Expectations on the mass determination using astrometric microlensing by Gaia

TL;DR

This study evaluates Gaia's potential to measure stellar masses through astrometric microlensing, showing it can accurately determine masses for a subset of predicted events despite low observation cadence.

Contribution

The paper presents a simulation-based analysis of Gaia's capability to determine stellar masses via astrometric microlensing, highlighting achievable precision levels for upcoming events.

Findings

Gaia can detect astrometric deflections in 114 events.

Mass can be determined with better than 15% precision in 13 events.

Mass can be determined with better than 30% precision in 34 events.

Abstract

Context. Astrometric gravitational microlensing can be used to determine the mass of a single star (the lens) with an accuracy of a few percent. To do so, precise measurements of the angular separations between lens and background star with an accuracy below 1 milli-arcsecond at different epochs are needed. Hence only the most accurate instruments can be used. However, since the timescale is in the order of months to years, the astrometric deflection might be detected by Gaia, even though each star is only observed on a low cadence. Aims. We want to show how accurately Gaia can determine the mass of the lensing star. Methods. Using conservative assumptions based on the results of the second Gaia Data release, we simulated the individual Gaia measurements for 501 predicted astrometric microlensing events during the Gaia era (2014.5 - 2026.5). For this purpose we use the astrometric parameters of Gaia DR2, as well as an approximative mass based on the absolute G magnitude. By fitting the motion of lens and source simultaneously we then reconstruct the 11 parameters of the lensing event. For lenses passing by multiple background sources, we also fit the motion of all background sources and the lens simultaneously. Using a Monte-Carlo simulation we determine the achievable precision of the mass determination. Results. We find that Gaia can detect the astrometric deflection for 114 events. Further, for 13 events Gaia can determine the mass of the lens with a precision better than 15% and for 13 + 21 = 34 events with a precision of 30% or better.