The first all-sky view of the Milky Way stellar halo with Gaia+2MASS RR
  Lyrae
  We exploit the first \gaia data release to study the properties of the
Galactic stellar halo as traced by RR Lyrae. We demonstrate that it is possible
to select a pure sample of RR Lyrae using only photometric information
available in the Gaia+2MASS catalogue. The final sample contains about 21600 RR
Lyrae covering an unprecedented fraction ($\sim60\%$) of the volume of the
Galactic inner halo ($\text{R}<28$ kpc). We study the morphology of the stellar
halo by analysing the RR Lyrae distribution with parametric and non-parametric
techniques. Taking advantage of the uniform all-sky coverage, we test halo
models more sophisticated than usually considered in the literature, such as
those with varying flattening, tilt and/or offset of the halo with respect to
the Galactic disc. A consistent picture emerges: the inner halo is well
reproduced by a smooth distribution of stars settled on triaxial ellipsoids.
The minor axis is perpendicular to the Milky Ways disc, while the major axis is
misaligned by $\sim20^{\circ}$ from the Galactic Y axis. The elongation along
the major axis is mild ($\text{p}=1.27$), and the vertical flattening is shown
to evolve from a squashed state with $\text{q}\approx0.57$ in the centre to a
more spherical $\text{q}\approx0.75$ at the outer edge of our dataset. The
density slope is well approximated by a single power-law with exponent
$\alpha=-2.96$. Within the range probed, we see no significant evidence for a
change of the radial density slope, out of the plane tilt or an offset of the
halo with respect to the Galaxy's centre.
