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

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 Δ t = 51 ± 4 days (95% confidence level). This is the fourth optical time delay ever measured. Adopting a Ω = 0.3, Λ = 0 universe and using the mass model of Maller et al., this time delay estimate yields a Hubble parameter of H 0 = 52 km s -1 Mpc -1 (95% confidence level), where the errors include time delay as well as model uncertainties. There are time-dependent offsets between the two (appropriately shifted) light curves that indicate the presence of external variations due to microlensing.