OGLE-2015-BLG-1771Lb: A Microlens Planet Orbiting an Ultracool Dwarf?

We report the discovery and the analysis of the short ( ${t}_{{\rm{E}}}\lt 5$ days) planetary microlensing event, OGLE-2015-BLG-1771. The event was discovered by the Optical Gravitational Lensing Experiment, and the planetary anomaly (at I ∼ 19) was captured by The Korea Microlensing Telescope Network. The event has three surviving planetary models that explain the observed light curves, with planet-host mass ratio q ∼ 5.4 × 10^-3, 4.5 × 10^-3 and 4.5 × 10^-2, respectively. The first model is the best-fit model, while the second model is disfavored by Δχ² ∼ 3. The last model is strongly disfavored by Δχ² ∼ 15 but not ruled out. A Bayesian analysis using a Galactic model indicates that the first two models are probably composed of a Saturn-mass planet orbiting a late M dwarf, while the third one could consist of a super-Jovian planet and a mid-mass brown dwarf. The source-lens relative proper motion is μ_rel ∼ 9 mas yr^-1, so the source and lens could be resolved by current adaptive-optics instruments in 2020 if the lens is luminous.