Astrometry-Only Detection of Microlensing Events with Gaia

Abstract

Astrometric microlensing events occur when a massive object passes between a distant source and the observer, causing a shift of the light centroid. The precise astrometric measurements of the Gaia mission provide an unprecedented opportunity to detect and analyze these events, revealing properties of lensing objects such as their mass and distance. We develop and test the Gaia Astrometric Microlensing Events (GAME) Filter, a software tool to identify astrometric microlensing events and derive lensing object properties. We generated mock Gaia observations for different magnitudes, number of Gaia visits, and events extending beyond Gaia's observational run. We applied GAME Filter to these datasets and validated its performance. We also assessed the rate of false positives where binary astrometric systems are misidentified as microlensing events. GAME Filter successfully recovers microlensing parameters for strong events. Parameters are more difficult to recover for short events and those extending beyond Gaia's run, where only a fraction of the events is observed. The astrometric effect breaks the degeneracy in the microlensing parallax present in photometric microlensing. For fainter sources, the observed signal weakens, reducing recovered events and increasing parameter errors. However, even for Gaia G-band magnitude 19, parameters can be recovered for Einstein radii above two mas. Observing regions with varying numbers of Gaia visits has minimal impact on filter accuracy when the number of visits exceeds 90. Additionally, even if the peak of a microlensing event lies outside Gaia's run, microlensing parameters can still be recovered. GAME Filter characterizes lenses with astrometry-only data for lens masses from approximately 1 to 20 solar masses and distances up to 6 kpc.