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We characterize the spatial density of the Pan-STARRS1 (PS1) sample of RRLyrae stars, to study the properties of the old Galactic stellar halo as tracedby 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 adistance precision of 3\% and thus is able to trace the halo out to largerdistances than most previous studies. After excising stars that are attributedto dense regions such as stellar streams, the Galactic disc and bulge as wellas halo globular clusters, the sample contains ${\sim}11,000$ sources within$20 \; \mathrm{kpc} \leq R_{\mathrm{gc}} \leq 131 \; \mathrm{kpc}$. We thenapply forward modeling using ellipsoidal stellar density models$\rho(l,b,R_{\mathrm{gc}})$ both with a constant and a radius-dependent haloflattening $q(R_{\mathrm{gc}})$. Assuming constant flattening $q$, thedistribution 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 wellby 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, wefind that the south Galactic hemisphere is more flattened than the northGalactic hemisphere. The results of our work are largely consistent with manyearlier results, e.g. \cite{Watkins2009}, \cite{Iorio2017}. We find that thestellar halo, as traced in RR Lyrae stars, exhibits a substantial number offurther significant over- and underdensities, even after all knownoverdensities have been masked.

The Profile of the Galactic Halo from Pan-STARRS1 3$\pi$ RR Lyrae