Time-Delay Measurement of the Lensed Quasar HE 1104-1805

We have measured the time delay between the two images of the gravitationally lensed quasar HE 1104-1805 by combining observations made with the Wise Observatory 1 m telescope and published observations of this system by Schechter et al., spanning a total of 5 years, from 1997 to 2002. Based on a variety of techniques, we find that the best-fit time delay is -161^+7,+34_-7,-11 days. The 68% and 95% confidence intervals include the systematic errors due to an observed component of uncorrelated variability between images. The delay is shorter than predicted by simple models and may indicate a large external shear or a large value of the Hubble parameter, h>0.75 (95% confidence). The residual light curve between the two images shows a long-term trend of ~0.04 mag yr^-1, presumably the result of microlensing by stars in the lens galaxy, but also short-timescale (~1 month) variability with a mean amplitude of about 0.07 mag.