FBQ 0951+2635: Time delay and structure of the main lensing galaxy

As there is a long-standing controversy over the time delay between the two images of the gravitationally lensed quasar FBQ 0951+2635, we combined early and new optical light curves to robustly measure a delay of 13.5 ± 1.6 d (1 σ interval). The new optical records covering the last 17 yr were also used to trace the long-timescale evolution of the microlensing variability. Additionally, the new time-delay interval and a relatively rich set of further observational constraints allowed us to discuss the mass structure of the main lensing galaxy at a redshift of 0.26. This lens system is of particular interest because the external shear from secondary gravitational deflectors is relatively low, but the external convergence is one of the highest known. When modelling the galaxy as a singular power-law ellipsoid without hypotheses or priors on the power-law index, ellipticity, and position angle, we demonstrated that its mass profile is close to isothermal, and there is a good agreement between the shape of the mass distribution and that of the near-IR light. We also recovered the true mass scale of the galaxy. Finally, a constant mass-to-light ratio model also worked acceptably well.