  In November 2019, the nearby single, isolated DQ-type white dwarf LAWD 37 (WD
1142-645) aligned closely with a distant background source and caused an
astrometric microlensing event. Leveraging astrometry from \Gaia{} and followup
data from the \textit{Hubble Space Telescope} we measure the astrometric
deflection of the background source and obtain a gravitational mass for
LAWD~37. The main challenge of this analysis is in extracting the lensing
signal of the faint background source whilst it is buried in the wings of
LAWD~37's point spread function. Removal of LAWD 37's point spread function
induces a significant amount of correlated noise which we find can mimic the
astrometric lensing signal. We find a deflection model including correlated
noise caused by the removal of LAWD~37's point spread function best explains
the data and yields a mass for LAWD 37 of $0.56\pm0.08 M_{\odot}$. This mass is
in agreement with the theoretical mass-radius relationship and cooling tracks
expected for CO core white dwarfs. Furthermore, the mass is consistent with no
or trace amounts of hydrogen that is expected for objects with helium-rich
atmospheres like LAWD 37. We conclude that further astrometric followup data on
the source is likely to improve the inference on LAWD 37's mass at the
$\approx3$ percent level and definitively rule out purely correlated noise
explanations of the data. This work provides the first semi-empirical test of
the white dwarf mass-radius relationship using a single, isolated white dwarf
and supports current model atmospheres of DQ white dwarfs and white dwarf
evolutionary theory.
