An optical time delay for the double gravitational lens system FBQ 0951+2635

We present optical R-band light curves of the double gravitationally lensed quasar FBQ 0951+2635 from observations obtained at the Nordic Optical Telescope between March 1999 and June 2001. A time delay of Δτ = 16 ± 2 days (1σ) is determined from the light curves. New constraints on the lensing geometry are provided by the position and ellipticity of the lensing galaxy. For a (Ω_m, Ω_Λ) = (0.3, 0.7) cosmology, the time delay yields a Hubble parameter of H₀ = 60 _-7⁺⁹ (random, 1σ) ±2 (systematic) km s ^-1 Mpc^-1 for a singular isothermal ellipsoid model and H₀ = 63_-7⁺⁹ (random, 1σ) ±1 (systematic) km s^-1 Mpc^-1 for a constant mass-to-light ratio model. In both models, the errors are mainly due to the time-delay uncertainties. Non-parametric models yield H₀ = 64_-7 ⁺⁹ (random, lσ) ±14 (systematic) km s^-1 Mpc^-1.