Gaia21blx: Complete Resolution of a Binary Microlensing Event in the Galactic Disk

Context. Gravitational microlensing is a method that is used to discoverplanet-hosting systems at distances of several kiloparsec in the Galactic diskand bulge. We present the analysis of a microlensing event reported by the Gaiaphotometric alert team that might have a bright lens. Aims. In order to inferthe mass and distance to the lensing system, the parallax measurement at theposition of Gaia21blx was used. In this particular case, the source and thelens have comparable magnitudes and we cannot attribute the parallax measuredby Gaia to the lens or source alone. Methods. Since the blending flux isimportant, we assumed that the Gaia parallax is the flux-weighted average ofthe parallaxes of the lens and source. Combining this assumption with theinformation from the microlensing models and the finite source effects we wereable to resolve all degeneracies and thus obtained the mass, distance,luminosities and projected kinematics of the binary lens and the source.Results. According to the best model, the lens is a binary system at 2.18 ±0.07 kpc from Earth. It is composed of a G star with 0.95± 0.17 M_⊙and a K star with 0.53 ± 0.07 M_⊙. The source is likely to be an Fsubgiant star at 2.38 ± 1.71 kpc with a mass of 1.10 ± 0.18 M_⊙. Both lenses and the source follow the kinematics of the thin-diskpopulation. We also discuss alternative models, that are disfavored by the dataor by prior expectations, however.