A time-delay determination from VLA light curves of the CLASS gravitational lens B1600+434

We present Very Large Array (VLA) 8.5-GHz light curves of the two lens imag es of the Cosmic Lens All Sky Survey (CLASS) gravitational lens B1600+434. We find a nearly linear decrease of 18-19% in the flux densities of both le ns images over a period of eight months (February-October) in 1998. Additio nally, the brightest image A shows modulations up to 11% peak-to-peak on sc ales of days to weeks over a large part of the observing period. Image B va ries significantly less on this time scale. We conclude that most of the sh ort-term variability in image A is not intrinsic source variability, but is most likely caused by microlensing in the lens galaxy. The alternative, sc intillation by the ionized Galactic ISM, is shown to be implausible based o n its strong opposite frequency dependent behavior compared with results fr om multi-frequency WSRT monitoring observations (Koopmans & de Bruyn 1999). From these VLA light curves we determine a median time delay between the le ns images of 47(-6)(+5), d (68%) or 47(-9)(+12) d (95%). We use two differe nt methods to derive the time delay; both give the same result within the e rrors. We estimate an additional systematic error between -8 and +7 d. If t he mass distribution of lens galaxy can be described by an isothermal model (Koopmans, de Bruyn & Jackson 1998), this time delay would give a value fo r the Hubble parameter H-0=57(-11)(+14) (95% statistical) (+26)(-15) (syste matic) km s(-1) Mpc(-1) (Omega(m)=1 and Omega(Lambda)=0). Similarly, the Mo dified-Hubble-Profile mass model would give H-0=74(-15)(+18) (95% statistic al) (+22)(-22) (systematic) km s(-1) Mpc(-1) For Omega(m)=0.3 and Omega(Lam bda)=0.7, these values increase by 5.4%. We emphasize that the slope of the radial mass profile of the lens-galaxy dark-matter halo in B1600+434 is ex tremely ill-constrained. Hence, an accurate determination of Ho from this s ystem is very difficult, if no additional constraints on the mass model are obtained. These values of Ho should therefore be regarded as indicative. Once H-0 (from independent methods) and the time delay have been determined with sufficient accuracy, it will prove more worthwhile to constrain the r adial mass profile of the dark-matter halo around the edge-on spiral lens g alaxy at z=0.4.