Axial Asymmetry Studies in Gaia Data Release 2 Yield the Pattern Speed
  of the Galactic Bar
  Our recent studies of axial-symmetry breaking in the nearby ($d <3 \,{\rm
kpc}$) star counts are sensitive to the distortions of stellar orbits
perpendicular and parallel to the orientation of the bar just within and beyond
the outer Lindblad resonance (OLR) radius. Using the location of the sign flip
in the left-right asymmetry in stars counts about the anticenter line to
determine the OLR radius $R_{\rm OLR}$, and treating the bar as if it were a
weakly non-axisymmetric effect, we use $R_{\rm OLR}$ and recent measurements of
the Galactic rotation curve and the Sun-Galactic-center distance $R_{0}$ to
determine the pattern speed $\Omega_{\rm p}$ of the Galactic bar, as well as
the Galactic corotation radius $R_{\rm CR}$. After removing the effect of the
Large and Small Magellanic clouds from our asymmetry measurement, we find that
$R_{\rm OLR}=(0.96 \pm 0.03)R_0 = 7.85 \pm 0.25 \ \rm kpc$, $\Omega_{\rm p} =
49.3 \pm 2.2 \ \rm km \ s^{-1} \ kpc^{-1}$, $R_{\rm CR}=(0.58 \pm 0.04)R_0 =
4.76 \pm 0.27 \ \rm kpc$, revealing, as we shall show, that the Milky Way's bar
is likely both weak and fast, though we also note possible evidence for
non-steady-state effects in the bar region.
