Hjorth, J., Burud, I., Jaunsen, A. O., Schechter, P. L., Kneib, J.-P., Andersen, M. I., Korhonen, H., Clasen, J. W., Kaas, A. A., Østensen, R., Pelt, J., and Pijpers, F. P.
The Astrophysical Journal, Volume 572, Issue 1, pp. L11-L14.
We present optical light curves of the gravitationally lensed components A (≡A1+A2+A3) and B of the quadruple quasar RX J0911.4+0551 (z=2.80). The observations were primarily obtained at the Nordic Optical Telescope between 1997 March and 2001 April and consist of 74 I-band data points for each component. The data allow the measurement of a time delay of 146+/-8 days (2 σ) between A and B, with B as the leading component. This value is significantly shorter than that predicted from simple models and indicates a very large external shear. Mass models including the main lens galaxy and the surrounding massive cluster of galaxies at z=0.77, responsible for the external shear, yield H0=71+/-4 (random, 2 σ)+/-8 (systematic) km s-1 Mpc-1. The systematic model uncertainty is governed by the surface-mass density (convergence) at the location of the multiple images. 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.