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 Δt=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 H₀=52⁺¹⁴_-8 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. Based on observations made with the Nordic Optical Telescope, operated on the island of La Palma jointly by Denmark, Finland, Iceland, Norway, and Sweden, in the Spanish Observatorio del Roque de los Muchachos of the Instituto de Astrofisica de Canarias.