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Fig. 1

image

Possible configurations of the two multiple images A and B generated from a source lying close to an almost point-like caustic of an axisymmetric gravitational lens: without perturbations, A and B are aligned with the centre of the lens, and have symmetric arc lengths and a radius close to the Einstein radius (a); adding a global perturbation, the alignment symmetry between A and B can be broken (b); small, local perturbations can cause asymmetric arcs in the angular direction (c); or in the radial direction (d).

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