COSMOGRAIL: the COSmological MOnitoring of GRAvItational Lenses

C. Vuissoz; F. Courbin; D. Sluse; G. Meylan; M. Ibrahimov; I. Asfandiyarov; E. Stoops; A. Eigenbrod; L. Le Guillou; H. van Winckel; P. Magain

doi:doi:10.1051/0004-6361:20065823

Free access

Issue		A&A Volume 464, Number 3, March IV 2007


Page(s)		845 - 851
Section		Cosmology (including clusters of galaxies)
DOI		http://dx.doi.org/10.1051/0004-6361:20065823

A&A 464, 845-851 (2007)
DOI: 10.1051/0004-6361:20065823

V. The time delay in SDSS J1650+4251

C. Vuissoz¹, F. Courbin¹, D. Sluse¹, G. Meylan¹, M. Ibrahimov², I. Asfandiyarov², E. Stoops^{1, 3}, A. Eigenbrod¹, L. Le Guillou⁴, H. van Winckel⁴, and P. Magain³

¹ Laboratoire d'Astrophysique, École Polytechnique Fédérale de Lausanne (EPFL), Observatoire, 1290 Sauverny, Switzerland
e-mail: christel.vuissoz@epfl.ch
² Ulugh Beg Astronomical Institute, Academy of Sciences, Tashkent, Uzbekistan
³ Institut d'Astrophysique et de Géophysique, Université de Liège, Allée du 6 août 17, Sart-Tilman, Bât. B5C, 4000 Liège, Belgium
⁴ Instituut voor Sterrenkunde, Katholieke Universiteit Leuven, Celestijnenlaan 200B, 3001 Heverlee, Belgium

(Received 14 June 2006 / Accepted 29 December 2006 )

Abstract
Aims.Our aim is to measure the time delay between the two gravitationally lensed images of the $z_{\rm qso}$ = 1.547 quasar SDSS J1650+4251, in order to estimate the Hubble constant $H_{\rm0}$ .
Methods.Our measurement is based on R-band light curves with 57 epochs obtained at Maidanak Observatory, in Uzbekistan, from May 2004 to September 2005. The photometry is performed using simultaneous deconvolution of the data, which provides the individual light curves of the otherwise blended quasar images. The time delay is determined from the light curves using two very different numerical techniques, i.e., polynomial fitting and direct cross-correlation. The time delay is converted into $H_{\rm0}$ following analytical modeling of the potential well.
Results.Our best estimate of the time delay is $\Delta t = 49.5$ $\pm$ 1.9 days, i.e., we reach a 3.8% accuracy. The R-band flux ratio between the quasar images, corrected for the time delay and for slow microlensing, is $F_{\rm A}/F_{\rm B} = 6.2$ $\pm$ 5%.
Conclusions.The accuracy reached on the time delay allows us to discriminate well between families of lens models. As for most other multiply imaged quasars, only models of the lensing galaxy that have a de Vaucouleurs mass profile plus external shear give a Hubble constant compatible with the current most popular value ( $H_{\rm0}$ = 72 $\pm$ 8 km s^-1 Mpc^-1). A more realistic singular isothermal sphere model plus external shear gives $H_{\rm0}$ = 51.7 ^+4.0_-3.0 km s^-1 Mpc^-1.

Key words: gravitational lensing -- galaxies: quasars: individual: J1650+4251 -- cosmology: cosmological parameters

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