P57	September 2000

¹ Institut d'Astrophysique et de Géophysique de Liège, Avenue de Cointe 5, B-4000 Liège, Belgium
² Astronomical Observatory, University of Copenhagen, Juliane Maries Vej 30, DK-2100 Copenhagen Ø, Denmark
³ Institute of Theoretical Astrophysics, University of Oslo, Pb. 1029 Blindern, N0315 Oslo, Norway
⁴ Division of Astronomy, University of Oulu, P.O. Box 3000, FIN-90014 Oulun Yliopisto, Finland
⁵ Nordic Optical Telescope, Apartado 474, E-38700 St. Cruz de la Palma, Canary Islande, Spain
⁶ Tartu Astrophysical Observatory, Tõravere, 61602, Estonia
⁷ Theoretical Astrophysics Center, University of Aarhus, DK-8000 Århus C, Denmark
⁸ Department of Physics, University of Tromsø, Norway

^* 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.

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 April 1998 and November 1999. 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 % confidence 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. (2000), this time-delay estimate yields a Hubble parameter of (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.