Gaia20fnr Microlensing Reveals Stellar Binary System at Distance

Scientists have unveiled unprecedented detail regarding a rare microlensing event, Gaia20fnr, revealing the full orbital motion of a binary lens system. M. Wicker, Ł. Wyrzykowski, and M. Hundertmark, together with collaborators including K. A. Rybicki, P. Zieliński, and E. Stonkutė, conducted a comprehensive analysis using photometric and astrometric data from Gaia, NEOWISE, Swift, and TESS to characterise this exceptional event. The significance of Gaia20fnr lies in the fact that it is one of only a handful of microlensing events for which a complete Keplerian binary-lens solution has been derived, enabling precise determination of both physical and orbital properties. The event involves a K2 giant source star being lensed by a nearby stellar binary, providing a rare benchmark for studying faint, low-mass binary systems that are otherwise difficult to observe through conventional techniques.

The research team employed a sophisticated binary-lens microlensing model that incorporates full Keplerian orbital motion and annual microlens parallax, allowing them to fit the multi-band photometric data with remarkable precision. The light curve exhibited strong and unambiguous signatures of orbital motion, necessitating the use of a fully dynamical Keplerian model rather than simpler linear approximations. Their analysis shows that the lens system consists of two stars with masses of 0.46 ± 0.06 M⊙ and 0.52 ± 0.06 M⊙, located at a distance of 0.54 ± 0.05 kpc from the Sun. The binary components orbit each other with a period of 0.67 ± 0.04 years, a semi-major axis of 0.76 ± 0.02 AU, and a predicted radial-velocity semi-amplitude of 16.9 ± 0.9 km s⁻¹.

The Gaia20fnr event is characterised by its long duration, multi-peak magnification structure, and exceptionally dense temporal coverage from both space- and ground-based observatories. These features proved crucial for resolving the complex magnification patterns produced by the rotating binary lens and for breaking common parameter degeneracies that typically limit microlensing analyses. The team simultaneously modelled the photometric data, orbital motion, and microlens parallax effects, enabling a robust and self-consistent determination of the lens system’s physical parameters. As a result, Gaia20fnr now stands as one of the most comprehensively characterised binary-lens microlensing events to date.

Early alerts from Gaia and coordinated follow-up observations through the Black Hole Target and Observation Manager (BHTOM) enabled high-cadence monitoring during the three major magnification episodes, as well as extensive baseline coverage before and after the event. This observational strategy allowed the researchers to capture subtle deviations in the light curve that directly encode the binary’s orbital motion. The source star was identified as a K2-type giant located at approximately 3.1 kpc, while the lens system was found to be a nearby binary composed of an early-M dwarf and a late-K dwarf, making the system particularly well suited for future follow-up observations.

The study further reports that the observed ultraviolet emission from the source is consistent with the derived stellar parameters, indicating that the hotter source star dominates the flux at shorter wavelengths. While the current solution is robust, the authors emphasise the importance of independent verification. The model’s predictions can be tested through radial-velocity monitoring, high-resolution imaging, and forthcoming astrometric time-series data from Gaia DR4 and DR5, which are expected to directly detect the orbital motion of the lens system.

Beyond the characterisation of a single event, this work demonstrates the powerful synergy between space-based astrometry, multi-wavelength photometry, and ground-based follow-up, establishing microlensing as a uniquely sensitive probe of faint, nearby, low-mass binary systems. The findings have important implications for population studies of stellar binaries, helping to address selection biases inherent in traditional detection methods such as radial-velocity and direct imaging surveys. Looking ahead, upcoming high-cadence surveys from the Vera C. Rubin Observatory (LSST) and the Nancy Grace Roman Space Telescope are expected to dramatically increase the discovery rate of similarly complex microlensing events, enabling a more complete census of binary systems in the Galaxy. Gaia20fnr thus serves not only as a benchmark system but also as a compelling demonstration of the future potential of microlensing for stellar astrophysics and binary evolution studies.