Gaia20fnr: A binary-lens microlensing event with full orbital motion revealed by four space telescopes

The microlensing event Gaia20fnr is a long-duration, non-caustic-crossing binary-lens event located at a high Galactic latitude. Triggered by a photometric rise detected by the Gaia space mission, the event was followed up with observations from multiple ground-based facilities and observed by four space telescopes: Gaia, NEOWISE, Swift, and TESS. We characterised the Gaia20fnr microlensing system by determining the physical and orbital properties of the binary lens, the nature of the luminous source, and the kinematics of both the source and the lens. We employed a binary-lens microlensing model including full Keplerian orbital motion and annual microlens parallax to fit the photometric data. The event is best explained by a K2 giant source at D S = 3.10 ± 0.10kpc lensed by a stellar binary composed of M L,1 = 0.46 ± 0.06 M ⊙ and M L,2 = 0.52 ± 0.06 M ⊙ at a distance of D L = 0.54 ± 0.05 kpc. The light curve exhibits strong signatures of orbital motion and requires a full Keplerian model with a period of P = 0.67 ± 0.04 yr and a predicted semi-amplitude of K 1 = 16.9 ± 0.9 km s −1 . Gaia20fnr is one of the few microlensing events for which a complete Keplerian binary-lens solution has been derived. The model may be further constrained with the forthcoming Gaia DR4 and DR5 astrometric time-series data, while high-angular-resolution imaging and radial-velocity follow-ups could become feasible once the source and lens are sufficiently separated, despite the observational challenges posed by the bright source. Its long duration, multi-peak structure, and extensive coverage from both space- and ground-based facilities make it a benchmark for studying faint nearby low-mass binaries through microlensing.