Fit of the lens galaxy’s surface brightness profile. Left: the HST/ACS imaging in the F814W filter of the lens system RXJ1131−1231 with the quasar images and the lensed arcs subtracted using the prediction from the best-fit lens model from Suyu et al. (2013), thus leaving only the lens galaxy’s light to be fitted. The orange circle shows the large circular region considered for fitting in our analysis, and the yellow square shows the smaller cutout used for lens modeling by Suyu et al. (2013). The cyan annulus contains the region where pixels were fitted to reconstruct the source by Suyu et al. (2013). Thus the lensed arcs from the quasar host galaxy were subtracted only within this annulus. The red contours mark quasar image positions with significant residuals due to saturated pixels, which we mask. Middle: the fitted light profile with a double Sérsic model. The black pixels correspond to masked pixels. The additional masked pixels within the orange circle not described above are randomly selected through an iterative process that performs outlier rejection while preserving the Gaussian tail (Sect. 5.1.2 for details). Right: normalized residual of the best-fit model.