What Makes Quadruply Lensed Quasars Quadruple?

Understand

Among known strongly lensed quasar systems, ~25% have gravitational potentials sufficiently flat (and sources sufficiently well aligned) to produce four images rather than two.

• The projected flattening of the lensing galaxy and tides from neighboring galaxies both contribute to the potential's quadrupole.
• Witt's hyperbola and Wynne's ellipse permit determination of the overall quadrupole from the positions of the quasar images.
• The position of the lensing galaxy resolves the distinct contributions of intrinsic ellipticity and tidal shear to that quadrupole.