2017ApJ...838..154C

We analyze the single microlensing event OGLE-2015-BLG-1482 simultaneously observed from two ground-based surveys and from Spitzer. The Spitzer data exhibit finite-source effects that are due to the passage of the lens close to or directly over the surface of the source star as seen from Spitzer. Such finite-source effects generally yield measurements of the angular Einstein radius, which when combined with the microlens parallax derived from a comparison between the ground-based and the Spitzer light curves yields the lens mass and lens-source relative parallax. From this analysis, we find that the lens of OGLE-2015-BLG-1482 is a very low-mass star with a mass 0.10±0.02 M_☉ or a brown dwarf with a mass 55±9 M_J, which are located at D_LS=0.80±0.19 kpc and D_LS=0.54±0.08 kpc, respectively, where D_LS is the distance between the lens and the source, and thus it is the first isolated low-mass microlens that has been decisively located in the Galactic bulge. The degeneracy between the two solutions is severe (Δχ²=0.3). The fundamental reason for the degeneracy is that the finite-source effect is seen only in a single data point from Spitzer, and this single data point gives rise to two solutions for ρ, the angular size of the source in units of the angular Einstein ring radius. Because the ρ degeneracy can be resolved only by relatively high-cadence observations around the peak, while the Spitzer cadence is typically ∼1 day^–1, we expect that events for which the finite-source effect is seen only in the Spitzer data may frequently exhibit this ρ degeneracy. For OGLE-2015-BLG-1482, the relative proper motion of the lens and source for the low-mass star is µ_rel=9.0±1.9 mas yr^–1, while for the brown dwarf it is 5.5±0.5 mas yr^–1. Hence, the degeneracy can be resolved within ∼10 years from direct-lens imaging by using next-generation instruments with high spatial resolution.