Gaia17bpp: A Giant Star with the Deepest and Longest Known Dimming Event

We report the serendipitous discovery of Gaia17bpp/2MASS J19372316+1759029, a binary star with a deep single large-amplitude dimming event of ∼4.5 mag that lasted over 6.5 yr. Using the optical-to-IR spectral energy distribution (SED), we constrain the primary star to be a cool giant M0III star with effective temperature T _eff = 3850 K and radius R = 58 R _⊙ . Based on the SED fitting, we obtained a bimodal posterior distribution of primary stellar masses with a stronger preference for a 1.5 M _⊙ mass star. Within the last 66 yr of photometric coverage, no other significant dimming events of this depth and duration were identified in the optical light curves. Using a Gaussian process, we fit a generalized Gaussian distribution to the optical and IR light curves and conclude that the dimming event exhibits moderate asymmetries from optical to IR. At the minimum of the dimming event, the Wide-field Infrared Survey Explorer color (W1–W2) differed by ∼0.2 mag relative to the primary star color outside the dimming event. The ingress and egress colors show a shallow reddening profile. We suggest that the main culprit of the dimming event is likely due to the presence of a large, optically thick disk transiting the primary giant star. By fitting a monochromatic transit model of an oblate disk transiting a star, we found good agreement with a slow-moving (0.005 km s ^−1 ) disk with a ∼1.4 au radius. We propose that Gaia17bpp belongs to a rare binary star population similar to the ϵ Aurigae system, which consists of a secondary star enshrouded by an optically thick debris disk.