The Fundamental Surface of Quad Lenses
  In a quadruply imaged lens system the angular distribution of images around
the lens center is completely described by three relative angles. We show
empirically that in the 3D space of these angles, spanning 180 x 180 x 90
degrees, quads from simple two-fold symmetric lenses of arbitrary radial
density profile and arbitrary radially dependent ellipticity or external shear
define a nearly invariant 2D surface. We give a fitting formula for the surface
using SIS+elliptical lensing potential. Various circularly symmetric mass
distributions with shear up to 0.4 deviate from it by typically, rms~0.1 deg,
while elliptical mass distributions with ellipticity of up 0.4 deviate from it
by rms~1.5 deg. The existence of a near invariant surface gives a new insight
into the lensing theory and provides a framework for studying quads. It also
allows one to gain information about the lens mass distribution from the image
positions alone, without any recourse to mass modeling. As an illustration, we
show that about 3/4 of observed galaxy-lens quads do not belong to this surface
within observational error, and so require additional external shear or
substructure to be modeled adequately.
