Abstract
Abstract The strong lensing modelling of gravitational ‘rings’ formed around massive galaxies is sensitive to the amplitude of the external shear and convergence produced by nearby mass condensations. In current wide-field surveys, it is now possible to find out a large number of rings, typically 10 gravitational rings per square degree. We propose here, to systematically study gravitational rings around galaxy clusters to probe the cluster mass profile beyond the cluster strong lensing regions. For cluster of galaxies with multiple arc systems, we show that rings found at various distances from the cluster centre can improve the modelling by constraining the slope of the cluster mass profile. We outline the principle of the method with simple numerical simulations and we apply it to three rings discovered recently in Abell 1689. In particular, the lens modelling of the three rings confirms that the cluster is bimodal, and favours a slope of the mass profile steeper than isothermal at a cluster radius ∼300 kpc. These results are compared with previous lens modelling of Abell 1689 including weak lensing analysis. Because of the difficulty arising from the complex mass distribution in Abell 1689, we argue that the ring method will be better implemented on simpler and relaxed clusters.
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