Microlens mass determination for Gaia's predicted photometric events
  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 $\approx 0.11 M_\odot$, while the event reaches maximum amplification of
$20^{+20}_{-10}$ mmag on November 3rd 2019 ($\pm$1d). For candidate 2, the lens
(l2) is a metal-poor M dwarf with mass $\approx 0.38 M_\odot$ derived from
spectral energy distribution (SED) fitting, and maximum amplification of
$10^{+40}_{-10}$ mmag occurs on June 3rd 2019 ($\pm$4d). This permits a new
algorithm for mass inference on the microlens. Given the predicted time, the
photometric lightcurve of these events can be densely sampled by ground-based
telescopes. The lightcurve is a function of the unknown lens mass, together
with 8 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, and assuming that a ground-based
campaign can provide 50 measurements at mmag precision, we show for example
that the mass of l1 can be recovered to within 20 per cent (68 per cent
confidence limit).
