COSMOGRAIL: the COSmological MOnitoring of GRAvItational Lenses

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Issue		A&A Volume 488, Number 2, September III 2008


Page(s)		481 - 490
Section		Cosmology (including clusters of galaxies)
DOI		http://dx.doi.org/10.1051/0004-6361:200809866
Published online		09 July 2008

A&A 488, 481-490 (2008)
DOI: 10.1051/0004-6361:200809866

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

C. Vuissoz¹, F. Courbin¹, D. Sluse¹, G. Meylan¹, V. Chantry², E. Eulaers², C. Morgan^{3, 4}, M. E. Eyler⁴, C. S. Kochanek³, J. Coles⁵, P. Saha⁵, P. Magain², and E. E. Falco⁶

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 $\Delta t_{B-A}$ = 35.5 $\pm$ 1.4 days (3.8%) and $\Delta t_{B-C} = 62.6^{+\,4.1}_{-\,2.3}~{\rm days} ~\ (^{+\,6.5\%}_{-\,3.7\%})$ , where A is a composite of the close, merging image pair. After correcting for the time delays, we find R-band flux ratios of F_A/F_B = 2.88 $\pm$ 0.04, F_A/F_C = 3.38 $\pm$ 0.06, and F_A1/F_A2 = 1.37 $\pm$ 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 ⁺¹³_-10 km s^-1 Mpc^-1, while the Single Isothermal Sphere model yields H₀ = 63 ⁺⁷_-3 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.

Key words: gravitational lensing -- cosmology: cosmological parameters -- quasars: individual: WFI J2033-4723

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