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Astron. Astrophys. 325, 877-880

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Gravitational microlensing of large sources
including shear term effects

S. Refsdal1 and R. Stabell2

1Hamburger Sternwarte, Gojenbergsweg 112, D-21029 Hamburg, Germany
2Institute of Theoretical Astrophysics, University of Oslo, P.O. Box 1029, Blindern, N-0315 Oslo, Norway

Received 17 February 1997 / Accepted 3 April 1997


The standard deviation FORMULA of the variations in magnitude due to microlensing is calculated for the case when the angular radius ( FORMULA ) of the source is much larger than that of the Einstein ring ( FORMULA ) for the microlens, ( FORMULA ). An external shear different from zero is included in the calcuations. We find that a shear term usually has the effect of reducing the value of FORMULA , and in all cases FORMULA is smaller than the analytical approximation for the case = 0 given by Refsdal and Stabell (1991), (RS). Thus the approximation can still be used to estimate an upper limit of the source size if FORMULA is available from observations. We argue that microlensing effects on the radio core of QSO 0957+561 may be of importance for the modelling of that system, and hence for the value of the Hubble parameter, even if the core radius is as large as one light year.

Key words: quasars: general - QSO 0957+561 - gravitational lensing

Send offprint requests to: S. Refsdal

© European Southern Observatory (ESO) 1997

Online publication: September 9, 1997
Last change: April 28, 1998