The edge-on spiral gravitational lens B1600+434

We present new observations of the gravitational lens (GL) system B1600+434, strongly suggesting that the lens is an edge-on spiral galaxy. These observations are used to constrain the mass model of the system, in particular the oblateness and velocity dispersion of the dark matter halo around the lensing galaxy. From an analytical model we find a lower limit on the halo oblateness $q_\rmn{halo}=(c/a)_{\rho}\ga0.4$; more detailed numerical models give a lower limit $q_\rmn{h}\ga0.5$. We determine an average halo velocity dispersion of $\sigma_{halo}=190\pm 15$ km/s over all non-singular isothermal elliptical (NIE) halo models. Constraining the models to larger and more massive disks, decreases this average by only 10 km/s. A lower limit of $\sigma_{halo}\ga 150$ km/s is found, even for disk masses larger then the mass inside the Einstein radius. This lower limit indicates the need for a massive dark matter halo, contributing at least half of the mass inside the Einstein radius. Time delay calculations give $(54\pm3)/h_{50}$ days for the NIE halo model and $(70\pm4)/h_{50}$ days for the modified Hubble profile (MHP) halo model. Although the time delay for both NIE and MHP halo models is well constrained on our parameter grid, it strongly depends on the halo surface density profile. We furthermore find that the presence of a flat luminous mass distribution can severely alter the statistical properties of the lens.