Analysis of Time Delays in the Gravitational Lens PG1115+080

Abstract

We present a new method for determining time delays among the light curves of various images in a gravitational lens. The method is based on constructing a simple model for the source variation and forming a chi squared measure of the agreement of this same variation with all of the lightcurves. While inspired by Press et al. (1992a, b) our approach is different since we do not assume a Gaussian process for the source variation. We apply our method to the light curves of the quadruple gravitational lens PG1115+080 measured by Schechter et al. (1997). Unlike Schechter et al. we include correlated measurement errors in the analysis, as well as the possibility that microlensing may cause different images 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 of the two intermediate delays is poorly determined at 1.13 (+.18,-.17) (68% confidence), close to the value predicted by lens models (1.4) unlike the Schechter et al. value (0.7). The variation ratios of C with respect to A and of 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 a microlensing gradient, and this type of microlensing may allow us to conclude that the size of the quasar optical emission region is about 1000 AU.