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

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+/-12 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+/-0.01. 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 H0=66 +/- 8 km s-1 Mpc-1 (1sigma error) with an estimated systematic error of +/-3 km s-1 Mpc-1. Using non-parametric models yields H0=65 +/- 8 km s-1 Mpc-1 (1sigma 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.