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

TL;DR

Gaia20fnr is a long-duration binary-lens microlensing event whose rich, multi-mission photometry plus spectroscopic constraints enable a full Keplerian orbital solution for the lens. The authors combine space-based parallax, orbital motion, and blend-light analysis to derive a lens composed of two near-solar-mass late-type stars at about $D_L\approx 0.54$ kpc, lens masses $M_{L,1}\approx0.46\,M_\odot$ and $M_{L,2}\approx0.52\,M_\odot$, and a period $P\approx 0.67$ yr with $K_1\approx 16.9$ km s$^{-1}$. The source is a K2 giant at $D_S\approx3.1$ kpc, and the event yields an angular Einstein radius $\theta_E\approx3.50$ mas, a physical Einstein radius $R_E\approx1.87$ AU, and a strong, testable lens–source geometry via future RV and high-resolution imaging as well as Gaia DR4/DR5 astrometric time-series data. This system serves as a benchmark for characterizing faint nearby binaries with microlensing and highlights the value of combining space-based spacecraft with ground-based networks for complex, long-duration events."

Abstract

The microlensing event Gaia20fnr is a long-duration, non-caustic-crossing binary-lens event at 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 four space telescopes: Gaia, NEOWISE, Swift, and TESS. We characterize 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 $D_{\rm S} = 3.10 \pm 0.10\,\mathrm{kpc}$ lensed by a stellar binary composed of $M_{\rm L,1} = 0.46 \pm 0.06\,M_\odot$ and $M_{\rm L,2} = 0.52 \pm 0.06\,M_\odot$ at a distance of $D_{\rm L} = 0.54 \pm 0.05\,\mathrm{kpc}$. The light curve exhibits strong signatures of orbital motion and requires a full Keplerian model with a period of $P = 0.67 \pm 0.04\,\mathrm{yr}$ and a radial-velocity semi-amplitude of $K_1 = 16.9 \pm 0.9\,\mathrm{km\,s^{-1}}$. Gaia20fnr is one of the few microlensing events for which a complete Keplerian binary-lens solution has been derived. The model can be tested with follow-up radial-velocity and high-resolution imaging observations as well as forthcoming Gaia DR4 and DR5 astrometric time-series data. Its long duration, multi-peak structure, and extensive coverage make it a benchmark for studying faint nearby low-mass binaries through microlensing.

Figures (11)

• Figure 1: Sky map view of the Milky Way with the location of Gaia20fnr marked with an orange circle. This illustrates the unique location of the microlensing event. The figure was made with MW-Plota and Gaia EDR3 data GaiaEDR3. On the right, a finding chart made from Pan-STARRS1 Panstarrs1 data via the ALADIN Tool aladinSky.
• Figure 2: Photometric observations of Gaia20fnr combining space-based observations, survey data, and follow-up observations gathered by BHTOM. The Swift UV data in the UVW1 and UVW2 filter are not shown for visibility reasons. Their data are at the same timestamps as the Swift U-band data, but with 1.6 mag and 2.8 mag higher magnitude values for UVW1 and UVW2, respectively. The dashed grey lines mark the times at which spectroscopic observations were obtained.
• Figure 3: Low-resolution spectra of the Gaia20fnr event observed by the SPRAT, GMOS-N and FLOYDS instruments. Vertical dashed lines present two Balmer lines, Mg I, and Ca II.
• Figure 4: Spectrum of the Gaia20fnr event (blue) observed by the SALT HRS spectrograph on the 06.02.2021 (left) and 04.09.2021 (right). Shown in red is the synthetic spectrum calculated for the best-fit atmospheric parameters.
• Figure 5: Spectrum of the Gaia20fnr event (blue) observed by the X-Shooter instrument on the VLT on the 02.10.2021. Shown in red is the synthetic spectrum calculated for the best-fit atmospheric parameters.