2001ApJ...549L..33R

Most gravitational lens systems consist of two or four observable images. The absence of detectable odd images allows us to place a lower limit on the power-law slope of the inner mass profile of lensing galaxies. Using a sample of six two-image radio-loud lens systems and assuming a singular power-law surface density (Σ∝r^–β) for the inner several kiloparsecs of the mass distribution, we find that there is less than a 10% probability that the data are consistent with profile slopes β<0.80. Furthermore, individual mass modeling yields β>0.85 for B0739+366 and β>0.91 for B1030+074. Modeling central black holes as additional point masses changes the constraints in these systems to β>0.84 and β>0.83, respectively. The inner mass profiles of lensing galaxies are therefore not much shallower than isothermal.