Gravitational lensing potential reconstruction in quadruply imaged systems

We develop a semi-analytical method to reconstruct the lensing potential in quadruply imaged gravitational lens systems. Assuming that the potential belongs to a broad class of boxy non-elliptical models, we show how it is possible to write down a system of equations which can be numerically solved to recover the potential parameters directly from image positions and using physical constraints. We also describe a code developed to search for solutions of the system previously found and test it on simulated cases. Finally, we apply the method to the quadruple lens PG 1115+080 which allows us to get also an estimate of the Hubble constant H-0 from the measured time delay as H-0 = 56(-11)(+12) km s(-1) Mpc(-1).