Analysis of Time Delays in the Gravitational Lens PG 1115+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 s2 measure of the agreement of this same variation with all of the light curves. While inspired by Press, Rybicki, & Hewitt, our approach is di erent since we do not assume a Gaussian process for the source variation. Our method has a number of desirable properties : Ðrst, it yields an approximate reconstruction of the source variation and of other parameters such as relative time delays ; second, it easily incorporates di erent assumptions about the relations among the light curves and about the data measurement errors ; Ðnally, it can be applied to more than two light curves by the addition of s2. We apply this method to the light curves of the quadruple gravitational lens PG 1115]080 measured by Schechter et al. Unlike Schechter et al., we include correlated measurement errors in the analysis, as well as the possibility that microlensing may cause di erent images to vary by di erent factors in Ñux. We Ðnd a value of days (95% conÐdence) for the delay between components B and C (close to the 25.0 ~3.3.3 24-day value of Schechter et al., and so leading to a similar value of the Hubble constant for a given lens model). However, the ratio of the two intermediate delays is poorly determined at (68% t AC /t BA