The Sagittarius Stream and Halo Triaxiality
  We present a mass model for the Milky Way, which is fit to observations of
the Sagittarius stream together with constraints derived from a wide range of
photometric and kinematic data. The model comprises a S\'{e}rsic bulge, an
exponential disk, and an Einasto halo. Our Bayesian analysis is accomplished
using an affine-invariant Markov chain Monte Carlo algorithm. We find that the
best-fit dark matter halo is triaxial with axis ratios of $3.3\pm 0.7$ and
$2.7\pm 0.4$ and with the short axis approximately aligned with the
Sun-Galactic centre line. Our results are consistent with those presented in
Law and Majewski (2010). Such a strongly aspherical halo is disfavoured by the
standard cold dark matter scenario for structure formation.
