Infrared echo and late-stage rebrightening of nuclear transient
  PS1-10adi: exploring Torus by tidal disruption event in active galactic
  nuclei
  Tidal disruption events (TDEs) in active galactic nuclei (AGNs) have been
overlooked for a long time but tentatively been investigated recently. We
report the discovery of a long-lasting luminous mid-infrared (mid-IR) flare in
PS1-10adi, which is a newly-identified highly energetic transient event
occurred in AGN. The IR luminosity of PS1-10adi, as well as other analogous
events, are at least one order of magnitude higher than all known supernova,
but can be well interpreted as the dust echoes of TDEs, whose ultra-high IR
energy is reprocessed from the dusty torus around the black hole. The torus
dust is sublimating during the early stage of the outburst and probably lead to
the observed rapid emergence of Fe II lines. Moreover, the UV-optical
rebrightening and contemporaneous X-ray onset after $\sim1500$ rest-frame days
since the optical peak is also an intriguing feature of PS1-10adi, which could
be attributed to the interaction between the high-velocity outflow and torus.
We suggest that the luminous IR echo is a very typical phenomenon of TDEs in
AGNs and may provide us an ideal opportunity to explore the torus properties.
