Optical discovery of probable stellar tidal disruption flares (1009.1627v2)

Abstract: Using archival SDSS multi-epoch imaging data (Stripe 82), we have searched for the tidal disruption of stars by super-massive black holes in non-active galaxies. Two candidate tidal disruption events (TDEs) are identified. They have optical black-body temperatures 2 10^4 K and observed peak luminosities M_g=-18.3 and -20.4; their cooling rates are very low, qualitatively consistent with expectations for tidal disruption flares. Their properties are examined using i) SDSS imaging to compare them to other flares observed in the search, ii) UV emission measured by GALEX and iii) spectra of the hosts and of one of the flares. Our pipeline excludes optically identifiable AGN hosts, and our variability monitoring over 9 years provides strong evidence that these are not flares in hidden AGNs. The spectra and color evolution of the flares are unlike any SN observed to date, their strong late-time UV emission is particularly distinctive, and they are nuclear at high resolution, arguing against their being first cases of a previously-unobserved class of SNe or more extreme examples of known SN types. Taken together, the observed properties are difficult to reconcile with a SN or AGN-flare explanation, although an entirely new process specific to the inner few-hundred parsecs of non-active galaxies cannot be excluded. Our observed rate and method show the feasibility of obtaining a candidate TDE sample of hundreds of events and O(1) purity, using geometric resolution and host and flare color alone. A by-product of this work is quantification of the power-spectrum of extreme flares in AGNs.

• The paper identified two probable stellar tidal disruption flares using archival optical data from SDSS Stripe 82 with a methodology robust against SN and AGN contamination.
• The discovered flares exhibited properties inconsistent with known SNe or AGN, showing characteristic optical blackbody temperatures and unique luminosity/color evolution.
• This optical discovery method suggests that future large-scale optical surveys like LSST have the potential to detect hundreds or thousands of these tidal disruption events.

Optical Discovery of Probable Stellar Tidal Disruption Flares

The paper provides an in-depth analysis and identification of tidal disruption events (TDEs) using archival multi-epoch imaging data from the Sloan Digital Sky Survey (SDSS) Stripe 82. The primary objective of this research was to identify flares resulting from the tidal disruption of stars by supermassive black holes in dormant galaxies. This paper successfully identified two candidates for TDEs through a robust selection methodology designed to mitigate contamination from supernovae (SNe) and active galactic nuclei (AGN) flares.

Initially, the investigation employed a pipeline with significant rejection efficiency for non-TDE events. Upon selection, the candidates exhibited properties consistent with hypothesized characteristics of TDE flares, such as optical blackbody temperatures around $20,000\, K$ and relatively low cooling rates. These flare events were scrutinized through a comprehensive methodology comprising SDSS imaging, UV emission data from \textsl{GALEX}, and spectral analysis of hosts and flares.

Key Results

1. Observational Features:
   • The two TDE candidates reported peak optical luminosities corresponding to $M_g = -18.3$ and $-20.4$.
   • Analyzed flares demonstrated luminosity and color evolution inconsistent with any documented supernovae or AGN flares. The robust late-time UV emission further differentiated these events from known phenomena.
2. Rate and Detection Strategies:
   • An inferred rate suggests the potential for hundreds or even thousands of TDEs to be detected by current and forthcoming optical transient surveys, such as the Large Synoptic Survey Telescope (LSST).
3. Geometric and Host Properties:
   • The pipeline effectively utilized host galaxy properties to exclude AGNs. Additionally, the proximity of the flares to the galactic nucleus, along with a lack of host variability, supported the TDE nature of the candidates. This was further substantiated through spectral analysis, which indicated no significant AGN activity.
4. Characterization and Theoretical Conformance:
   • The flare properties, such as luminosity decay and cooling rates, challenged existing models of tidal disruption dynamics. The paper cited the necessity for further theoretical development to reconcile these observational discrepancies, indicating the current models might require refinement in parameters or additional facets of the disruption process.

Implications and Future Directions

The paper emphasizes that the capabilities demonstrated by the SDSS Stripe 82 in discovering TDEs through optical means suggest a promising avenue for future exploration of black hole physics and accretion phenomena. The paper exemplifies the utility of optical surveys in the identification of TDEs and encourages further development of survey methodologies and theoretical models to enhance the identification and understanding of such events.

Furthermore, the results compel a reevaluation of tidal disruption models, as observed features like the flare luminosities and decay rates deviate from extant predictions. This reassessment is critical for advancing the theoretical frameworks that describe the complex interactions underlying TDEs, with anticipated contributions expected from upcoming large-scale synoptic surveys, which are poised to yield a more extensive and diverse sample of these uncommon cosmic occurrences.