Predictions of Gaia's prize microlensing events are flawed

TL;DR

This paper reveals that many Gaia-predicted microlensing events are likely spurious due to data issues, and provides methods to improve event candidate selection, impacting future gravitational mass measurements.

Contribution

It identifies the high rate of false positives in Gaia microlensing predictions and offers quality criteria to refine candidate lists.

Findings

61% of predicted events are likely spurious

Background sources often duplicates or binary companions

Half of the events during Gaia's mission remain unidentified

Abstract

Precision astrometry from the second Gaia data release has allowed astronomers to predict 5,787 microlensing events, with 528 of these having maximums within the extended Gaia mission (J2014.5 - J2026.5). Future analysis of the Gaia time-series astrometry of these events will, in some cases, lead to precise gravitational mass measurements of the lens. We find that 61% of events predicted during the extended Gaia mission with sources brighter than G = 18 are likely to be spurious, with the background source in these cases commonly being either a duplicate detection or a binary companion of the lens. We present quality cuts to identify these spurious events and a revised list of microlensing event candidates. Our findings imply that half of the predictable astrometric microlensing events during the Gaia mission have yet to be identified.