  In this article, we apply strong lensing techniques in Abell 1703, a massive
X-ray luminous galaxy cluster at z=0.28. Our analysis is based on imaging data
both from space and ground in 8 bands, complemented with a spectroscopic
survey. Abell 1703 looks rather circular from the general shape of its multiply
imaged systems and present a dominant giant elliptical cD galaxy in its centre.
This cluster exhibits a remarkable bright 'central ring' formed by 4 bright
images at z_{spec}=0.888 located very close to the cD galaxy, providing
observational constraints that are potentially very interesting to probe the
central mass distribution. The stellar contribution from the cD galaxy (~1.25
10^{12} M_{sun} within 7") is accounted for in our parametric mass modelling,
and the underlying smooth dark matter component distribution is described using
a generalized NFW profile parametrized with a central logarithmic slope \alpha.
We find that within the range where observational constraints are present (from
~5" to ~50"), the slope of the dark matter distribution in Abell1703 is equal
to 1.09^{+0.05}_{-0.11} (3\sigma confidence level). The concentration parameter
is equal to c_{200} ~ 3.5, and the scale radius is constrained to be larger
than the region where observational constraints are available. Within this
radius, the 2D mass is equal to M(50")=2.4 10^{14} M_{\sun}. We cannot draw any
conclusions on cosmological models at this point since we lack results from
realistic numerical simulations containing baryons to make a proper comparison.
We advocate the need for a sample of observed and simulated unimodal relaxed
galaxy clusters in order to make reliable comparisons, and potentially provide
a test of cosmological models.
