Context : We present a detailed analysis of the inner mass structure of the Cosmic Horseshoe ( J1148+1930 ) strong gravitational lens system observed with the Hubble Space Telescope ( HST ) Wide Field Camera 3 ( WFC3 ) .
In addition to the spectacular Einstein ring , this systems shows a radial arc .
We obtained the redshift of the radial arc counter image z _ { \text { s,r } } = 1.961 \pm 0.001 from Gemini observations .
To disentangle the dark and luminous matter , we consider three different profiles for the dark matter distribution : a power-law profile , the NFW , and a generalized version of the NFW profile .
For the luminous matter distribution , we base it on the observed light distribution that is fitted with three components : a point mass for the central light component resembling an active galactic nucleus , and the remaining two extended light components scaled by a constant M/L .
To constrain the model further , we include published velocity dispersion measurements of the lens galaxy and perform a self-consistent lensing and axisymmetric Jeans dynamical modeling .
Our model fits well to the observations including the radial arc , independent of the dark matter profile .
Depending on the dark matter profile , we get a dark matter fraction between 60 % and 70 % .
With our composite mass model we find that the radial arc helps to constrain the inner dark matter distribution of the Cosmic Horseshoe independently of the dark matter profile .

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