Analysis of Time Delays in the Gravitational Lens PG 1115+080

We present a new method for determining time delays among the light curves ofvarious images in a gravitational lens. The method is based on constructing asimple model for the source variation and forming a chi squared measure of theagreement of this same variation with all of the lightcurves. While inspired byPress et al. (1992a, b) our approach is different since we do not assume aGaussian process for the source variation. We apply our method to the lightcurves of the quadruple gravitational lens PG1115+080 measured by Schechter etal. (1997). Unlike Schechter et al. we include correlated measurement errors inthe analysis, as well as the possibility that microlensing may cause differentimages to vary by different factors in flux. We find a value of 25.0(+3.3,-3.8) days (95% confidence) for the delay between components B and C(close to the 24 day value of Schechter et al). However, the ratio t_AC/t_BA ofthe two intermediate delays is poorly determined at 1.13 (+.18,-.17) (68%confidence), close to the value predicted by lens models (1.4) unlike theSchechter et al. value (0.7). The variation ratios of C with respect to A andof A with respect to B are both different from 1, 1.39 (+.16,-.20) and .79(+.10,-.12) (95% confidence), respectively. This is an indication of amicrolensing gradient, and this type of microlensing may allow us to concludethat the size of the quasar optical emission region is about 1000 AU.Comment: Many minor revisions, 16 pages, 4 Postscript figures, to appear in Ap