  We model the extremely massive and luminous lens galaxy in the Cosmic
Horseshoe Einstein ring system, recently discovered in the Sloan Digital Sky
Survey. We use the semi-linear method of Warren & Dye (2003), which pixelises
the source surface brightness distribution, to invert the Einstein ring for
sets of parameterised lens models. Here, the method is refined by exploiting
Bayesian inference to optimise adaptive pixelisation of the source plane and to
choose between three differently parameterised models: a singular isothermal
ellipsoid, a power law model and a NFW profile. The most probable lens model is
the power law with a volume mass density that scales as r^(-1.96+/-0.02) and an
axis ratio of ~0.8. The mass within the Einstein ring (i.e., within a cylinder
with projected distance of ~30 kpc from the centre of the lens galaxy) is
(5.02+/-0.09)*10^12 M_solar, and the mass-to-light ratio is ~30. Even though
the lens lies in a group of galaxies, the preferred value of the external shear
is almost zero. This makes the Cosmic Horseshoe unique amongst large separation
lenses, as almost all the deflection comes from a single, very massive galaxy
with little boost from the environment.
