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 images of the Cosmic Lens All Sky Sur- vey (CLASS) gravitational lens B1600+434. We find a nearly linear decrease of 18-19% in the flux densities of both lens im- ages over a period of eight months (February-October) in 1998. Additionally, the brightest image A shows modulations up to 11% peak-to-peak on scales of days to weeks over a large part of the observing period. Image B varies significantly less on this time scale. We conclude that most of the short-term vari- ability in image A is not intrinsic source variability, but is most likely caused by microlensing in the lens galaxy. The alterna- tive, scintillation by the ionized Galactic ISM, is shown to be implausible based on its strong opposite frequency dependent behavior compared with results from multi-frequency WSRT monitoring observations (Koopmans & de Bruyn 1999). From these VLA light curves we determine a median time delay between the lens images of 47 +5 6 d (68%) or 47 +12 9 d (95%). We use two different methods to derive the time de- lay; both give the same result within the errors. We estimate an additional systematic error between 8 and +7 d. If the mass distribution of lens galaxy can be described by an isothermal model (Koopmans, de Bruyn & Jackson 1998), this time delay would give a value for the Hubble parameter, H0=57 +14 11 (95% statistical) +26 15 (systematic) km s 1 Mpc 1 (m=1 and=0). Similarly, the Modified-Hubble-Profile mass model would give H0=74 +18 15 (95% statistical) +22 22 (systematic) km s 1 Mpc 1 . Form=0.3 and=0.7, these values increase by 5.4%. We em- phasize that the slope of the radial mass profile of the lens-galaxy dark-matter halo in B1600+434 is extremely ill-constrained. Hence, an accurate determination of H0 from this system is very difficult, if no additional constraints on the mass model are obtained. These values of H0 should therefore be regarded as indicative. Once H0 (from independent methods) and the time delay have been determined with sufficient accuracy, it will prove more worthwhile to constrain the radial mass profile of the dark- matter halo around the edge-on spiral lens galaxy at z=0.4.