Quantifying torque from the Milky Way bar using Gaia DR2
  We determine the mass of the Milky Way bar and the torque it causes, using
Gaia DR2, by applying the orbital arc method. Based on this, we have found that
the gravitational acceleration is not directed towards the centre of our Galaxy
but a few degrees away from it. We propose that the tangential acceleration
component is caused by the bar of the Galaxy. Calculations based on our model
suggest that the torque experienced by the region around the Sun is $\approx
2400\, km^2 s^{-2}$ per solar mass. The mass estimate for the bar is $\sim
1.6\pm0.3\times10^{10} M_\odot$. Using greatly improved data from Gaia DR2, we
have computed the acceleration field to great accuracy by adapting the oPDF
method (Han et al. 2016) locally and used the phase space coordinates of $\sim
4\times10^5$ stars within a distance of 0.5 kpc from the Sun. In the orbital
arc method, the first step is to guess an acceleration field and then
reconstruct the stellar orbits using this acceleration for all the stars within
a specified region. Next, the stars are redistributed along orbits to check if
the overall phase space distribution has changed. We repeat this process until
we find an acceleration field that results in a new phase space distribution
that is the same as the one that we started with; we have then recovered the
true underlying acceleration.
