Constraints on source and lens parameters from microlensing variability in QSO 0957

From regular monitoring of the Double Quasar QSO 0957+561 A,B there is now general agreement on a time delay of about 416 days. This has made it possible to determine the microlensing residual in the light-curve, see Pelt et al. (1998). We have used two significant microlensing features: 1) A " quiet " period with a variability less than 0.05 mag lasting about 8 years, and 2) A change in the residual of 0.25 mag during a time interval of about 5 years. The first feature gives a lower limit for the lens mass, M , for a given normalized source radius, r, whereas the second feature gives an upper limit. We have considered the amount of mass in a continuum to be a free parameter with possible values between 0% (all mass in lenses) and 90%. At a significance level of 1% the mass can only be constrained within a rather wide range (10 −6 M ⊙ to 5M ⊙). For the radius R of the source an upper limit of 10 16 cm is found, whereas the normalized source radius r is restricted to be smaller than 30. At a level of 10% , however, the range of possible masses is much narrower (2 · 10 −3 M ⊙ to 0.5M ⊙), and the upper limit of R is about 6 · 10 15 cm, whereas the value of r is restricted to be less than 2. We have used an effective transverse velocity V equal to 600 km s −1 .