Analyzing the Flux Anomalies of the Large-Separation Lensed Quasar SDSS
  J1029+2623
  Using a high resolution radio image, we successfully resolve the two fold
image components B and C of the quasar lens system SDSS J1029+2623. The flux
anomalies associated with these two components in the optical regime persist,
albeit less strongly, in our radio observations, suggesting that the cluster
must be modeled by something more than a single central potential. We argue
that placing substructure close to one of the components can account for a flux
anomaly with negligible changes in the component positions. Our best fit model
has a substructure mass of ~10^8 solar masses up to the mass-sheet degeneracy,
located roughly 0.1 arcsecs West and 0.1 arcsecs North of component B. We
demonstrate that a positional offset between the centers of the source
components can explain the differences between the optical and radio flux
ratios.
