COSMOGRAIL: the COSmological MOnitoring of GRAvItational Lenses ^*,**

VII. Time delays and the Hubble constant from WFI J2033–4723

¹ Laboratoire d'Astrophysique, École Polytechnique Fédérale de Lausanne (EPFL), Observatoire de Sauverny, 1290 Versoix, Switzerland e-mail: christel.vuissoz@epfl.ch
² Institut d'Astrophysique et de Géophysique, Université de Liège, Allée du 6 août 17, Sart-Tilman, Bât. B5C, 4000 Liège, Belgium
³ Department of Astronomy and the Center for Cosmology and Astroparticle Physics, The Ohio State University, Columbus, OH 43210, USA
⁴ Department of Physics, United States Naval Academy, 572C Holloway Road, Annapolis MD 21402, USA
⁵ Institute of Theoretical Physics, University of Zürich, Winterthurerstrasse 190, 8057 Zürich, Switzerland
⁶ Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge MA 02138, USA

Received: 28 March 2008
Accepted: 2 July 2008

Abstract

Gravitationally lensed quasars can be used to map the mass distribution in lensing galaxies and to estimate the Hubble constant H₀ by measuring the time delays between the quasar images. Here we report the measurement of two independent time delays in the quadruply imaged quasar WFI J2033-4723  (z = 1.66). Our data consist of R-band images obtained with the Swiss 1.2 m EULER telescope located at La Silla and with the 1.3 m SMARTS telescope located at Cerro Tololo. The light curves have 218 independent epochs spanning 3 full years of monitoring between March 2004 and May 2007, with a mean temporal sampling of one observation every 4th day. We measure the time delays using three different techniques, and we obtain = 35.5 ± 1.4 days (3.8%) and , where A is a composite of the close, merging image pair. After correcting for the time delays, we find R-band flux ratios of  ± 0.04,  ± 0.06, and  ± 0.05 with no evidence for microlensing variability over a time scale of three years. However, these flux ratios do not agree with those measured in the quasar emission lines, suggesting that longer term microlensing is present. Our estimate of H₀ agrees with the concordance value: non-parametric modeling of the lensing galaxy predicts H₀ = 67 km s^-1 Mpc^-1, while the Single Isothermal Sphere model yields H₀ = 63 km s^-1 Mpc^-1  (68% confidence level). More complex lens models using a composite de Vaucouleurs plus NFW galaxy mass profile show twisting of the mass isocontours in the lensing galaxy, as do the non-parametric models. As all models also require a significant external shear, this suggests that the lens is a member of the group of galaxies seen in field of view of WFI J2033-4723.