On April 18, 2019, the European Space Agency's Gaia Mission alerted astronomers worldwide to an unusually bright but fleeting celestial event: the gravitational microlensing event Gaia19bld. The temporary, chance alignment between two unrelated star systems produced twin images of the background star and gave scientists their first opportunity to actually observe the arc-shaped images move in real time, unlocking key information.

Follow-up photometric and spectroscopic observations performed by LCO instruments gave the angular separation between the arcs. In parallel, the PIONIER instrument at the European Southern Observatory's Very Large Telescope Interferometer (VLTI) observed the images evolving with time during the event peak. Combining all these data for the first time enabled the determination of the mass of the microlens object to an unprecedented accuracy.

Gravitational microlensing is a powerful technique for measuring the mass of isolated, faint or non-luminous objects in the Milky Way. In Galactic microlensing, a foreground massive object (the 'lens') crosses the line-of-sight of a background source star and splits its disk-shaped image into multiple and distorted images. The separation of these images for stellar-mass lenses is so small that astronomers normally cannot distinguish them, measuring instead the overall increase of the total light received from the source (called 'magnification') as the star systems move into and then out of alignment.

The distinct images produced by a stellar lens have only been detected once before and their real-time motion has never before been observed, despite having been predicted decades earlier. As microlensing does not rely on the light emitted by the lenses themselves, it can be used to discover distant planets (even those without a host star), faint brown dwarfs and white dwarfs, as well as neutron stars and stellar mass black holes.

In most cases, a combination of photometric and spectroscopic observations are required to measure accurately the mass of the microlens.

In a recent paper published in Astronomy and Astrophysics, LCO postdoc Dr. Etienne Bachelet details the use of spectroscopic and imaging observations from LCO for the calculation of the mass and distance of the Gaia19bld lens. Both low- and high-resolution spectroscopy from multiple sites around the world were obtained during the event.

Gaia19bld is a spectacular showcase of the possibilities flowing from multi-faceted observations of microlensing events. Using two space satellites, ground-based survey telescopes, a follow-up network of smaller telescopes, high resolution spectroscopy and interferometric measurements, it was possible to accurately characterise the lens and the source, which would not be possible with any of these channels individually.

This work demonstrates the potential of the spectroscopic follow-up of microlensing events. It allows the precise characterization of the source star stellar parameters, which are critical to unlock the physical characteristics of the hidden lensing system. This is especially useful for events in the Galactic Disk, where the distance to the source and how much it might be dimmed by intervening dust are not well known.

It is expected that the methods described in this work will be used routinely in the era of the new generation of all-sky surveys currently under development.In particular, the Legacy Survey of Space and Time will detect thousands of events every year that will require spectroscopic monitoring to better characterize their properties and will ultimately increase our understanding of faint objects in the entire Milky Way.

Dr. Bachelet shared his enthusiasm about the discovery, "For the first time, we saw the motion of the two images created by the gravity field of the microlens! More importantly, these interferometric observations allow an independent estimation of the properties of the microlens, which was in perfect agreement with the measurements made with other telescopes."

Research Report: "A spectroscopic follow-up for Gaia19bld"

Related Links
Las Cumbres Observatory.
The Physics of Time and Space

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