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

We report an exceptional mid-infrared flare in the Seyfert 1.8 NGC 3786. In the multiepoch data from the Wide-field Infrared Survey Explorer, the nuclear mid-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 significant variation in the corresponding light curve of the optical band from the Zwicky Transient Facility. This implies that the flare may have been heavily obscured by nuclear dust. Through follow-up spectroscopic observations with Gemini-North after the flare, we find that broad emission lines in Paα and Paβ newly appear, while the broad Hβ emission is marginally detected in the postflare spectrum. In addition, their central wavelengths are systematically redshifted up to 900 km s⁻¹ with respect to the narrow emission lines. This reveals that the flare is associated with the changing-look phenomenon from type 1.8 to type 1. Based on these findings, we argue that the flare is likely to originate from an obscured tidal disruption event, although extreme variation in the accretion rate may not be ruled out completely.