An Optical Time Delay Estimate for the Double Gravitational Lens System B1600+434

Abstract

We present optical I-band light curves of the gravitationally lensed double QSO B1600]434 from observations obtained at the Nordic Optical Telescope (NOT) between 1998 April and 1999 November. The photometry has been performed by simultaneous deconvolution of all the data frames, involving a numerical lens galaxy model. Four methods have been applied to determine the time delay between the two QSO components, giving a mean estimate of *t \ 51 ^ 4 days (95% conÐdence level). This is the fourth optical time delay ever measured. Adopting a) \ 0.3, " \ 0 universe and using the mass model of Maller et al., this time delay estimate yields a Hubble parameter of km s~1 Mpc~1 (95% H 0 \ 52 ~8`14 ~8`14 conÐdence level), where the errors include time delay as well as model uncertainties. There are time-dependent o †sets between the two (appropriately shifted) light curves that indicate the presence of external variations due to microlensing.