Paper detail

The light curve and the time delay of QSO 0957+561

We present a new analysis of the presently available photometric data for the gravitationally lensed quasar 0957+561 A,B with the aim of determining the time delay between its two images. The basic method used is the dispersion estimation technique. Even by using the simplest non-parametric form of our method we can convincingly rule out a time delay near 536 days and show that the time delay is in the vicinity of 420 days. We then introduce refinements to our method in order to get a stable and reliable result for the time delay independent of high frequency noise in the data and sampling errors. Our best result for the time delay, checked by various statistical tests and using bootstrap error estimates, is 423 +/- 6 days. We furthermore confirm our earlier result that the radio data are compatible with this value. Using the best available model for the mass distribution in the lensing galaxy and cluster, our result for the time delay constrains the Hubble parameter to be smaller than 70 km/(s Mpc).