A likely decade-long sustained tidal disruption event
  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.
