A spectroscopic follow-up for Gaia19bld
  Due to their scarcity, microlensing events in the Galactic disk are of great
interest and high-cadence photometric observations, supplemented by
spectroscopic follow-up, are necessary for constraining the physical parameters
of the lensing system. In particular, a precise estimate of the source
characteristics is required to accurately measure the lens distance and mass.
We conducted a spectroscopic follow-up of microlensing event Gaia19bld to
derive the properties of the microlensing source and, ultimately, to estimate
the mass and distance of the lens. We obtained low- and high-resolution
spectroscopy from multiple sites around the world during the course of the
event. The spectral lines and template matching analysis has led to two
independent, consistent characterizations of the source. We found that the
source is a red giant located at about 8.5 kpc from the Earth. Combining our
results with the photometric analysis has led to a lens mass of Ml=1.1 M at a
distance of D l = 5.5 kpc. We did not find any significant blend light in the
spectra (with an upper detection limit of V < 17 mag), which is in agreement
with photometric observations. Therefore, we cannot exclude the possibility
that the lens is a main-sequence star. Indeed, we predict in this scenario a
lens brightness of V about 20 mag, a value that would make it much fainter than
the detection limit.
