EDP Sciences
Free access
Issue
A&A
Volume 383, Number 1, February III 2002
Page(s) 71 - 81
Section Extragalactic astronomy
DOI http://dx.doi.org/10.1051/0004-6361:20011731


A&A 383, 71-81 (2002)
DOI: 10.1051/0004-6361:20011731

An optical time-delay for the lensed BAL quasar HE 2149-2745

I. Burud1, 2, F. Courbin1, 3, 4, P. Magain1, C. Lidman5, D. Hutsemékers5, J.-P. Kneib6, J. Hjorth7, J. Brewer5, E. Pompei5, L. Germany5, J. Pritchard5, A. O. Jaunsen5, G. Letawe1 and G. Meylan2

1  Institut d'Astrophysique et de Géophysique, Université de Liège, Avenue de Cointe 5, 4000 Liège, Belgium
2  Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218, USA
3  Universidad Católica de Chile, Departamento de Astronomia y Astrofisica, Casilla 306, Santiago 22, Chile
4  GEPI, Observatoire de Paris-Meudon, Place Jules Janssen, 92195 Meudon, France
5  European Southern Observatory, Casilla 19, Santiago, Chile
6  Observatoire Midi-Pyrénées (LAS), UMR 5572, 14 avenue E. Belin, 31400 Toulouse, France
7  Astronomical Observatory, University of Copenhagen, Juliane Maries Vej 30, 2100 Copenhagen Ø, Denmark

(Received 1 June 2001 / Accepted 26 November 2001 )

Abstract
We present optical V and i-band light curves of the gravitationally lensed BAL quasar HE 2149-2745. The data, obtained with the 1.5 m Danish Telescope (ESO-La Silla) between October 1998 and December 2000, are the first from a long-term project aimed at monitoring selected lensed quasars in the Southern Hemisphere. A time delay of $103\pm12$ days is determined from the light curves. In addition, VLT/FORS1 spectra of HE 2149-2745 are deconvolved in order to obtain the spectrum of the faint lensing galaxy, free of any contamination by the bright nearby two quasar images. By cross-correlating the spectrum with galaxy-templates we obtain a tentative redshift estimate of $z=0.495\pm0.01$. Adopting this redshift, a $\Omega=0.3$, $\Lambda=0.7$ cosmology, and a chosen analytical lens model, our time-delay measurement yields a Hubble constant of $H_{0}=66 \pm 8\,
{\rm km}~{\rm s^{-1}}~{\rm Mpc^{-1}}$ (1 $\sigma$ error) with an estimated systematic error of $\pm3\, {\rm km}~{\rm s^{-1}}~{\rm Mpc^{-1}}$. Using non-parametric models yields $H_{0}=65 \pm 8\, {\rm km}~{\rm s^{-1}}~{\rm Mpc^{-1}}$ (1 $\sigma$ error) and confirms that the lens exhibits a very dense/concentrated mass profile. Finally, we note, as in other cases, that the flux ratio between the two quasar components is wavelength dependent. While the flux ratio in the broad emission lines - equal to 3.7 - remains constant with wavelength, the continuum of the brighter component is bluer. Although the data do not rule out extinction of one quasar image relative to the other as a possible explanation, the effect could also be produced by differential microlensing by stars in the lensing galaxy.


Key words: gravitational lensing -- quasars: individual: HE 2149-2745 -- cosmological parameters

Offprint request: I. Burud, burud@stsci.edu

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