We report the discovery of two concentric partial Einstein rings around the gravitational lens SDSSJ0946+1006 , as part of the Sloan Lens ACS Survey .
The main lens is at redshift z _ { l } = 0.222 , while the inner ring ( 1 ) is at redshift z _ { s 1 } = 0.609 and Einstein radius { R _ { Ein } } _ { 1 } = 1.43 \pm 0.01 \arcsec .
The wider image separation ( { R _ { Ein } } _ { 2 } = 2.07 \pm 0.02 \arcsec ) of the outer ring ( 2 ) implies that it is at higher redshift than Ring 1 .
Although no spectroscopic feature was detected in \sim 9 hours of spectroscopy at the Keck I Telescope , the detection of Ring 2 in the F814W ACS filter implies an upper limit on the redshift of z _ { s 2 } \lesssim 6.9 .
The lens configuration can be well described by a power law total mass density profile for the main lens \rho _ { tot } \propto r ^ { - \gamma ^ { \prime } } with logarithmic slope \gamma ^ { \prime } = 2.00 \pm 0.03 ( i.e .
close to isothermal ) , velocity dispersion \sigma _ { SIE } = 287 \pm 5 { km s } ^ { -1 } ( in good agreement with the stellar velocity dispersion \sigma _ { v,* } = 284 \pm 24 { km s } ^ { -1 } ) with little dependence upon cosmological parameters or the redshift of Ring 2 .
Using strong lensing constraints only we show that the enclosed mass to light ratio increases as a function of radius , inconsistent with mass following light .
Adopting a prior on the stellar mass to light ratio from previous SLACS work we infer that 73 \pm 9 \% of the mass is in form of dark matter within the cylinder of radius equal to the effective radius of the lens .
We consider whether the double source plane configuration can be used to constrain cosmological parameters exploiting the ratios of angular distance ratios entering the set of lens equations .
We find that constraints for SDSSJ0946+1006 are uninteresting due to the sub-optimal lens and source redshifts for this application .
We then consider the perturbing effect of the mass associated with Ring 1 ( modeled as a singular isothermal sphere ) building a double lens plane compound lens model .
This introduces minor changes to the mass of the main lens , allows to estimate the redshift of the Ring 2 ( z _ { s 2 } = 3.1 ^ { +2.0 } _ { -1.0 } ) , and the mass of the source responsible for Ring 1 ( \sigma _ { SIE,s 1 } = 94 ^ { +27 } _ { -47 } { km s } ^ { -1 } ) .
We conclude by examining the prospects of doing cosmography with a sample of 50 double source plane gravitational lenses , expected from future space based surveys such as DUNE or JDEM .
Taking full account of the uncertainty in the mass density profile of the main lens , and of the effect of the perturber , and assuming known redshifts for both sources , we find that such a sample could be used to measure \Omega _ { m } and w with 10 % accuracy , assuming a flat cosmological model .