A likely decade-long sustained tidal disruption event

Abstract: Multiwavelength flares from tidal disruption and accretion of stars can be used to find and study otherwise dormant massive black holes in galactic nuclei. Previous well-monitored candidate flares are short-lived, with most emission confined to within ~1 year. Here we report the discovery of a well observed super-long (>11 years) luminous soft X-ray flare from the nuclear region of a dwarf starburst galaxy. After an apparently fast rise within ~4 months a decade ago, the X-ray luminosity, though showing a weak trend of decay, has been persistently high at around the Eddington limit (when the radiation pressure balances the gravitational force). The X-ray spectra are generally soft (steeply declining towards higher energies) and can be described with Comptonized emission from an optically thick low-temperature corona, a super-Eddington accretion signature often observed in accreting stellar-mass black holes. Dramatic spectral softening was also caught in one recent observation, implying either a temporary transition from the super-Eddington accretion state to the standard thermal state or the presence of a transient highly blueshifted (~0.36c) warm absorber. All these properties in concert suggest a tidal disruption event of an unusually long super-Eddington accretion phase that has never been observed before.

• The paper reveals an unprecedented, eleven-year tidal disruption event marked by a fast-rise and slow-decay soft X-ray flare.
• It employs multi-year observations from Chandra and XMM-Newton to track detailed spectral and luminosity evolution near the Eddington limit.
• The study implies that sustained super-Eddington accretion from a disrupted massive star challenges existing TDE models and black hole growth theories.

A Likely Decade-long Sustained Tidal Disruption Event: An Insightful Overview

The research paper entitled "A likely decade-long sustained tidal disruption event" presents the intriguing discovery of a luminous and prolonged soft X-ray flare emanating from the nuclear region of a dwarf starburst galaxy. The event, hypothesized as a tidal disruption event (TDE), stands out with its remarkable duration of over eleven years, a characteristic atypical for TDEs which are usually short-lived phenomena.

Background and Observation Details

The X-ray source, designated 3XMM J150052.0+015452 or XJ1500+0154, was detected across multiple observations conducted by advanced X-ray observatories such as Chandra and XMM-Newton over several years. Notably, these observations span from 2005 to 2016, allowing for a comprehensive analysis of its X-ray spectral and luminosity evolution. The galaxy hosting the source, observed at a redshift of 0.145, shows no evidence of persistent galactic nuclear activity, aligning with the starburst profile.

Key Findings and Spectral Characteristics

The most striking feature of XJ1500+0154 is its fast-rise and very-slow-decay luminosity profile. Following an intense rise in soft X-ray luminosity within months in 2005, the event plateaued at near-Eddington luminosity levels, demonstrating a mere gradual decline over the subsequent decades. This pattern is supported by spectral analyses revealing generally soft characteristics consistent with Eddington-limit accretion, further characterized by a transient phase of harder spectral qualities implying super-Eddington accretion signatures.

Implications of a Super-Eddington Tidal Disruption Event

The paper posits that the prolonged super-Eddington accretion phase, evidenced by the decade-spanning flare, likely results from the tidal disruption of a stellar body, potentially a very massive star, by a supermassive black hole (SMBH) with an estimated mass of about $10^6$ solar masses. This is unique, as traditional TDE models predict much shorter durations for Eddington-limited accretion. The phenomenon, supported by spectral data indicative of a Comptonized optically thick, low-temperature corona, furnishes a new observational perspective on black hole accretion physics, necessitating models that account for prolonged super-Eddington effects.

Challenges in Interpretation

Though the TDE hypothesis is robust, alternative explanations such as a highly variable active galactic nucleus (AGN) cannot be definitively ruled out. However, the unusually high degree of variability in X-ray emissions and swift spectral transitions, not matched by traditional AGNs, supports the TDE scenario. The lack of optical emission indicative of AGN activity further aligns this event with the characteristics of a TDE.

Broader Astrophysical Context and Future Prospects

The implications of observing such prolonged super-Eddington accretion include the need to reconsider black hole growth models, especially in the context of highly energetic early universe quasars. Understanding these accretion mechanisms could elucidate pathways for rapid black hole growth under high accretion efficiency constraints.

Future observations will test these interpretations, focusing on observing spectral transitions that may resolve between competing theoretical models. The study advocates for continued exploration of TDE candidates, encouraging a broadened search in X-ray wavelengths as a means to uncover the diverse behaviors exhibited by accretion phenomena around SMBHs.

In conclusion, this research spotlights XJ1500+0154 as an exemplar of both observational novelty and theoretical challenge, prompting the astrophysics community to reevaluate and expand existing frameworks governing TDEs and accretion onto SMBHs.