*Astron. Astrophys. 325, 877-880*
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## Gravitational microlensing of large sources including
shear term effects
**S. Refsdal**^{1} and R. Stabell^{2}
^{1}Hamburger Sternwarte, Gojenbergsweg 112, D-21029
Hamburg,
Germany
^{2}Institute of Theoretical Astrophysics, University of Oslo,
P.O. Box 1029, Blindern, N-0315 Oslo, Norway
*Received 17 February 1997 / Accepted 3 April 1997*
**Abstract**
The standard deviation of the variations in magnitude due
to microlensing is calculated for the case when the angular
radius ( ) of the source is much larger than that of the
Einstein ring ( ) for the microlens,
( ).
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 , and in all cases 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 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
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