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We report an exceptional mid-infrared flare in the Seyfert 1.8 NGC 3786. Inthe multi-epoch data from the Wide-field Infrared Survey Explorer, the nuclearmid-infrared brightness of NGC 3786 appears to vary substantially up to$0.5-0.8$ mag around mid-2020. However, there is no evidence of significantvariation in the corresponding light curve of the optical band from the ZwickyTransient Facility. This implies that the flare may have been heavily obscuredby nuclear dust. Through follow-up spectroscopic observations with Gemini-Northafter the flare, we find that broad emission lines in ${\rm Pa}\alpha$ and${\rm Pa}\beta$ newly appear, while the broad ${\rm H}\beta$ emission ismarginally detected in the post-flare spectrum. In addition, their centralwavelengths are systematically redshifted up to 900 km s$^{-1}$ with respect tothe narrow emission lines. This reveals that the flare is associated with thechanging-look phenomenon from type 1.8 to type 1. Based on these findings, weargue that the flare is likely to originate from an obscured tidal disruptionevent, although extreme variation in the accretion rate may not be ruled outcompletely.

A Mid-infrared Flare in the Seyfert Galaxy NGC 3786: A Changing-look Event Triggered by an Obscured Tidal Disruption Event?