Characterizing the Galactic warp with Gaia: I. The tilted ring model
  with a twist
  We explore the possibility of detecting and characterizing the warp of the
stellar disc of our Galaxy using synthetic Gaia data. The availability of
proper motions and, for the brightest stars radial velocities, adds a new
dimension to this study. A family of Great Circle Cell Counts (GC3) methods is
used. They are ideally suited to find the tilt and twist of a collection of
rings, which allow us to detect and measure the warp parameters. To test them,
we use random realizations of test particles which evolve in a realistic
Galactic potential warped adiabatically to various final configurations. In
some cases a twist is introduced additionally. The Gaia selection function, its
errors model and a realistic 3D extinction map are applied to mimic three
tracer populations: OB, A and Red Clump stars. We show how the use of
kinematics improves the accuracy in the recovery of the warp parameters. The OB
stars are demonstrated to be the best tracers determining the tilt angle with
accuracy better than $\sim0.5$ up to Galactocentric distance of $\sim16$ kpc.
Using data with good astrometric quality, the same accuracy is obtained for A
type stars up to $\sim13$ kpc and for Red Clump up to the expected stellar
cut-off. Using OB stars the twist angle is recovered to within $< 3^\circ$ for
all distances.
