Constraints on the Inner Mass Profiles of Lensing Galaxies from Missing Odd Images

Most gravitational lens systems consist of two or four observable images. Theabsence of detectable odd images allows us to place a lower limit on thepower-law slope of the inner mass profile of lensing galaxies. Using a sampleof six two-image radio-loud lens systems and assuming a singular power-lawsurface density (Sigma proportional to r^{-beta}) for the inner several kpc ofthe mass distribution, we find that there is less than a 10% probability thatthe data are consistent with profile slopes beta < 0.80. Furthermore,individual mass modeling yields beta > 0.85 for B0739+366 and beta > 0.91 forB1030+074. Modeling central black holes as additional point masses changes theconstraints in these systems to beta > 0.84 and beta > 0.83, respectively. Theinner mass profiles of lensing galaxies are therefore not much shallower thanisothermal.Comment: Final published version, minor typos corrected, 13 page