410 Gravitational lensing
Figure 14.8. Light curves of the two images of the gravitationally lensed quasar
Q0957+561. Note the sudden decrease in image A at the beginning in the 1995 season
(taken from T Kundi ́cet al1997 [60]).
(2) every cluster produces weakly distorted images of a large number of
background galaxies (weak lensing) (A nice example is in figure 14.11).Both of these cases have been observed and have provided important
information on the distribution of the matter in galaxy clusters. For the analysis
of giant arcs, we have to use parametrized lens models which are fitted to the
observational data. The situation is much better for weak lensing, because
there now exist several parameter-free reconstruction methods of projected mass
distributions from weak lensing data now exist.