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We report the discovery of an exceptional MIR flare in a Type 2 AGN, SDSSJ165726.81+234528.1, at $z=0.059$. This object brightened by 3 mag in theWide-field Infrared Survey Explorer (WISE) $W1$ and $W2$ bands between 2015 and2017 (and is fading since 2018), without significant changes ($\lesssim$ 0.2mag) in the optical over the same period of time. Based on the WISE lightcurves and near-IR imaging, the flare is more significant at longerwavelengths, suggesting an origin of hot dust emission. The estimated blackhole mass ($\sim 10^{6.5}\,M_\odot$) from different methods places its peakbolometric luminosity around the Eddington limit. The high luminosity of theMIR flare and its multi-year timescale suggest that it most likely originatedfrom reprocessed dust radiation in an extended torus surrounding the AGN,instead of from stellar explosions. The MIR color variability is consistentwith known changing-look AGN and tidal disruption events (TDEs), butinconsistent with normal supernovae. We suggest that it is a turning-on Type 2AGN or TDE, where the optical variability is obscured by the dust torus duringthe transition. This MIR flare event reveals a population of dramatic nucleartransients that are missed in the optical.

An Unusual Mid-Infrared Flare in a Type 2 AGN: An Obscured Turning-on AGN or Tidal Disruption Event?