Probing Axial Symmetry Breaking in the Galaxy with Gaia Data Release 2
  We study a set of solar neighborhood ($d < 3$ kpc) stars from Gaia Data
Release 2 to determine azimuthal star count differences, i.e., left and right
of the line from the Galactic center through the sun - and compare these
differences north and south. In this companion paper to Gardner et al. (2020),
we delineate our procedures to remove false asymmetries from sampling effects,
incompleteness, and/or interloper populations, as this is crucial to tests of
axisymmetry. Particularly, we have taken care to make appropriate selections of
magnitude, color, in-plane Galactocentric radius and Galactic $|b|$ and $|z|$.
We find that requiring parallax determinations of high precision induces
sampling biases, so that we eschew such requirements and exclude, e.g., regions
around the lines of sight to the Magellanic clouds, along with their
mirror-image lines of sight, to ensure well-matched data sets. After making
conservative cuts, we demonstrate the existence of azimuthal asymmetries, and
find differences in those, north and south. These asymmetries give key insights
into the nature and origins of the perturbations on Galactic matter, allowing
us to assess the relative influence of the Magellanic Clouds (LMC & SMC), the
Galactic bar, and other masses on the Galactic mass distribution, as described
in Gardner et al. (2020). The asymmetry's radial dependence reveals variations
that we attribute to the Galactic bar, and it changes sign at a radius of
$(0.95 \pm 0.03) R_0$, with $R_0$ the Sun-Galactic-Center (GC) distance, to
give us the first direct assessment of the outer Lindblad resonant radius.
