Multiwavelength observations of the extraordinary accretion event AT2021lwx

Abstract

We present observations from X-ray to mid-infrared wavelengths of the most energetic non-quasar transient ever observed, AT2021lwx. Our data show a single optical brightening by a factor $>100$ to a luminosity of $7\times 10^{45}$ erg s${}^{-1}$, and a total radiated energy of $1.5\times 10^{53}$ erg, both greater than any known optical transient. The decline is smooth and exponential and the ultra-violet - optical spectral energy distribution resembles a black body with temperature $1.2\times 10^4$ K. Tentative X-ray detections indicate a secondary mode of emission, while a delayed mid-infrared flare points to the presence of dust surrounding the transient. The spectra are similar to recently discovered optical flares in known active galactic nuclei but lack some characteristic features. The lack of emission for the previous seven years is inconsistent with the short-term, stochastic variability observed in quasars, while the extreme luminosity and long timescale of the transient disfavour the disruption of a single solar-mass star. The luminosity could be generated by the disruption of a much more massive star, but the likelihood of such an event occurring is small. A plausible scenario is the accretion of a giant molecular cloud by a dormant black hole of $10^8-10^9$ solar masses. AT2021lwx thus represents an extreme extension of the known scenarios of black hole accretion.

11 pages, 5 figures, Accepted for publication in MNRAS

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• AT2021lwx: Another Neutrino-Coincident Tidal Disruption Event with a Strong Dust Echo?
• Scary Barbie: An Extremely Energetic, Long-Duration Tidal Disruption Event Candidate Without a Detected Host Galaxy at z = 0.995
• TiDES: The 4MOST Time Domain Extragalactic Survey
• AT2018dyk Revisited: a Tidal Disruption Event Candidate with Prominent Infrared Echo and Delayed X-ray Emission in a LINER Galaxy
• AT 2018dyk: tidal disruption event or active galactic nucleus? Follow-up observations of an extreme coronal line emitter with the Dark Energy Spectroscopic Instrument
• Discovery of a Featureless Tidal Disruption Event at z~1 with the Wide Field Survey Telescope