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 Δ τ = 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_b, Ω_Λ)= (0.3, 0.7) cosmology, the time delay yields a Hubble parameter of H₀ = 60⁺⁹_-7 (random, 1 σ) ± 2 (systematic) km s^-1 Mpc^-1 for a singular isothermal ellipsoid model and H₀ = 63⁺⁹_-7 (random, 1 σ) ± 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⁺⁹_-7 (random, 1 σ) ± 14 (systematic) km s^-1 Mpc^-1.

Based on observations made with the Nordic Optical Telescope, operated on the island of La Palma jointly by Denmark, Finland, Iceland, Norway, and Sweden, in the Spanish Observatorio del Roque de los Muchachos of the Instituto de Astrofisica de Canarias. Based on observations made with ESO Telescopes at the Paranal Observatories under programme ID 66.A-0062(A). Based on observations made with the NASA/ESA Hubble Space Telescope, obtained from the Data Archive at the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS 5-26555. These observations are associated with program #7887.