SIMBAD references

A statistical analysis of an extended data set on a time basis of 17 years of optical observations of the gravitationally lensed quasar 0957+561 is presented. We use all available photometry to estimate the time delay between A and B images as 416.3±1.7 days. Using this refined value we investigate basic statistical properties of the difference curve between the A and the time shifted B image. We find a significant variability of 0.25 mag on a time scale of 5 years, which we interpret as microlensing by compact objects in the lensing galaxy. We show that masses down to about 10^–5M_☉ may explain this microlensing event. Solar mass objects alone can more or less be excluded. However, a distribution of compact objects with a small fraction (≲5%) in solar mass objects, seems to be possible. The best constrained parameter is the radius of the source which roughly should be 3x10¹⁵cm (within a factor of 3). With the present noise level an additional variability due to the microlensing on shorter time scales (days to months) is difficult to prove with a reasonable significance. All steps of the analysis are carried out by using simple statistical tests and with as few a priori assumptions as possible .