenging to disentangle the TDE emission from the AGN's baseline activity.
Observational Impacts:

Increased Variability: AGNs themselves are intrinsically variable sources. This inherent variability can obscure or mimic the observational signatures of a TDE, making it difficult to distinguish between the two phenomena.
Spectral Contamination: The AGN emission, particularly in the optical and ultraviolet bands, can contaminate the spectrum of the TDE. This contamination can mask key spectral features used to identify and characterize TDEs, such as broad emission lines and the characteristic thermal continuum.
Radio Emission Confusion: Radio-loud AGNs are powerful sources of radio emission. If the TDE also produces significant radio emission, it can be challenging to separate the contributions from the AGN and the TDE.
In essence, the presence of a radio-loud AGN introduces a significant level of complexity when searching for and studying TDEs. It requires careful analysis of multi-wavelength data, detailed modeling of the AGN environment, and a good understanding of the potential interplay between AGN activity and TDE signatures.

Could the observed flares be explained by a different mechanism altogether, such as a massive stellar outburst or a previously unknown type of AGN activity?

While the context focuses on TDEs, it's crucial to consider alternative explanations for the observed flares in AT 2021aeuk. Here are some possibilities:
Massive Stellar Outbursts:

Luminous Blue Variables (LBVs): LBVs are massive, evolved stars prone to episodic outbursts that can release tremendous amounts of energy, mimicking supernovae in brightness. However, LBVs typically exhibit characteristic spectral features during their outbursts, such as strong hydrogen and helium emission lines with P Cygni profiles, which may or may not be present in AT 2021aeuk's spectra.
Supernova Imposters: Certain types of supernovae, like Type IIn, can exhibit complex light curves with multiple peaks and extended durations. These events are often associated with significant circumstellar interaction, which could potentially explain some of the observed features in AT 2021aeuk.
Unusual AGN Activity:

Variable Accretion Rate: AGN accretion disks are not static structures. Variations in the accretion rate onto the black hole can lead to significant changes in the AGN's luminosity across all wavelengths. It's possible that the observed flares are a manifestation of an unusual or episodic accretion event onto the central black hole in J1612.
Disk Instabilities: Accretion disks can be subject to various instabilities that lead to changes in their structure and emission properties. These instabilities could potentially produce flares with the observed timescales and amplitudes.
Binary Black Hole Interactions: If J1612 harbors a binary black hole system, the orbital interactions between the two black holes could periodically perturb the accretion flow, leading to outbursts of energy.
Challenges and Considerations:

Rarity of Events: Many of these alternative scenarios involve relatively rare events. The probability of observing three such events within a five-year timeframe in a single object is statistically low.
Spectral and Multi-wavelength Constraints: Obtaining high-quality spectra during the flares and securing comprehensive multi-wavelength coverage (e.g., X-ray, radio) would be crucial to differentiate between these possibilities.
Further observations and detailed modeling are essential to definitively determine the physical mechanism responsible for the flares in AT 2021aeuk.

What are the implications of finding a potential TDE in an active galaxy for our understanding of the demographics and occurrence rates of such events?

Finding a potential TDE in an active galaxy like AT 2021aeuk has significant implications for our understanding of TDE demographics and occurrence rates:

Challenging Current Selection Biases: Current TDE searches often intentionally exclude active galaxies to avoid confusion with AGN variability. This introduces a selection bias that could lead to an underestimation of the true TDE rate. If TDEs are indeed more common in active galaxies, as some models suggest, then our current understanding of their occurrence rates needs revision.
Insights into Nuclear Star Clusters: TDE rates are closely linked to the dynamics and properties of nuclear star clusters surrounding supermassive black holes. Finding TDEs in active galaxies provides a unique opportunity to study these clusters in environments significantly different from quiescent galaxies.
Understanding TDE-AGN Connections: The discovery could point towards a potential connection between TDEs and AGN activity. It raises the question of whether TDEs could be triggered by or even contribute to the fueling of AGNs.
Refining TDE Models: Modeling TDEs in active galaxies presents a new set of challenges due to the complex and energetic environment. Successfully modeling such events would lead to more robust and comprehensive TDE models.
Overall Impact:
Confirming a TDE in an active galaxy would necessitate a reevaluation of our current understanding of TDE demographics and their relationship with AGN activity. It would open up new avenues of research into the interplay between these energetic phenomena and provide valuable insights into the dynamics of galactic nuclei.