2002ApJ...572L..11H

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.