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Astron. Astrophys. 336, 829-839 (1998)

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Microlensing on different timescales in the lightcurves of QSO 0957+561 A,B

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

1 Tartu Observatory, EE-2444 Tõravere, Estonia
2 Harvard-Smithsonian Center for Astrophysics, MS-19 60 Garden Street, Cambridge, MA 02138, USA
3 Hamburger Sternwarte, Gojenbergsweg 112, D-21029 Hamburg-Bergedorf, Germany
4 Institute of Theoretical Astrophysics, P.O. Box 1029, Blindern, N-0315 Oslo, Norway
5 Centre for Advanced Study, Drammensveien 78, N-0271, Oslo, Norway

Received 20 February 1998 / Accepted 26 May 1998


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 [FORMULA] 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 [FORMULA] 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 ([FORMULA]) in solar mass objects, seems to be possible. The best constrained parameter is the radius of the source which roughly should be [FORMULA] (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.

Key words: methods: statistical – methods: data analysis – quasars: individual: 0957+561A,B – gravitational lensing

Send offprint requests to: R. Stabell4

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© European Southern Observatory (ESO) 1998

Online publication: July 27, 1998