Astrometry-only detection of microlensing events with Gaia

¹ Institute of Physics of the Czech Academy of Sciences, Na Slovance 1999/2,182 21 Praha 8, Prague, Czech Republic
² Center for Astrophysics and Cosmology, University of Nova Gorica, Vipavska 11c, 5270 Ajdovščina, Slovenia
³ Astronomical Observatory, University of Warsaw, Al. Ujazdowskie 4, 00-478 Warszawa, Poland
⁴ Aalta Lab d.o.o., Soška cesta 17, 5250 Solkan, Slovenia
⁵ Astronomical Observatory, Volgina 7, 11000 Belgrade, Serbia
⁶ Argelander-Institut für Astronomie, Universität Bonn, Auf dem Hügel 71, 53121 Bonn, Germany
⁷ Research School of Astronomy and Astrophysics, Australian National University, Mount Stromlo Observatory, Cotter Road Weston Creek, ACT 2611, Australia
⁸ Zentrum für Astronomie der Universität Heidelberg, Astronomisches Rechen-Institut, Mönchhofstr. 12-14, 69120 Heidelberg, Germany

^★ Corresponding author: jankovic@fzu.cz

Received: 19 December 2024
Accepted: 24 May 2025

Abstract

Context. 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 analyse these events, revealing properties of lensing objects such as their mass and distance.

Aims. We developed and tested the Gaia Astrometric Microlensing Events (GAME) Filter, a software tool for identifying astrometric microlensing events and deriving lensing object properties.

Methods. We generated mock Gaia observations for different magnitudes, different numbers 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 involving binary astrometric systems misidentified as microlensing events.

Results. 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 are 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, the parameters can be recovered for Einstein radii ≳2 mas. Observing regions with varying numbers of Gaia visits has a minimal impact on filter accuracy when the number of visits is ≳90. Additionally, even if the peak of a microlensing event lies outside Gaia’s run, microlensing parameters can still be recovered. GAME filter characterises lenses with astrometry-only data for lens masses ranging from approximately 1–20 M_⊙ and distances of up to ≈6 kpc.

Conclusions. GAME Filter reveals the astrometric potential in microlensing events. Our results indicate that astrometric data can enhance photometric detections and refine constraints on microlensing parameters.