Single-lens mass measurement in the high-magnification microlensing
  event Gaia19bld located in the Galactic disc
  We present the photometric analysis of Gaia19bld, a high-magnification
($A\approx60$) microlensing event located in the southern Galactic plane, which
exhibited finite source and microlensing parallax effects. Due to a prompt
detection by the Gaia satellite and the very high brightness of $I = 9.05~$mag
at the peak, it was possible to collect a complete and unique set of
multi-channel follow-up observations, which allowed us to determine all
parameters vital for the characterisation of the lens and the source in the
microlensing event. Gaia19bld was discovered by the Gaia satellite and was
subsequently intensively followed up with a network of ground-based
observatories and the Spitzer Space Telescope. We collected multiple
high-resolution spectra with Very Large Telescope (VLT)/X-Shooter to
characterise the source star. The event was also observed with VLT
Interferometer (VLTI)/PIONIER during the peak. Here we focus on the photometric
observations and model the light curve composed of data from Gaia, Spitzer, and
multiple optical, ground-based observatories. We find the best-fitting solution
with parallax and finite source effects. We derived the limit on the luminosity
of the lens based on the blended light model and spectroscopic distance. We
compute the mass of the lens to be $1.13 \pm 0.03~M_{\odot}$ and derive its
distance to be $5.52^{+0.35}_{-0.64}~\mathrm{kpc}$. The lens is likely a main
sequence star, however its true nature has yet to be verified by future
high-resolution observations. Our results are consistent with interferometric
measurements of the angular Einstein radius, emphasising that interferometry
can be a new channel for determining the masses of objects that would otherwise
remain undetectable, including stellar-mass black holes.
