Context : Strong gravitational lensing is a unique tool that can be used to model with great accuracy the inner mass distribution of massive galaxy clusters .
In particular , clusters with large Einstein radii provide a wealth of multiply imaged systems in the cluster core .
Measuring the redshift of these multiple images provide strong constraints to determine precisely the shape of the central dark matter profile .

Aims : This paper presents a spectroscopic survey of strongly lensed galaxies in the massive cluster lens Abell 1703 , displaying a large Einstein radius ( 28″at z = 2.8 ) and numerous strongly-lensed systems including a central ring -like configuration .

Methods : We used the LRIS spectrograph on Keck to target multiple images and lensed galaxy candidates , and use the measured spectroscopic redshifts to constrain the mass distribution of the cluster using a parametric model .

Results : The spectroscopic data enable us to measure accurate redshifts for 7 sources at z > 2 , all of which in good agreement with their photometric redshifts .
We update the identification of multiply imaged systems by discovering 3 new systems and identifying a radial counter image .
We also report the discovery of a remarkably bright \sim 3.6 L ^ { * } i -band dropout at z = 5.827 in our mask that is only moderately magnified by the cluster ( \mu \sim 3.0 \pm 0.08 ) .
The improved parametric mass model , including 16 multiple systems with 10 spectroscopic redshifts , further constrain the smooth cluster-scale mass distribution with a generalized NFW profile of best-fit logarithmic slope \alpha = 0.92 \pm 0.04 , concentration c _ { 200 } = 4.72 \pm 0.40 and scale radius r _ { s } = 476 \pm 45 kpc .
The overall RMS in the image plane is 1.3″ .

Conclusions : Using our strong-lensing model , we predict a large scale shear signal that is consistent with weak-lensing measurements inferred from Subaru data out to 4 Mpc h ^ { -1 } .
Together with the fact that the strong-lensing modeling requires a single dark matter clump , this suggests that Abell 1703 is be a relaxed , unimodal cluster .
This unique cluster could to be probed further using deep X-ray , SZ and dynamics analysis , for a detailed study of the physics in a relaxed cluster .