Mid-infrared flare of TDE candidate PS16dtm: dust echo and implications
  for the spectral evolution
  PS16dtm was classified as a candidate tidal disruption event (TDE) in a dwarf
Seyfert 1 galaxy with low-mass black hole ($\sim10^6M\odot$) and has presented
various intriguing photometric and spectra characteristics. Using the archival
WISE and the newly released NEOWISE data, we found PS16dtm is experiencing a
mid-infrared (MIR) flare which started $\sim11$ days before the first optical
detection. Interpreting the MIR flare as a dust echo requires close
pre-existing dust with a high covering factor, and suggests the optical flare
may have brightened slowly for some time before it became bright detectable
from the ground. More evidence is given at the later epochs. At the peak of the
optical light curve, the new inner radius of the dust torus has grown to much
larger size, a factor of 7 of the initial radius due to strong radiation field.
At $\sim150$ days after the first optical detection, the dust temperature has
dropped well below the sublimation temperature. Other peculiar spectral
features shown by PS16dtm are the transient, prominent FeII emission lines and
outflows indicated by broad absorption lines detected during the optical flare.
Our model explains the enhanced FeII emission from iron newly released from the
evaporated dust. The observed broad absorption line outflow could be explained
by accelerated gas in the dust torus due to the radiation pressure.
