On measuring the Galactic dark matter halo with hypervelocity stars
  Hypervelocity stars (HVSs) travel from the Galactic Centre across the dark
matter halo of the Milky Way, where they are observed with velocities in excess
of the Galactic escape speed. Because of their quasi-radial trajectories, they
represent a unique probe of the still poorly constrained dark matter component
of the Galactic potential. In this paper, we present a new method to produce
such constraints. Our likelihood is based on the local HVS density obtained by
back-propagating the observed phase space position and quantifies the ejection
probability along the orbit. To showcase our method, we apply it to simulated
Gaia samples of $\sim200$ stars in three realistic Galactic potentials with
dark matter components parametrized by spheroidal NFW profiles. We find that
individual HVSs exhibit a degeneracy in the scale mass-scale radius plane
($M_s-r_s$) and are able to measure only the combination $\alpha = M_s/r_s^2$.
Likewise, a degeneracy is also present between $\alpha$ and the spheroidal
axis-ratio $q$. In the absence of observational errors, we show the whole
sample can nail down both parameters with {\it sub-per cent} precision (about
$1\%$ and $0.1\%$ for $\alpha$ and $q$ respectively) with no systematic bias.
This remarkable power to constrain deviations from a symmetric halo is a
consequence of the Galactocentric origin of HVSs. To compare our results with
other probes, we break the degeneracy in the scale parameters and impose a
mass-concentration relation. The result is a competitive precision on the
virial mass $M_{200}$ of about $10\%$.
