Microlensing Events in the Galactic Plane Using the Zwicky Transient
  Facility
  Microlensing is a powerful technique to study the Galactic population of
"dark" objects such as exoplanets both bound and unbound, brown dwarfs,
low-luminosity stars, old white dwarfs, neutron stars, and almost the only way
to study isolated stellar-mass black holes. The majority of previous efforts to
search for gravitational microlensing events have concentrated towards
high-density fields such as the Galactic bulge. Microlensing events in the
Galactic plane have the advantage of closer proximity and better constrained
relative proper motions, leading to better constrained lens mass estimates at
the expense of a lower optical depth compared to events towards the Galactic
bulge. We use the Zwicky Transient Facility (ZTF) Data Release 5 (DR5) compiled
from 2018--2021 to survey the Galactic plane in the region of $|b| < 20^\circ$.
We find a total of 60 candidate microlensing events including three that show a
strong microlensing parallax effect. The rate of events traces Galactic
structure, decreasing exponentially as a function Galactic longitude with scale
length $\ell_0 \sim 37^\circ$. On average, we find Einstein timescales of our
microlensing events to be about three times as long ($\sim60$ days) compared to
those towards the Galactic bulge ($\sim20$ days). This pilot project
demonstrates that microlensing towards the Galactic plane shows strong promise
for characterization of dark objects within the Galatic disk.
