Non-parametric reconstruction of the galaxy lens in PG 1115+080

We describe a new, non-parametric method for reconstructing lensing mass distributions in multiple-image systems, and apply it to PG 1115 + 080, for which time delays have recently been measured. It turns out that the image positions and the ratio of time delays between different pairs of images constrain the mass distribution in a linear fashion. Since observational errors on image positions and time-delay ratios are constantly improving, we use these data as a rigid constraint in our modelling. In addition, we require the projected mass distributions to be inversion-symmetric and to have inward-pointing density gradients. With these realistic yet non-restrictive conditions it is very easy to produce mass distributions that fit the data precisely. We then present models, for H-0 = 42, 63 and 84 km s(-1) Mpc(-1), that in each case minimize mass-to-light variations while strictly obeying the lensing constraints. (Only a very rough light distribution is available at present.) All three values of H-0 are consistent with the lensing data, but require quite different morphologies for the lensing galaxy. If H-0 is low, the main lensing galaxy could be early-type. If H-0 is high, the galaxy is reconstructed as a late elliptical or nearly edge-on disc; a binary or merging system is also plausible.