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Astron. Astrophys. 360, 10-14 (2000)

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Constraints on source and lens parameters from microlensing variability in QSO 0957+561 A,B

S. Refsdal 1,5, R. Stabell 2,5, J. Pelt 3,5 and R. Schild 4,5

1 Hamburger Sternwarte, Gojenbergsweg 112, 21029 Hamburg, Germany
2 University of Oslo, Institute of Theoretical Astrophysics, P.O. Box 1029, Blindern, 0315 Oslo, Norway
3 Tartu Observatory, 61602 Toravere, Estonia
4 Harvard-Smithsonian Center for Astrophysics, MS-19, 60 Garden Street, Cambridge, MA 02138, USA
5 Centre for Advanced Study, Drammensveien 78, 0271 Oslo, Norway

Received 28 February 2000 / Accepted 9 May 2000


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 ([FORMULA] to [FORMULA]). For the radius R of the source an upper limit of [FORMULA] 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 ([FORMULA] to [FORMULA]), and the upper limit of R is about [FORMULA] 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.

Key words: cosmology: gravitational lensing – galaxies: quasars: individual: QSO 0957+561

Send offprint requests to: rolf.stabell@astro.uio.no

© European Southern Observatory (ESO) 2000

Online publication: July 27, 2000