The Profile of the Galactic Halo from Pan-STARRS1 3$\pi$ RR Lyrae
  We characterize the spatial density of the Pan-STARRS1 (PS1) sample of RR
Lyrae stars, to study the properties of the old Galactic stellar halo as traced
by RRab stars. This sample of 44,403 sources spans Galactocentric radii of
$0.55 \; \mathrm{kpc} \leq R_{\mathrm{gc}} \leq 141 \; \mathrm{kpc}$ with a
distance precision of 3\% and thus is able to trace the halo out to larger
distances than most previous studies. After excising stars that are attributed
to dense regions such as stellar streams, the Galactic disc and bulge as well
as halo globular clusters, the sample contains ${\sim}11,000$ sources within
$20 \; \mathrm{kpc} \leq R_{\mathrm{gc}} \leq 131 \; \mathrm{kpc}$. We then
apply forward modeling using ellipsoidal stellar density models
$\rho(l,b,R_{\mathrm{gc}})$ both with a constant and a radius-dependent halo
flattening $q(R_{\mathrm{gc}})$. Assuming constant flattening $q$, the
distribution of the sources is reasonably well fit from $20 \; \mathrm{kpc}$ to
$131 \; \mathrm{kpc}$ by a single power law with $n=4.40^{+0.05}_{-0.04}$ and
$q=0.918^{+0.016}_{-0.014}$. The distance distribution is fit comparably well
by an Einasto profile with $n=9.53^{+0.27}_{-0.28}$, an effective radius
$r_{\mathrm{eff}}=1.07 \pm 0.10 \; \mathrm{kpc}$ and a halo flattening of
$q=0.923 \pm 0.007$. If we allow for a radius-dependent flattening
$q(R_{\mathrm{gc}})$, we find evidence for a distinct flattening of
$q{\sim}0.8$ of the inner halo at ${\sim} 25 \; \mathrm{kpc}$. Additionally, we
find that the south Galactic hemisphere is more flattened than the north
Galactic hemisphere. The results of our work are largely consistent with many
earlier results, e.g. \cite{Watkins2009}, \cite{Iorio2017}. We find that the
stellar halo, as traced in RR Lyrae stars, exhibits a substantial number of
further significant over- and underdensities, even after all known
overdensities have been masked.
