  SDSS J120602.09+514229.5 is a gravitational lens system formed by a group of
galaxies at redshift z=0.422 lensing a bright background galaxy at redshift
z=2.001. The main peculiarity of this system is the presence of a luminous
satellite near the Einstein radius, that slightly deforms the giant arc. This
makes SDSS J120602.09+514229.5 the ideal system to test our grid-based Bayesian
lens modelling method, designed to detect galactic satellites independently
from their mass-to-light ratio, and to measure the mass of this dwarf galaxy
despite its high redshift. Thanks to the pixelized source and potential
reconstruction technique of Vegetti and Koopmans 2009a we are able to detect
the luminous satellite as a local positive surface density correction to the
overall smooth potential. Assuming a truncated Pseudo-Jaffe density profile,
the satellite has a mass M=(2.75+-0.04)10^10 M_sun inside its tidal radius of
r_t=0.68". We determine for the satellite a luminosity of L_B=(1.6+-0.8)10^9
L_sun, leading to a total mass-to-light ratio within the tidal radius of
(M/L)_B=(17.2+-8.5) M_sun/L_sun. The central galaxy has a sub-isothermal
density profile as in general is expected for group members. From the SDSS
spectrum we derive for the central galaxy a velocity dispersion of
sigma_kinem=380+-60 km/s within the SDSS aperture of diameter 3". The
logarithmic density slope of gamma=1.7+0.25-0.30 (68% CL), derived from this
measurement, is consistent within 1-sigma with the density slope of the
dominant lens galaxy gamma~1.6, determined from the lens model. This paper
shows how powerful pixelized lensing techniques are in detecting and
constraining the properties of dwarf satellites at high redshift.
