The plots on the left show the source trajectories (solid lines) relative to the lens (plus sign), projected onto the sky plane, as seen from the perspective of the observer on Earth (top panel, green line) and Spitzer Space Telescope (bottom panel, red line). The Gaia satellite is relatively close to our planet, so its perspective is very similar to terrestrial observer’s.
During the microlensing event, the light coming from the source splits into two images (yellow, elongated shapes), which, however, are so close to each other that during flux measurements we are unable to separate them and we see a single object.
The right panel shows what we measure in practice, i.e. the source brightness changes over time. It results from the fact that the total flux coming from two images is larger than the source flux outside of the event. The smaller the observed separation between the source and lens, the larger flux increase. The points on the right panel represent the measurements collected by various telescopes, and solid lines are fitted models. The perspective of Spitzer is shown in red, while ground-based and Gaia observations in green. In Spitzer's point of view, the source and the lens do not get as close to each other as they do for an observer on Earth, and therefore the amplification seen by this satellite is much lower.
In the lower part, we present the image reconstructions based on the interferometric measurements, which were made around the maximum of the event. Observations were made with four 8-meter telescopes operating simultaneously as a single instrument (VLTI). Thanks to them, we could directly see - for the first time in history - the rotation of the source images around the lens. More information on the Gaia19bld event can be found in Cassan et al. 2021, Rybicki et al. 2021 and Bachelet et al. 2021