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

¹ Niels Bohr Institute, Astronomical Observatory, University of Copenhagen, Juliane Maries Vej 30, 2100 Copenhagen Ø, Denmark e-mail: pallja@astro.ku.dk
² Science Institute, University of Iceland, Dunhaga 3, 107 Reykjavík, Iceland
³ Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218, USA
⁴ Institut d'Astrophysique et de Géophysique, ULg, Allée du 6 Août 17, B5C, 4000 Sart Tilman (Liège), Belgium
⁵ European Southern Observatory, Casilla 19, Santiago, Chile
⁶ Laboratoire d'Astrophysique, École Polytechnique Fédérale de Lausanne, Observatoire, 1290 Chavannes-des-bois, Switzerland

Received: 8 June 2004
Accepted: 13 October 2004

Abstract

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  ± 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 cosmology, the time delay yields a Hubble parameter of  (random, )  (systematic) km s^-1 Mpc^-1 for a singular isothermal ellipsoid model and  (random, )  (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  (random, )  (systematic) km s^-1 Mpc^-1.