2019MNRAS.483.4210M

We used Gaia Data Release 2 to search for upcoming photometric microlensing events, identifying two candidates with high amplification. In the case of candidate 1, a spectrum of the lens (l1) confirms it is a usdM3 subdwarf with mass ≃0.11M_☉, while the event reaches maximum amplification of 20⁺²⁰_–10 mmag on 2019 November 3 (±1d). For candidate 2, the lens (l2) is a metal-poor M dwarf with mass ≃0.38M_☉ derived from spectral energy distribution fitting, and maximum amplification of 10⁺⁴⁰_–10 mmag occurs on 2019 June 3 (±4d). This permits the use of a new algorithm for mass inference on the microlens. Given the predicted time, the photometric light curve of these events can be sampled by targeted photometric campaigns. The light curve is a function of the unknown lens mass, together with eight other parameters for all of which Gaia provides measurements and uncertainties. Leveraging this prior information on the source and lens provided by Gaia's astrometric solution, combined with a photometric campaign, we show that the masses of 11 and l2 are likely to be inferred with 20-27 and 30-42 per cent precision, respectively. This mass measurement for l1 will provide an important benchmark for low-mass stars in the halo. We show for both l1 and l2 the precision at which their masses can be inferred will likely be limited by the photometric measurements and not by the priors provided by Gaia Data Release 2.