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.
